LCOV - code coverage report
Current view: top level - src/backend/storage/ipc - procarray.c (source / functions) Hit Total Coverage
Test: PostgreSQL 19devel Lines: 1159 1319 87.9 %
Date: 2025-07-29 03:18:01 Functions: 71 76 93.4 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : /*-------------------------------------------------------------------------
       2             :  *
       3             :  * procarray.c
       4             :  *    POSTGRES process array code.
       5             :  *
       6             :  *
       7             :  * This module maintains arrays of PGPROC substructures, as well as associated
       8             :  * arrays in ProcGlobal, for all active backends.  Although there are several
       9             :  * uses for this, the principal one is as a means of determining the set of
      10             :  * currently running transactions.
      11             :  *
      12             :  * Because of various subtle race conditions it is critical that a backend
      13             :  * hold the correct locks while setting or clearing its xid (in
      14             :  * ProcGlobal->xids[]/MyProc->xid).  See notes in
      15             :  * src/backend/access/transam/README.
      16             :  *
      17             :  * The process arrays now also include structures representing prepared
      18             :  * transactions.  The xid and subxids fields of these are valid, as are the
      19             :  * myProcLocks lists.  They can be distinguished from regular backend PGPROCs
      20             :  * at need by checking for pid == 0.
      21             :  *
      22             :  * During hot standby, we also keep a list of XIDs representing transactions
      23             :  * that are known to be running on the primary (or more precisely, were running
      24             :  * as of the current point in the WAL stream).  This list is kept in the
      25             :  * KnownAssignedXids array, and is updated by watching the sequence of
      26             :  * arriving XIDs.  This is necessary because if we leave those XIDs out of
      27             :  * snapshots taken for standby queries, then they will appear to be already
      28             :  * complete, leading to MVCC failures.  Note that in hot standby, the PGPROC
      29             :  * array represents standby processes, which by definition are not running
      30             :  * transactions that have XIDs.
      31             :  *
      32             :  * It is perhaps possible for a backend on the primary to terminate without
      33             :  * writing an abort record for its transaction.  While that shouldn't really
      34             :  * happen, it would tie up KnownAssignedXids indefinitely, so we protect
      35             :  * ourselves by pruning the array when a valid list of running XIDs arrives.
      36             :  *
      37             :  * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
      38             :  * Portions Copyright (c) 1994, Regents of the University of California
      39             :  *
      40             :  *
      41             :  * IDENTIFICATION
      42             :  *    src/backend/storage/ipc/procarray.c
      43             :  *
      44             :  *-------------------------------------------------------------------------
      45             :  */
      46             : #include "postgres.h"
      47             : 
      48             : #include <signal.h>
      49             : 
      50             : #include "access/subtrans.h"
      51             : #include "access/transam.h"
      52             : #include "access/twophase.h"
      53             : #include "access/xact.h"
      54             : #include "access/xlogutils.h"
      55             : #include "catalog/catalog.h"
      56             : #include "catalog/pg_authid.h"
      57             : #include "commands/dbcommands.h"
      58             : #include "miscadmin.h"
      59             : #include "pgstat.h"
      60             : #include "port/pg_lfind.h"
      61             : #include "storage/proc.h"
      62             : #include "storage/procarray.h"
      63             : #include "utils/acl.h"
      64             : #include "utils/builtins.h"
      65             : #include "utils/rel.h"
      66             : #include "utils/snapmgr.h"
      67             : 
      68             : #define UINT32_ACCESS_ONCE(var)      ((uint32)(*((volatile uint32 *)&(var))))
      69             : 
      70             : /* Our shared memory area */
      71             : typedef struct ProcArrayStruct
      72             : {
      73             :     int         numProcs;       /* number of valid procs entries */
      74             :     int         maxProcs;       /* allocated size of procs array */
      75             : 
      76             :     /*
      77             :      * Known assigned XIDs handling
      78             :      */
      79             :     int         maxKnownAssignedXids;   /* allocated size of array */
      80             :     int         numKnownAssignedXids;   /* current # of valid entries */
      81             :     int         tailKnownAssignedXids;  /* index of oldest valid element */
      82             :     int         headKnownAssignedXids;  /* index of newest element, + 1 */
      83             : 
      84             :     /*
      85             :      * Highest subxid that has been removed from KnownAssignedXids array to
      86             :      * prevent overflow; or InvalidTransactionId if none.  We track this for
      87             :      * similar reasons to tracking overflowing cached subxids in PGPROC
      88             :      * entries.  Must hold exclusive ProcArrayLock to change this, and shared
      89             :      * lock to read it.
      90             :      */
      91             :     TransactionId lastOverflowedXid;
      92             : 
      93             :     /* oldest xmin of any replication slot */
      94             :     TransactionId replication_slot_xmin;
      95             :     /* oldest catalog xmin of any replication slot */
      96             :     TransactionId replication_slot_catalog_xmin;
      97             : 
      98             :     /* indexes into allProcs[], has PROCARRAY_MAXPROCS entries */
      99             :     int         pgprocnos[FLEXIBLE_ARRAY_MEMBER];
     100             : } ProcArrayStruct;
     101             : 
     102             : /*
     103             :  * State for the GlobalVisTest* family of functions. Those functions can
     104             :  * e.g. be used to decide if a deleted row can be removed without violating
     105             :  * MVCC semantics: If the deleted row's xmax is not considered to be running
     106             :  * by anyone, the row can be removed.
     107             :  *
     108             :  * To avoid slowing down GetSnapshotData(), we don't calculate a precise
     109             :  * cutoff XID while building a snapshot (looking at the frequently changing
     110             :  * xmins scales badly). Instead we compute two boundaries while building the
     111             :  * snapshot:
     112             :  *
     113             :  * 1) definitely_needed, indicating that rows deleted by XIDs >=
     114             :  *    definitely_needed are definitely still visible.
     115             :  *
     116             :  * 2) maybe_needed, indicating that rows deleted by XIDs < maybe_needed can
     117             :  *    definitely be removed
     118             :  *
     119             :  * When testing an XID that falls in between the two (i.e. XID >= maybe_needed
     120             :  * && XID < definitely_needed), the boundaries can be recomputed (using
     121             :  * ComputeXidHorizons()) to get a more accurate answer. This is cheaper than
     122             :  * maintaining an accurate value all the time.
     123             :  *
     124             :  * As it is not cheap to compute accurate boundaries, we limit the number of
     125             :  * times that happens in short succession. See GlobalVisTestShouldUpdate().
     126             :  *
     127             :  *
     128             :  * There are three backend lifetime instances of this struct, optimized for
     129             :  * different types of relations. As e.g. a normal user defined table in one
     130             :  * database is inaccessible to backends connected to another database, a test
     131             :  * specific to a relation can be more aggressive than a test for a shared
     132             :  * relation.  Currently we track four different states:
     133             :  *
     134             :  * 1) GlobalVisSharedRels, which only considers an XID's
     135             :  *    effects visible-to-everyone if neither snapshots in any database, nor a
     136             :  *    replication slot's xmin, nor a replication slot's catalog_xmin might
     137             :  *    still consider XID as running.
     138             :  *
     139             :  * 2) GlobalVisCatalogRels, which only considers an XID's
     140             :  *    effects visible-to-everyone if neither snapshots in the current
     141             :  *    database, nor a replication slot's xmin, nor a replication slot's
     142             :  *    catalog_xmin might still consider XID as running.
     143             :  *
     144             :  *    I.e. the difference to GlobalVisSharedRels is that
     145             :  *    snapshot in other databases are ignored.
     146             :  *
     147             :  * 3) GlobalVisDataRels, which only considers an XID's
     148             :  *    effects visible-to-everyone if neither snapshots in the current
     149             :  *    database, nor a replication slot's xmin consider XID as running.
     150             :  *
     151             :  *    I.e. the difference to GlobalVisCatalogRels is that
     152             :  *    replication slot's catalog_xmin is not taken into account.
     153             :  *
     154             :  * 4) GlobalVisTempRels, which only considers the current session, as temp
     155             :  *    tables are not visible to other sessions.
     156             :  *
     157             :  * GlobalVisTestFor(relation) returns the appropriate state
     158             :  * for the relation.
     159             :  *
     160             :  * The boundaries are FullTransactionIds instead of TransactionIds to avoid
     161             :  * wraparound dangers. There e.g. would otherwise exist no procarray state to
     162             :  * prevent maybe_needed to become old enough after the GetSnapshotData()
     163             :  * call.
     164             :  *
     165             :  * The typedef is in the header.
     166             :  */
     167             : struct GlobalVisState
     168             : {
     169             :     /* XIDs >= are considered running by some backend */
     170             :     FullTransactionId definitely_needed;
     171             : 
     172             :     /* XIDs < are not considered to be running by any backend */
     173             :     FullTransactionId maybe_needed;
     174             : };
     175             : 
     176             : /*
     177             :  * Result of ComputeXidHorizons().
     178             :  */
     179             : typedef struct ComputeXidHorizonsResult
     180             : {
     181             :     /*
     182             :      * The value of TransamVariables->latestCompletedXid when
     183             :      * ComputeXidHorizons() held ProcArrayLock.
     184             :      */
     185             :     FullTransactionId latest_completed;
     186             : 
     187             :     /*
     188             :      * The same for procArray->replication_slot_xmin and
     189             :      * procArray->replication_slot_catalog_xmin.
     190             :      */
     191             :     TransactionId slot_xmin;
     192             :     TransactionId slot_catalog_xmin;
     193             : 
     194             :     /*
     195             :      * Oldest xid that any backend might still consider running. This needs to
     196             :      * include processes running VACUUM, in contrast to the normal visibility
     197             :      * cutoffs, as vacuum needs to be able to perform pg_subtrans lookups when
     198             :      * determining visibility, but doesn't care about rows above its xmin to
     199             :      * be removed.
     200             :      *
     201             :      * This likely should only be needed to determine whether pg_subtrans can
     202             :      * be truncated. It currently includes the effects of replication slots,
     203             :      * for historical reasons. But that could likely be changed.
     204             :      */
     205             :     TransactionId oldest_considered_running;
     206             : 
     207             :     /*
     208             :      * Oldest xid for which deleted tuples need to be retained in shared
     209             :      * tables.
     210             :      *
     211             :      * This includes the effects of replication slots. If that's not desired,
     212             :      * look at shared_oldest_nonremovable_raw;
     213             :      */
     214             :     TransactionId shared_oldest_nonremovable;
     215             : 
     216             :     /*
     217             :      * Oldest xid that may be necessary to retain in shared tables. This is
     218             :      * the same as shared_oldest_nonremovable, except that is not affected by
     219             :      * replication slot's catalog_xmin.
     220             :      *
     221             :      * This is mainly useful to be able to send the catalog_xmin to upstream
     222             :      * streaming replication servers via hot_standby_feedback, so they can
     223             :      * apply the limit only when accessing catalog tables.
     224             :      */
     225             :     TransactionId shared_oldest_nonremovable_raw;
     226             : 
     227             :     /*
     228             :      * Oldest xid for which deleted tuples need to be retained in non-shared
     229             :      * catalog tables.
     230             :      */
     231             :     TransactionId catalog_oldest_nonremovable;
     232             : 
     233             :     /*
     234             :      * Oldest xid for which deleted tuples need to be retained in normal user
     235             :      * defined tables.
     236             :      */
     237             :     TransactionId data_oldest_nonremovable;
     238             : 
     239             :     /*
     240             :      * Oldest xid for which deleted tuples need to be retained in this
     241             :      * session's temporary tables.
     242             :      */
     243             :     TransactionId temp_oldest_nonremovable;
     244             : } ComputeXidHorizonsResult;
     245             : 
     246             : /*
     247             :  * Return value for GlobalVisHorizonKindForRel().
     248             :  */
     249             : typedef enum GlobalVisHorizonKind
     250             : {
     251             :     VISHORIZON_SHARED,
     252             :     VISHORIZON_CATALOG,
     253             :     VISHORIZON_DATA,
     254             :     VISHORIZON_TEMP,
     255             : } GlobalVisHorizonKind;
     256             : 
     257             : /*
     258             :  * Reason codes for KnownAssignedXidsCompress().
     259             :  */
     260             : typedef enum KAXCompressReason
     261             : {
     262             :     KAX_NO_SPACE,               /* need to free up space at array end */
     263             :     KAX_PRUNE,                  /* we just pruned old entries */
     264             :     KAX_TRANSACTION_END,        /* we just committed/removed some XIDs */
     265             :     KAX_STARTUP_PROCESS_IDLE,   /* startup process is about to sleep */
     266             : } KAXCompressReason;
     267             : 
     268             : 
     269             : static ProcArrayStruct *procArray;
     270             : 
     271             : static PGPROC *allProcs;
     272             : 
     273             : /*
     274             :  * Cache to reduce overhead of repeated calls to TransactionIdIsInProgress()
     275             :  */
     276             : static TransactionId cachedXidIsNotInProgress = InvalidTransactionId;
     277             : 
     278             : /*
     279             :  * Bookkeeping for tracking emulated transactions in recovery
     280             :  */
     281             : static TransactionId *KnownAssignedXids;
     282             : static bool *KnownAssignedXidsValid;
     283             : static TransactionId latestObservedXid = InvalidTransactionId;
     284             : 
     285             : /*
     286             :  * If we're in STANDBY_SNAPSHOT_PENDING state, standbySnapshotPendingXmin is
     287             :  * the highest xid that might still be running that we don't have in
     288             :  * KnownAssignedXids.
     289             :  */
     290             : static TransactionId standbySnapshotPendingXmin;
     291             : 
     292             : /*
     293             :  * State for visibility checks on different types of relations. See struct
     294             :  * GlobalVisState for details. As shared, catalog, normal and temporary
     295             :  * relations can have different horizons, one such state exists for each.
     296             :  */
     297             : static GlobalVisState GlobalVisSharedRels;
     298             : static GlobalVisState GlobalVisCatalogRels;
     299             : static GlobalVisState GlobalVisDataRels;
     300             : static GlobalVisState GlobalVisTempRels;
     301             : 
     302             : /*
     303             :  * This backend's RecentXmin at the last time the accurate xmin horizon was
     304             :  * recomputed, or InvalidTransactionId if it has not. Used to limit how many
     305             :  * times accurate horizons are recomputed. See GlobalVisTestShouldUpdate().
     306             :  */
     307             : static TransactionId ComputeXidHorizonsResultLastXmin;
     308             : 
     309             : #ifdef XIDCACHE_DEBUG
     310             : 
     311             : /* counters for XidCache measurement */
     312             : static long xc_by_recent_xmin = 0;
     313             : static long xc_by_known_xact = 0;
     314             : static long xc_by_my_xact = 0;
     315             : static long xc_by_latest_xid = 0;
     316             : static long xc_by_main_xid = 0;
     317             : static long xc_by_child_xid = 0;
     318             : static long xc_by_known_assigned = 0;
     319             : static long xc_no_overflow = 0;
     320             : static long xc_slow_answer = 0;
     321             : 
     322             : #define xc_by_recent_xmin_inc()     (xc_by_recent_xmin++)
     323             : #define xc_by_known_xact_inc()      (xc_by_known_xact++)
     324             : #define xc_by_my_xact_inc()         (xc_by_my_xact++)
     325             : #define xc_by_latest_xid_inc()      (xc_by_latest_xid++)
     326             : #define xc_by_main_xid_inc()        (xc_by_main_xid++)
     327             : #define xc_by_child_xid_inc()       (xc_by_child_xid++)
     328             : #define xc_by_known_assigned_inc()  (xc_by_known_assigned++)
     329             : #define xc_no_overflow_inc()        (xc_no_overflow++)
     330             : #define xc_slow_answer_inc()        (xc_slow_answer++)
     331             : 
     332             : static void DisplayXidCache(void);
     333             : #else                           /* !XIDCACHE_DEBUG */
     334             : 
     335             : #define xc_by_recent_xmin_inc()     ((void) 0)
     336             : #define xc_by_known_xact_inc()      ((void) 0)
     337             : #define xc_by_my_xact_inc()         ((void) 0)
     338             : #define xc_by_latest_xid_inc()      ((void) 0)
     339             : #define xc_by_main_xid_inc()        ((void) 0)
     340             : #define xc_by_child_xid_inc()       ((void) 0)
     341             : #define xc_by_known_assigned_inc()  ((void) 0)
     342             : #define xc_no_overflow_inc()        ((void) 0)
     343             : #define xc_slow_answer_inc()        ((void) 0)
     344             : #endif                          /* XIDCACHE_DEBUG */
     345             : 
     346             : /* Primitives for KnownAssignedXids array handling for standby */
     347             : static void KnownAssignedXidsCompress(KAXCompressReason reason, bool haveLock);
     348             : static void KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
     349             :                                  bool exclusive_lock);
     350             : static bool KnownAssignedXidsSearch(TransactionId xid, bool remove);
     351             : static bool KnownAssignedXidExists(TransactionId xid);
     352             : static void KnownAssignedXidsRemove(TransactionId xid);
     353             : static void KnownAssignedXidsRemoveTree(TransactionId xid, int nsubxids,
     354             :                                         TransactionId *subxids);
     355             : static void KnownAssignedXidsRemovePreceding(TransactionId removeXid);
     356             : static int  KnownAssignedXidsGet(TransactionId *xarray, TransactionId xmax);
     357             : static int  KnownAssignedXidsGetAndSetXmin(TransactionId *xarray,
     358             :                                            TransactionId *xmin,
     359             :                                            TransactionId xmax);
     360             : static TransactionId KnownAssignedXidsGetOldestXmin(void);
     361             : static void KnownAssignedXidsDisplay(int trace_level);
     362             : static void KnownAssignedXidsReset(void);
     363             : static inline void ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId latestXid);
     364             : static void ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid);
     365             : static void MaintainLatestCompletedXid(TransactionId latestXid);
     366             : static void MaintainLatestCompletedXidRecovery(TransactionId latestXid);
     367             : 
     368             : static inline FullTransactionId FullXidRelativeTo(FullTransactionId rel,
     369             :                                                   TransactionId xid);
     370             : static void GlobalVisUpdateApply(ComputeXidHorizonsResult *horizons);
     371             : 
     372             : /*
     373             :  * Report shared-memory space needed by ProcArrayShmemInit
     374             :  */
     375             : Size
     376        3998 : ProcArrayShmemSize(void)
     377             : {
     378             :     Size        size;
     379             : 
     380             :     /* Size of the ProcArray structure itself */
     381             : #define PROCARRAY_MAXPROCS  (MaxBackends + max_prepared_xacts)
     382             : 
     383        3998 :     size = offsetof(ProcArrayStruct, pgprocnos);
     384        3998 :     size = add_size(size, mul_size(sizeof(int), PROCARRAY_MAXPROCS));
     385             : 
     386             :     /*
     387             :      * During Hot Standby processing we have a data structure called
     388             :      * KnownAssignedXids, created in shared memory. Local data structures are
     389             :      * also created in various backends during GetSnapshotData(),
     390             :      * TransactionIdIsInProgress() and GetRunningTransactionData(). All of the
     391             :      * main structures created in those functions must be identically sized,
     392             :      * since we may at times copy the whole of the data structures around. We
     393             :      * refer to this size as TOTAL_MAX_CACHED_SUBXIDS.
     394             :      *
     395             :      * Ideally we'd only create this structure if we were actually doing hot
     396             :      * standby in the current run, but we don't know that yet at the time
     397             :      * shared memory is being set up.
     398             :      */
     399             : #define TOTAL_MAX_CACHED_SUBXIDS \
     400             :     ((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
     401             : 
     402        3998 :     if (EnableHotStandby)
     403             :     {
     404        3974 :         size = add_size(size,
     405             :                         mul_size(sizeof(TransactionId),
     406        3974 :                                  TOTAL_MAX_CACHED_SUBXIDS));
     407        3974 :         size = add_size(size,
     408        3974 :                         mul_size(sizeof(bool), TOTAL_MAX_CACHED_SUBXIDS));
     409             :     }
     410             : 
     411        3998 :     return size;
     412             : }
     413             : 
     414             : /*
     415             :  * Initialize the shared PGPROC array during postmaster startup.
     416             :  */
     417             : void
     418        2152 : ProcArrayShmemInit(void)
     419             : {
     420             :     bool        found;
     421             : 
     422             :     /* Create or attach to the ProcArray shared structure */
     423        2152 :     procArray = (ProcArrayStruct *)
     424        2152 :         ShmemInitStruct("Proc Array",
     425             :                         add_size(offsetof(ProcArrayStruct, pgprocnos),
     426             :                                  mul_size(sizeof(int),
     427        2152 :                                           PROCARRAY_MAXPROCS)),
     428             :                         &found);
     429             : 
     430        2152 :     if (!found)
     431             :     {
     432             :         /*
     433             :          * We're the first - initialize.
     434             :          */
     435        2152 :         procArray->numProcs = 0;
     436        2152 :         procArray->maxProcs = PROCARRAY_MAXPROCS;
     437        2152 :         procArray->maxKnownAssignedXids = TOTAL_MAX_CACHED_SUBXIDS;
     438        2152 :         procArray->numKnownAssignedXids = 0;
     439        2152 :         procArray->tailKnownAssignedXids = 0;
     440        2152 :         procArray->headKnownAssignedXids = 0;
     441        2152 :         procArray->lastOverflowedXid = InvalidTransactionId;
     442        2152 :         procArray->replication_slot_xmin = InvalidTransactionId;
     443        2152 :         procArray->replication_slot_catalog_xmin = InvalidTransactionId;
     444        2152 :         TransamVariables->xactCompletionCount = 1;
     445             :     }
     446             : 
     447        2152 :     allProcs = ProcGlobal->allProcs;
     448             : 
     449             :     /* Create or attach to the KnownAssignedXids arrays too, if needed */
     450        2152 :     if (EnableHotStandby)
     451             :     {
     452        2140 :         KnownAssignedXids = (TransactionId *)
     453        2140 :             ShmemInitStruct("KnownAssignedXids",
     454             :                             mul_size(sizeof(TransactionId),
     455        2140 :                                      TOTAL_MAX_CACHED_SUBXIDS),
     456             :                             &found);
     457        2140 :         KnownAssignedXidsValid = (bool *)
     458        2140 :             ShmemInitStruct("KnownAssignedXidsValid",
     459        2140 :                             mul_size(sizeof(bool), TOTAL_MAX_CACHED_SUBXIDS),
     460             :                             &found);
     461             :     }
     462        2152 : }
     463             : 
     464             : /*
     465             :  * Add the specified PGPROC to the shared array.
     466             :  */
     467             : void
     468       34888 : ProcArrayAdd(PGPROC *proc)
     469             : {
     470       34888 :     int         pgprocno = GetNumberFromPGProc(proc);
     471       34888 :     ProcArrayStruct *arrayP = procArray;
     472             :     int         index;
     473             :     int         movecount;
     474             : 
     475             :     /* See ProcGlobal comment explaining why both locks are held */
     476       34888 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
     477       34888 :     LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
     478             : 
     479       34888 :     if (arrayP->numProcs >= arrayP->maxProcs)
     480             :     {
     481             :         /*
     482             :          * Oops, no room.  (This really shouldn't happen, since there is a
     483             :          * fixed supply of PGPROC structs too, and so we should have failed
     484             :          * earlier.)
     485             :          */
     486           0 :         ereport(FATAL,
     487             :                 (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
     488             :                  errmsg("sorry, too many clients already")));
     489             :     }
     490             : 
     491             :     /*
     492             :      * Keep the procs array sorted by (PGPROC *) so that we can utilize
     493             :      * locality of references much better. This is useful while traversing the
     494             :      * ProcArray because there is an increased likelihood of finding the next
     495             :      * PGPROC structure in the cache.
     496             :      *
     497             :      * Since the occurrence of adding/removing a proc is much lower than the
     498             :      * access to the ProcArray itself, the overhead should be marginal
     499             :      */
     500       83108 :     for (index = 0; index < arrayP->numProcs; index++)
     501             :     {
     502       75068 :         int         this_procno = arrayP->pgprocnos[index];
     503             : 
     504             :         Assert(this_procno >= 0 && this_procno < (arrayP->maxProcs + NUM_AUXILIARY_PROCS));
     505             :         Assert(allProcs[this_procno].pgxactoff == index);
     506             : 
     507             :         /* If we have found our right position in the array, break */
     508       75068 :         if (this_procno > pgprocno)
     509       26848 :             break;
     510             :     }
     511             : 
     512       34888 :     movecount = arrayP->numProcs - index;
     513       34888 :     memmove(&arrayP->pgprocnos[index + 1],
     514       34888 :             &arrayP->pgprocnos[index],
     515             :             movecount * sizeof(*arrayP->pgprocnos));
     516       34888 :     memmove(&ProcGlobal->xids[index + 1],
     517       34888 :             &ProcGlobal->xids[index],
     518             :             movecount * sizeof(*ProcGlobal->xids));
     519       34888 :     memmove(&ProcGlobal->subxidStates[index + 1],
     520       34888 :             &ProcGlobal->subxidStates[index],
     521             :             movecount * sizeof(*ProcGlobal->subxidStates));
     522       34888 :     memmove(&ProcGlobal->statusFlags[index + 1],
     523       34888 :             &ProcGlobal->statusFlags[index],
     524             :             movecount * sizeof(*ProcGlobal->statusFlags));
     525             : 
     526       34888 :     arrayP->pgprocnos[index] = GetNumberFromPGProc(proc);
     527       34888 :     proc->pgxactoff = index;
     528       34888 :     ProcGlobal->xids[index] = proc->xid;
     529       34888 :     ProcGlobal->subxidStates[index] = proc->subxidStatus;
     530       34888 :     ProcGlobal->statusFlags[index] = proc->statusFlags;
     531             : 
     532       34888 :     arrayP->numProcs++;
     533             : 
     534             :     /* adjust pgxactoff for all following PGPROCs */
     535       34888 :     index++;
     536       96824 :     for (; index < arrayP->numProcs; index++)
     537             :     {
     538       61936 :         int         procno = arrayP->pgprocnos[index];
     539             : 
     540             :         Assert(procno >= 0 && procno < (arrayP->maxProcs + NUM_AUXILIARY_PROCS));
     541             :         Assert(allProcs[procno].pgxactoff == index - 1);
     542             : 
     543       61936 :         allProcs[procno].pgxactoff = index;
     544             :     }
     545             : 
     546             :     /*
     547             :      * Release in reversed acquisition order, to reduce frequency of having to
     548             :      * wait for XidGenLock while holding ProcArrayLock.
     549             :      */
     550       34888 :     LWLockRelease(XidGenLock);
     551       34888 :     LWLockRelease(ProcArrayLock);
     552       34888 : }
     553             : 
     554             : /*
     555             :  * Remove the specified PGPROC from the shared array.
     556             :  *
     557             :  * When latestXid is a valid XID, we are removing a live 2PC gxact from the
     558             :  * array, and thus causing it to appear as "not running" anymore.  In this
     559             :  * case we must advance latestCompletedXid.  (This is essentially the same
     560             :  * as ProcArrayEndTransaction followed by removal of the PGPROC, but we take
     561             :  * the ProcArrayLock only once, and don't damage the content of the PGPROC;
     562             :  * twophase.c depends on the latter.)
     563             :  */
     564             : void
     565       34836 : ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
     566             : {
     567       34836 :     ProcArrayStruct *arrayP = procArray;
     568             :     int         myoff;
     569             :     int         movecount;
     570             : 
     571             : #ifdef XIDCACHE_DEBUG
     572             :     /* dump stats at backend shutdown, but not prepared-xact end */
     573             :     if (proc->pid != 0)
     574             :         DisplayXidCache();
     575             : #endif
     576             : 
     577             :     /* See ProcGlobal comment explaining why both locks are held */
     578       34836 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
     579       34836 :     LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
     580             : 
     581       34836 :     myoff = proc->pgxactoff;
     582             : 
     583             :     Assert(myoff >= 0 && myoff < arrayP->numProcs);
     584             :     Assert(ProcGlobal->allProcs[arrayP->pgprocnos[myoff]].pgxactoff == myoff);
     585             : 
     586       34836 :     if (TransactionIdIsValid(latestXid))
     587             :     {
     588             :         Assert(TransactionIdIsValid(ProcGlobal->xids[myoff]));
     589             : 
     590             :         /* Advance global latestCompletedXid while holding the lock */
     591         590 :         MaintainLatestCompletedXid(latestXid);
     592             : 
     593             :         /* Same with xactCompletionCount  */
     594         590 :         TransamVariables->xactCompletionCount++;
     595             : 
     596         590 :         ProcGlobal->xids[myoff] = InvalidTransactionId;
     597         590 :         ProcGlobal->subxidStates[myoff].overflowed = false;
     598         590 :         ProcGlobal->subxidStates[myoff].count = 0;
     599             :     }
     600             :     else
     601             :     {
     602             :         /* Shouldn't be trying to remove a live transaction here */
     603             :         Assert(!TransactionIdIsValid(ProcGlobal->xids[myoff]));
     604             :     }
     605             : 
     606             :     Assert(!TransactionIdIsValid(ProcGlobal->xids[myoff]));
     607             :     Assert(ProcGlobal->subxidStates[myoff].count == 0);
     608             :     Assert(ProcGlobal->subxidStates[myoff].overflowed == false);
     609             : 
     610       34836 :     ProcGlobal->statusFlags[myoff] = 0;
     611             : 
     612             :     /* Keep the PGPROC array sorted. See notes above */
     613       34836 :     movecount = arrayP->numProcs - myoff - 1;
     614       34836 :     memmove(&arrayP->pgprocnos[myoff],
     615       34836 :             &arrayP->pgprocnos[myoff + 1],
     616             :             movecount * sizeof(*arrayP->pgprocnos));
     617       34836 :     memmove(&ProcGlobal->xids[myoff],
     618       34836 :             &ProcGlobal->xids[myoff + 1],
     619             :             movecount * sizeof(*ProcGlobal->xids));
     620       34836 :     memmove(&ProcGlobal->subxidStates[myoff],
     621       34836 :             &ProcGlobal->subxidStates[myoff + 1],
     622             :             movecount * sizeof(*ProcGlobal->subxidStates));
     623       34836 :     memmove(&ProcGlobal->statusFlags[myoff],
     624       34836 :             &ProcGlobal->statusFlags[myoff + 1],
     625             :             movecount * sizeof(*ProcGlobal->statusFlags));
     626             : 
     627       34836 :     arrayP->pgprocnos[arrayP->numProcs - 1] = -1; /* for debugging */
     628       34836 :     arrayP->numProcs--;
     629             : 
     630             :     /*
     631             :      * Adjust pgxactoff of following procs for removed PGPROC (note that
     632             :      * numProcs already has been decremented).
     633             :      */
     634      102146 :     for (int index = myoff; index < arrayP->numProcs; index++)
     635             :     {
     636       67310 :         int         procno = arrayP->pgprocnos[index];
     637             : 
     638             :         Assert(procno >= 0 && procno < (arrayP->maxProcs + NUM_AUXILIARY_PROCS));
     639             :         Assert(allProcs[procno].pgxactoff - 1 == index);
     640             : 
     641       67310 :         allProcs[procno].pgxactoff = index;
     642             :     }
     643             : 
     644             :     /*
     645             :      * Release in reversed acquisition order, to reduce frequency of having to
     646             :      * wait for XidGenLock while holding ProcArrayLock.
     647             :      */
     648       34836 :     LWLockRelease(XidGenLock);
     649       34836 :     LWLockRelease(ProcArrayLock);
     650       34836 : }
     651             : 
     652             : 
     653             : /*
     654             :  * ProcArrayEndTransaction -- mark a transaction as no longer running
     655             :  *
     656             :  * This is used interchangeably for commit and abort cases.  The transaction
     657             :  * commit/abort must already be reported to WAL and pg_xact.
     658             :  *
     659             :  * proc is currently always MyProc, but we pass it explicitly for flexibility.
     660             :  * latestXid is the latest Xid among the transaction's main XID and
     661             :  * subtransactions, or InvalidTransactionId if it has no XID.  (We must ask
     662             :  * the caller to pass latestXid, instead of computing it from the PGPROC's
     663             :  * contents, because the subxid information in the PGPROC might be
     664             :  * incomplete.)
     665             :  */
     666             : void
     667     1096324 : ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
     668             : {
     669     1096324 :     if (TransactionIdIsValid(latestXid))
     670             :     {
     671             :         /*
     672             :          * We must lock ProcArrayLock while clearing our advertised XID, so
     673             :          * that we do not exit the set of "running" transactions while someone
     674             :          * else is taking a snapshot.  See discussion in
     675             :          * src/backend/access/transam/README.
     676             :          */
     677             :         Assert(TransactionIdIsValid(proc->xid));
     678             : 
     679             :         /*
     680             :          * If we can immediately acquire ProcArrayLock, we clear our own XID
     681             :          * and release the lock.  If not, use group XID clearing to improve
     682             :          * efficiency.
     683             :          */
     684      265998 :         if (LWLockConditionalAcquire(ProcArrayLock, LW_EXCLUSIVE))
     685             :         {
     686      265712 :             ProcArrayEndTransactionInternal(proc, latestXid);
     687      265712 :             LWLockRelease(ProcArrayLock);
     688             :         }
     689             :         else
     690         286 :             ProcArrayGroupClearXid(proc, latestXid);
     691             :     }
     692             :     else
     693             :     {
     694             :         /*
     695             :          * If we have no XID, we don't need to lock, since we won't affect
     696             :          * anyone else's calculation of a snapshot.  We might change their
     697             :          * estimate of global xmin, but that's OK.
     698             :          */
     699             :         Assert(!TransactionIdIsValid(proc->xid));
     700             :         Assert(proc->subxidStatus.count == 0);
     701             :         Assert(!proc->subxidStatus.overflowed);
     702             : 
     703      830326 :         proc->vxid.lxid = InvalidLocalTransactionId;
     704      830326 :         proc->xmin = InvalidTransactionId;
     705             : 
     706             :         /* be sure this is cleared in abort */
     707      830326 :         proc->delayChkptFlags = 0;
     708             : 
     709      830326 :         proc->recoveryConflictPending = false;
     710             : 
     711             :         /* must be cleared with xid/xmin: */
     712             :         /* avoid unnecessarily dirtying shared cachelines */
     713      830326 :         if (proc->statusFlags & PROC_VACUUM_STATE_MASK)
     714             :         {
     715             :             Assert(!LWLockHeldByMe(ProcArrayLock));
     716      242730 :             LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
     717             :             Assert(proc->statusFlags == ProcGlobal->statusFlags[proc->pgxactoff]);
     718      242730 :             proc->statusFlags &= ~PROC_VACUUM_STATE_MASK;
     719      242730 :             ProcGlobal->statusFlags[proc->pgxactoff] = proc->statusFlags;
     720      242730 :             LWLockRelease(ProcArrayLock);
     721             :         }
     722             :     }
     723     1096324 : }
     724             : 
     725             : /*
     726             :  * Mark a write transaction as no longer running.
     727             :  *
     728             :  * We don't do any locking here; caller must handle that.
     729             :  */
     730             : static inline void
     731      265998 : ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId latestXid)
     732             : {
     733      265998 :     int         pgxactoff = proc->pgxactoff;
     734             : 
     735             :     /*
     736             :      * Note: we need exclusive lock here because we're going to change other
     737             :      * processes' PGPROC entries.
     738             :      */
     739             :     Assert(LWLockHeldByMeInMode(ProcArrayLock, LW_EXCLUSIVE));
     740             :     Assert(TransactionIdIsValid(ProcGlobal->xids[pgxactoff]));
     741             :     Assert(ProcGlobal->xids[pgxactoff] == proc->xid);
     742             : 
     743      265998 :     ProcGlobal->xids[pgxactoff] = InvalidTransactionId;
     744      265998 :     proc->xid = InvalidTransactionId;
     745      265998 :     proc->vxid.lxid = InvalidLocalTransactionId;
     746      265998 :     proc->xmin = InvalidTransactionId;
     747             : 
     748             :     /* be sure this is cleared in abort */
     749      265998 :     proc->delayChkptFlags = 0;
     750             : 
     751      265998 :     proc->recoveryConflictPending = false;
     752             : 
     753             :     /* must be cleared with xid/xmin: */
     754             :     /* avoid unnecessarily dirtying shared cachelines */
     755      265998 :     if (proc->statusFlags & PROC_VACUUM_STATE_MASK)
     756             :     {
     757        1276 :         proc->statusFlags &= ~PROC_VACUUM_STATE_MASK;
     758        1276 :         ProcGlobal->statusFlags[proc->pgxactoff] = proc->statusFlags;
     759             :     }
     760             : 
     761             :     /* Clear the subtransaction-XID cache too while holding the lock */
     762             :     Assert(ProcGlobal->subxidStates[pgxactoff].count == proc->subxidStatus.count &&
     763             :            ProcGlobal->subxidStates[pgxactoff].overflowed == proc->subxidStatus.overflowed);
     764      265998 :     if (proc->subxidStatus.count > 0 || proc->subxidStatus.overflowed)
     765             :     {
     766        1150 :         ProcGlobal->subxidStates[pgxactoff].count = 0;
     767        1150 :         ProcGlobal->subxidStates[pgxactoff].overflowed = false;
     768        1150 :         proc->subxidStatus.count = 0;
     769        1150 :         proc->subxidStatus.overflowed = false;
     770             :     }
     771             : 
     772             :     /* Also advance global latestCompletedXid while holding the lock */
     773      265998 :     MaintainLatestCompletedXid(latestXid);
     774             : 
     775             :     /* Same with xactCompletionCount  */
     776      265998 :     TransamVariables->xactCompletionCount++;
     777      265998 : }
     778             : 
     779             : /*
     780             :  * ProcArrayGroupClearXid -- group XID clearing
     781             :  *
     782             :  * When we cannot immediately acquire ProcArrayLock in exclusive mode at
     783             :  * commit time, add ourselves to a list of processes that need their XIDs
     784             :  * cleared.  The first process to add itself to the list will acquire
     785             :  * ProcArrayLock in exclusive mode and perform ProcArrayEndTransactionInternal
     786             :  * on behalf of all group members.  This avoids a great deal of contention
     787             :  * around ProcArrayLock when many processes are trying to commit at once,
     788             :  * since the lock need not be repeatedly handed off from one committing
     789             :  * process to the next.
     790             :  */
     791             : static void
     792         286 : ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
     793             : {
     794         286 :     int         pgprocno = GetNumberFromPGProc(proc);
     795         286 :     PROC_HDR   *procglobal = ProcGlobal;
     796             :     uint32      nextidx;
     797             :     uint32      wakeidx;
     798             : 
     799             :     /* We should definitely have an XID to clear. */
     800             :     Assert(TransactionIdIsValid(proc->xid));
     801             : 
     802             :     /* Add ourselves to the list of processes needing a group XID clear. */
     803         286 :     proc->procArrayGroupMember = true;
     804         286 :     proc->procArrayGroupMemberXid = latestXid;
     805         286 :     nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
     806             :     while (true)
     807             :     {
     808         286 :         pg_atomic_write_u32(&proc->procArrayGroupNext, nextidx);
     809             : 
     810         286 :         if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
     811             :                                            &nextidx,
     812             :                                            (uint32) pgprocno))
     813         286 :             break;
     814             :     }
     815             : 
     816             :     /*
     817             :      * If the list was not empty, the leader will clear our XID.  It is
     818             :      * impossible to have followers without a leader because the first process
     819             :      * that has added itself to the list will always have nextidx as
     820             :      * INVALID_PROC_NUMBER.
     821             :      */
     822         286 :     if (nextidx != INVALID_PROC_NUMBER)
     823             :     {
     824          22 :         int         extraWaits = 0;
     825             : 
     826             :         /* Sleep until the leader clears our XID. */
     827          22 :         pgstat_report_wait_start(WAIT_EVENT_PROCARRAY_GROUP_UPDATE);
     828             :         for (;;)
     829             :         {
     830             :             /* acts as a read barrier */
     831          22 :             PGSemaphoreLock(proc->sem);
     832          22 :             if (!proc->procArrayGroupMember)
     833          22 :                 break;
     834           0 :             extraWaits++;
     835             :         }
     836          22 :         pgstat_report_wait_end();
     837             : 
     838             :         Assert(pg_atomic_read_u32(&proc->procArrayGroupNext) == INVALID_PROC_NUMBER);
     839             : 
     840             :         /* Fix semaphore count for any absorbed wakeups */
     841          22 :         while (extraWaits-- > 0)
     842           0 :             PGSemaphoreUnlock(proc->sem);
     843          22 :         return;
     844             :     }
     845             : 
     846             :     /* We are the leader.  Acquire the lock on behalf of everyone. */
     847         264 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
     848             : 
     849             :     /*
     850             :      * Now that we've got the lock, clear the list of processes waiting for
     851             :      * group XID clearing, saving a pointer to the head of the list.  Trying
     852             :      * to pop elements one at a time could lead to an ABA problem.
     853             :      */
     854         264 :     nextidx = pg_atomic_exchange_u32(&procglobal->procArrayGroupFirst,
     855             :                                      INVALID_PROC_NUMBER);
     856             : 
     857             :     /* Remember head of list so we can perform wakeups after dropping lock. */
     858         264 :     wakeidx = nextidx;
     859             : 
     860             :     /* Walk the list and clear all XIDs. */
     861         550 :     while (nextidx != INVALID_PROC_NUMBER)
     862             :     {
     863         286 :         PGPROC     *nextproc = &allProcs[nextidx];
     864             : 
     865         286 :         ProcArrayEndTransactionInternal(nextproc, nextproc->procArrayGroupMemberXid);
     866             : 
     867             :         /* Move to next proc in list. */
     868         286 :         nextidx = pg_atomic_read_u32(&nextproc->procArrayGroupNext);
     869             :     }
     870             : 
     871             :     /* We're done with the lock now. */
     872         264 :     LWLockRelease(ProcArrayLock);
     873             : 
     874             :     /*
     875             :      * Now that we've released the lock, go back and wake everybody up.  We
     876             :      * don't do this under the lock so as to keep lock hold times to a
     877             :      * minimum.  The system calls we need to perform to wake other processes
     878             :      * up are probably much slower than the simple memory writes we did while
     879             :      * holding the lock.
     880             :      */
     881         550 :     while (wakeidx != INVALID_PROC_NUMBER)
     882             :     {
     883         286 :         PGPROC     *nextproc = &allProcs[wakeidx];
     884             : 
     885         286 :         wakeidx = pg_atomic_read_u32(&nextproc->procArrayGroupNext);
     886         286 :         pg_atomic_write_u32(&nextproc->procArrayGroupNext, INVALID_PROC_NUMBER);
     887             : 
     888             :         /* ensure all previous writes are visible before follower continues. */
     889         286 :         pg_write_barrier();
     890             : 
     891         286 :         nextproc->procArrayGroupMember = false;
     892             : 
     893         286 :         if (nextproc != MyProc)
     894          22 :             PGSemaphoreUnlock(nextproc->sem);
     895             :     }
     896             : }
     897             : 
     898             : /*
     899             :  * ProcArrayClearTransaction -- clear the transaction fields
     900             :  *
     901             :  * This is used after successfully preparing a 2-phase transaction.  We are
     902             :  * not actually reporting the transaction's XID as no longer running --- it
     903             :  * will still appear as running because the 2PC's gxact is in the ProcArray
     904             :  * too.  We just have to clear out our own PGPROC.
     905             :  */
     906             : void
     907         576 : ProcArrayClearTransaction(PGPROC *proc)
     908             : {
     909             :     int         pgxactoff;
     910             : 
     911             :     /*
     912             :      * Currently we need to lock ProcArrayLock exclusively here, as we
     913             :      * increment xactCompletionCount below. We also need it at least in shared
     914             :      * mode for pgproc->pgxactoff to stay the same below.
     915             :      *
     916             :      * We could however, as this action does not actually change anyone's view
     917             :      * of the set of running XIDs (our entry is duplicate with the gxact that
     918             :      * has already been inserted into the ProcArray), lower the lock level to
     919             :      * shared if we were to make xactCompletionCount an atomic variable. But
     920             :      * that doesn't seem worth it currently, as a 2PC commit is heavyweight
     921             :      * enough for this not to be the bottleneck.  If it ever becomes a
     922             :      * bottleneck it may also be worth considering to combine this with the
     923             :      * subsequent ProcArrayRemove()
     924             :      */
     925         576 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
     926             : 
     927         576 :     pgxactoff = proc->pgxactoff;
     928             : 
     929         576 :     ProcGlobal->xids[pgxactoff] = InvalidTransactionId;
     930         576 :     proc->xid = InvalidTransactionId;
     931             : 
     932         576 :     proc->vxid.lxid = InvalidLocalTransactionId;
     933         576 :     proc->xmin = InvalidTransactionId;
     934         576 :     proc->recoveryConflictPending = false;
     935             : 
     936             :     Assert(!(proc->statusFlags & PROC_VACUUM_STATE_MASK));
     937             :     Assert(!proc->delayChkptFlags);
     938             : 
     939             :     /*
     940             :      * Need to increment completion count even though transaction hasn't
     941             :      * really committed yet. The reason for that is that GetSnapshotData()
     942             :      * omits the xid of the current transaction, thus without the increment we
     943             :      * otherwise could end up reusing the snapshot later. Which would be bad,
     944             :      * because it might not count the prepared transaction as running.
     945             :      */
     946         576 :     TransamVariables->xactCompletionCount++;
     947             : 
     948             :     /* Clear the subtransaction-XID cache too */
     949             :     Assert(ProcGlobal->subxidStates[pgxactoff].count == proc->subxidStatus.count &&
     950             :            ProcGlobal->subxidStates[pgxactoff].overflowed == proc->subxidStatus.overflowed);
     951         576 :     if (proc->subxidStatus.count > 0 || proc->subxidStatus.overflowed)
     952             :     {
     953         206 :         ProcGlobal->subxidStates[pgxactoff].count = 0;
     954         206 :         ProcGlobal->subxidStates[pgxactoff].overflowed = false;
     955         206 :         proc->subxidStatus.count = 0;
     956         206 :         proc->subxidStatus.overflowed = false;
     957             :     }
     958             : 
     959         576 :     LWLockRelease(ProcArrayLock);
     960         576 : }
     961             : 
     962             : /*
     963             :  * Update TransamVariables->latestCompletedXid to point to latestXid if
     964             :  * currently older.
     965             :  */
     966             : static void
     967      267886 : MaintainLatestCompletedXid(TransactionId latestXid)
     968             : {
     969      267886 :     FullTransactionId cur_latest = TransamVariables->latestCompletedXid;
     970             : 
     971             :     Assert(FullTransactionIdIsValid(cur_latest));
     972             :     Assert(!RecoveryInProgress());
     973             :     Assert(LWLockHeldByMe(ProcArrayLock));
     974             : 
     975      267886 :     if (TransactionIdPrecedes(XidFromFullTransactionId(cur_latest), latestXid))
     976             :     {
     977      243232 :         TransamVariables->latestCompletedXid =
     978      243232 :             FullXidRelativeTo(cur_latest, latestXid);
     979             :     }
     980             : 
     981             :     Assert(IsBootstrapProcessingMode() ||
     982             :            FullTransactionIdIsNormal(TransamVariables->latestCompletedXid));
     983      267886 : }
     984             : 
     985             : /*
     986             :  * Same as MaintainLatestCompletedXid, except for use during WAL replay.
     987             :  */
     988             : static void
     989       44272 : MaintainLatestCompletedXidRecovery(TransactionId latestXid)
     990             : {
     991       44272 :     FullTransactionId cur_latest = TransamVariables->latestCompletedXid;
     992             :     FullTransactionId rel;
     993             : 
     994             :     Assert(AmStartupProcess() || !IsUnderPostmaster);
     995             :     Assert(LWLockHeldByMe(ProcArrayLock));
     996             : 
     997             :     /*
     998             :      * Need a FullTransactionId to compare latestXid with. Can't rely on
     999             :      * latestCompletedXid to be initialized in recovery. But in recovery it's
    1000             :      * safe to access nextXid without a lock for the startup process.
    1001             :      */
    1002       44272 :     rel = TransamVariables->nextXid;
    1003             :     Assert(FullTransactionIdIsValid(TransamVariables->nextXid));
    1004             : 
    1005       88332 :     if (!FullTransactionIdIsValid(cur_latest) ||
    1006       44060 :         TransactionIdPrecedes(XidFromFullTransactionId(cur_latest), latestXid))
    1007             :     {
    1008       33942 :         TransamVariables->latestCompletedXid =
    1009       33942 :             FullXidRelativeTo(rel, latestXid);
    1010             :     }
    1011             : 
    1012             :     Assert(FullTransactionIdIsNormal(TransamVariables->latestCompletedXid));
    1013       44272 : }
    1014             : 
    1015             : /*
    1016             :  * ProcArrayInitRecovery -- initialize recovery xid mgmt environment
    1017             :  *
    1018             :  * Remember up to where the startup process initialized the CLOG and subtrans
    1019             :  * so we can ensure it's initialized gaplessly up to the point where necessary
    1020             :  * while in recovery.
    1021             :  */
    1022             : void
    1023         212 : ProcArrayInitRecovery(TransactionId initializedUptoXID)
    1024             : {
    1025             :     Assert(standbyState == STANDBY_INITIALIZED);
    1026             :     Assert(TransactionIdIsNormal(initializedUptoXID));
    1027             : 
    1028             :     /*
    1029             :      * we set latestObservedXid to the xid SUBTRANS has been initialized up
    1030             :      * to, so we can extend it from that point onwards in
    1031             :      * RecordKnownAssignedTransactionIds, and when we get consistent in
    1032             :      * ProcArrayApplyRecoveryInfo().
    1033             :      */
    1034         212 :     latestObservedXid = initializedUptoXID;
    1035         212 :     TransactionIdRetreat(latestObservedXid);
    1036         212 : }
    1037             : 
    1038             : /*
    1039             :  * ProcArrayApplyRecoveryInfo -- apply recovery info about xids
    1040             :  *
    1041             :  * Takes us through 3 states: Initialized, Pending and Ready.
    1042             :  * Normal case is to go all the way to Ready straight away, though there
    1043             :  * are atypical cases where we need to take it in steps.
    1044             :  *
    1045             :  * Use the data about running transactions on the primary to create the initial
    1046             :  * state of KnownAssignedXids. We also use these records to regularly prune
    1047             :  * KnownAssignedXids because we know it is possible that some transactions
    1048             :  * with FATAL errors fail to write abort records, which could cause eventual
    1049             :  * overflow.
    1050             :  *
    1051             :  * See comments for LogStandbySnapshot().
    1052             :  */
    1053             : void
    1054        1570 : ProcArrayApplyRecoveryInfo(RunningTransactions running)
    1055             : {
    1056             :     TransactionId *xids;
    1057             :     TransactionId advanceNextXid;
    1058             :     int         nxids;
    1059             :     int         i;
    1060             : 
    1061             :     Assert(standbyState >= STANDBY_INITIALIZED);
    1062             :     Assert(TransactionIdIsValid(running->nextXid));
    1063             :     Assert(TransactionIdIsValid(running->oldestRunningXid));
    1064             :     Assert(TransactionIdIsNormal(running->latestCompletedXid));
    1065             : 
    1066             :     /*
    1067             :      * Remove stale transactions, if any.
    1068             :      */
    1069        1570 :     ExpireOldKnownAssignedTransactionIds(running->oldestRunningXid);
    1070             : 
    1071             :     /*
    1072             :      * Adjust TransamVariables->nextXid before StandbyReleaseOldLocks(),
    1073             :      * because we will need it up to date for accessing two-phase transactions
    1074             :      * in StandbyReleaseOldLocks().
    1075             :      */
    1076        1570 :     advanceNextXid = running->nextXid;
    1077        1570 :     TransactionIdRetreat(advanceNextXid);
    1078        1570 :     AdvanceNextFullTransactionIdPastXid(advanceNextXid);
    1079             :     Assert(FullTransactionIdIsValid(TransamVariables->nextXid));
    1080             : 
    1081             :     /*
    1082             :      * Remove stale locks, if any.
    1083             :      */
    1084        1570 :     StandbyReleaseOldLocks(running->oldestRunningXid);
    1085             : 
    1086             :     /*
    1087             :      * If our snapshot is already valid, nothing else to do...
    1088             :      */
    1089        1570 :     if (standbyState == STANDBY_SNAPSHOT_READY)
    1090        1358 :         return;
    1091             : 
    1092             :     /*
    1093             :      * If our initial RunningTransactionsData had an overflowed snapshot then
    1094             :      * we knew we were missing some subxids from our snapshot. If we continue
    1095             :      * to see overflowed snapshots then we might never be able to start up, so
    1096             :      * we make another test to see if our snapshot is now valid. We know that
    1097             :      * the missing subxids are equal to or earlier than nextXid. After we
    1098             :      * initialise we continue to apply changes during recovery, so once the
    1099             :      * oldestRunningXid is later than the nextXid from the initial snapshot we
    1100             :      * know that we no longer have missing information and can mark the
    1101             :      * snapshot as valid.
    1102             :      */
    1103         212 :     if (standbyState == STANDBY_SNAPSHOT_PENDING)
    1104             :     {
    1105             :         /*
    1106             :          * If the snapshot isn't overflowed or if its empty we can reset our
    1107             :          * pending state and use this snapshot instead.
    1108             :          */
    1109           0 :         if (running->subxid_status != SUBXIDS_MISSING || running->xcnt == 0)
    1110             :         {
    1111             :             /*
    1112             :              * If we have already collected known assigned xids, we need to
    1113             :              * throw them away before we apply the recovery snapshot.
    1114             :              */
    1115           0 :             KnownAssignedXidsReset();
    1116           0 :             standbyState = STANDBY_INITIALIZED;
    1117             :         }
    1118             :         else
    1119             :         {
    1120           0 :             if (TransactionIdPrecedes(standbySnapshotPendingXmin,
    1121             :                                       running->oldestRunningXid))
    1122             :             {
    1123           0 :                 standbyState = STANDBY_SNAPSHOT_READY;
    1124           0 :                 elog(DEBUG1,
    1125             :                      "recovery snapshots are now enabled");
    1126             :             }
    1127             :             else
    1128           0 :                 elog(DEBUG1,
    1129             :                      "recovery snapshot waiting for non-overflowed snapshot or "
    1130             :                      "until oldest active xid on standby is at least %u (now %u)",
    1131             :                      standbySnapshotPendingXmin,
    1132             :                      running->oldestRunningXid);
    1133           0 :             return;
    1134             :         }
    1135             :     }
    1136             : 
    1137             :     Assert(standbyState == STANDBY_INITIALIZED);
    1138             : 
    1139             :     /*
    1140             :      * NB: this can be reached at least twice, so make sure new code can deal
    1141             :      * with that.
    1142             :      */
    1143             : 
    1144             :     /*
    1145             :      * Nobody else is running yet, but take locks anyhow
    1146             :      */
    1147         212 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    1148             : 
    1149             :     /*
    1150             :      * KnownAssignedXids is sorted so we cannot just add the xids, we have to
    1151             :      * sort them first.
    1152             :      *
    1153             :      * Some of the new xids are top-level xids and some are subtransactions.
    1154             :      * We don't call SubTransSetParent because it doesn't matter yet. If we
    1155             :      * aren't overflowed then all xids will fit in snapshot and so we don't
    1156             :      * need subtrans. If we later overflow, an xid assignment record will add
    1157             :      * xids to subtrans. If RunningTransactionsData is overflowed then we
    1158             :      * don't have enough information to correctly update subtrans anyway.
    1159             :      */
    1160             : 
    1161             :     /*
    1162             :      * Allocate a temporary array to avoid modifying the array passed as
    1163             :      * argument.
    1164             :      */
    1165         212 :     xids = palloc(sizeof(TransactionId) * (running->xcnt + running->subxcnt));
    1166             : 
    1167             :     /*
    1168             :      * Add to the temp array any xids which have not already completed.
    1169             :      */
    1170         212 :     nxids = 0;
    1171         220 :     for (i = 0; i < running->xcnt + running->subxcnt; i++)
    1172             :     {
    1173           8 :         TransactionId xid = running->xids[i];
    1174             : 
    1175             :         /*
    1176             :          * The running-xacts snapshot can contain xids that were still visible
    1177             :          * in the procarray when the snapshot was taken, but were already
    1178             :          * WAL-logged as completed. They're not running anymore, so ignore
    1179             :          * them.
    1180             :          */
    1181           8 :         if (TransactionIdDidCommit(xid) || TransactionIdDidAbort(xid))
    1182           0 :             continue;
    1183             : 
    1184           8 :         xids[nxids++] = xid;
    1185             :     }
    1186             : 
    1187         212 :     if (nxids > 0)
    1188             :     {
    1189           8 :         if (procArray->numKnownAssignedXids != 0)
    1190             :         {
    1191           0 :             LWLockRelease(ProcArrayLock);
    1192           0 :             elog(ERROR, "KnownAssignedXids is not empty");
    1193             :         }
    1194             : 
    1195             :         /*
    1196             :          * Sort the array so that we can add them safely into
    1197             :          * KnownAssignedXids.
    1198             :          *
    1199             :          * We have to sort them logically, because in KnownAssignedXidsAdd we
    1200             :          * call TransactionIdFollowsOrEquals and so on. But we know these XIDs
    1201             :          * come from RUNNING_XACTS, which means there are only normal XIDs
    1202             :          * from the same epoch, so this is safe.
    1203             :          */
    1204           8 :         qsort(xids, nxids, sizeof(TransactionId), xidLogicalComparator);
    1205             : 
    1206             :         /*
    1207             :          * Add the sorted snapshot into KnownAssignedXids.  The running-xacts
    1208             :          * snapshot may include duplicated xids because of prepared
    1209             :          * transactions, so ignore them.
    1210             :          */
    1211          16 :         for (i = 0; i < nxids; i++)
    1212             :         {
    1213           8 :             if (i > 0 && TransactionIdEquals(xids[i - 1], xids[i]))
    1214             :             {
    1215           0 :                 elog(DEBUG1,
    1216             :                      "found duplicated transaction %u for KnownAssignedXids insertion",
    1217             :                      xids[i]);
    1218           0 :                 continue;
    1219             :             }
    1220           8 :             KnownAssignedXidsAdd(xids[i], xids[i], true);
    1221             :         }
    1222             : 
    1223           8 :         KnownAssignedXidsDisplay(DEBUG3);
    1224             :     }
    1225             : 
    1226         212 :     pfree(xids);
    1227             : 
    1228             :     /*
    1229             :      * latestObservedXid is at least set to the point where SUBTRANS was
    1230             :      * started up to (cf. ProcArrayInitRecovery()) or to the biggest xid
    1231             :      * RecordKnownAssignedTransactionIds() was called for.  Initialize
    1232             :      * subtrans from thereon, up to nextXid - 1.
    1233             :      *
    1234             :      * We need to duplicate parts of RecordKnownAssignedTransactionId() here,
    1235             :      * because we've just added xids to the known assigned xids machinery that
    1236             :      * haven't gone through RecordKnownAssignedTransactionId().
    1237             :      */
    1238             :     Assert(TransactionIdIsNormal(latestObservedXid));
    1239         212 :     TransactionIdAdvance(latestObservedXid);
    1240         424 :     while (TransactionIdPrecedes(latestObservedXid, running->nextXid))
    1241             :     {
    1242           0 :         ExtendSUBTRANS(latestObservedXid);
    1243           0 :         TransactionIdAdvance(latestObservedXid);
    1244             :     }
    1245         212 :     TransactionIdRetreat(latestObservedXid);    /* = running->nextXid - 1 */
    1246             : 
    1247             :     /* ----------
    1248             :      * Now we've got the running xids we need to set the global values that
    1249             :      * are used to track snapshots as they evolve further.
    1250             :      *
    1251             :      * - latestCompletedXid which will be the xmax for snapshots
    1252             :      * - lastOverflowedXid which shows whether snapshots overflow
    1253             :      * - nextXid
    1254             :      *
    1255             :      * If the snapshot overflowed, then we still initialise with what we know,
    1256             :      * but the recovery snapshot isn't fully valid yet because we know there
    1257             :      * are some subxids missing. We don't know the specific subxids that are
    1258             :      * missing, so conservatively assume the last one is latestObservedXid.
    1259             :      * ----------
    1260             :      */
    1261         212 :     if (running->subxid_status == SUBXIDS_MISSING)
    1262             :     {
    1263           0 :         standbyState = STANDBY_SNAPSHOT_PENDING;
    1264             : 
    1265           0 :         standbySnapshotPendingXmin = latestObservedXid;
    1266           0 :         procArray->lastOverflowedXid = latestObservedXid;
    1267             :     }
    1268             :     else
    1269             :     {
    1270         212 :         standbyState = STANDBY_SNAPSHOT_READY;
    1271             : 
    1272         212 :         standbySnapshotPendingXmin = InvalidTransactionId;
    1273             : 
    1274             :         /*
    1275             :          * If the 'xids' array didn't include all subtransactions, we have to
    1276             :          * mark any snapshots taken as overflowed.
    1277             :          */
    1278         212 :         if (running->subxid_status == SUBXIDS_IN_SUBTRANS)
    1279          52 :             procArray->lastOverflowedXid = latestObservedXid;
    1280             :         else
    1281             :         {
    1282             :             Assert(running->subxid_status == SUBXIDS_IN_ARRAY);
    1283         160 :             procArray->lastOverflowedXid = InvalidTransactionId;
    1284             :         }
    1285             :     }
    1286             : 
    1287             :     /*
    1288             :      * If a transaction wrote a commit record in the gap between taking and
    1289             :      * logging the snapshot then latestCompletedXid may already be higher than
    1290             :      * the value from the snapshot, so check before we use the incoming value.
    1291             :      * It also might not yet be set at all.
    1292             :      */
    1293         212 :     MaintainLatestCompletedXidRecovery(running->latestCompletedXid);
    1294             : 
    1295             :     /*
    1296             :      * NB: No need to increment TransamVariables->xactCompletionCount here,
    1297             :      * nobody can see it yet.
    1298             :      */
    1299             : 
    1300         212 :     LWLockRelease(ProcArrayLock);
    1301             : 
    1302         212 :     KnownAssignedXidsDisplay(DEBUG3);
    1303         212 :     if (standbyState == STANDBY_SNAPSHOT_READY)
    1304         212 :         elog(DEBUG1, "recovery snapshots are now enabled");
    1305             :     else
    1306           0 :         elog(DEBUG1,
    1307             :              "recovery snapshot waiting for non-overflowed snapshot or "
    1308             :              "until oldest active xid on standby is at least %u (now %u)",
    1309             :              standbySnapshotPendingXmin,
    1310             :              running->oldestRunningXid);
    1311             : }
    1312             : 
    1313             : /*
    1314             :  * ProcArrayApplyXidAssignment
    1315             :  *      Process an XLOG_XACT_ASSIGNMENT WAL record
    1316             :  */
    1317             : void
    1318          42 : ProcArrayApplyXidAssignment(TransactionId topxid,
    1319             :                             int nsubxids, TransactionId *subxids)
    1320             : {
    1321             :     TransactionId max_xid;
    1322             :     int         i;
    1323             : 
    1324             :     Assert(standbyState >= STANDBY_INITIALIZED);
    1325             : 
    1326          42 :     max_xid = TransactionIdLatest(topxid, nsubxids, subxids);
    1327             : 
    1328             :     /*
    1329             :      * Mark all the subtransactions as observed.
    1330             :      *
    1331             :      * NOTE: This will fail if the subxid contains too many previously
    1332             :      * unobserved xids to fit into known-assigned-xids. That shouldn't happen
    1333             :      * as the code stands, because xid-assignment records should never contain
    1334             :      * more than PGPROC_MAX_CACHED_SUBXIDS entries.
    1335             :      */
    1336          42 :     RecordKnownAssignedTransactionIds(max_xid);
    1337             : 
    1338             :     /*
    1339             :      * Notice that we update pg_subtrans with the top-level xid, rather than
    1340             :      * the parent xid. This is a difference between normal processing and
    1341             :      * recovery, yet is still correct in all cases. The reason is that
    1342             :      * subtransaction commit is not marked in clog until commit processing, so
    1343             :      * all aborted subtransactions have already been clearly marked in clog.
    1344             :      * As a result we are able to refer directly to the top-level
    1345             :      * transaction's state rather than skipping through all the intermediate
    1346             :      * states in the subtransaction tree. This should be the first time we
    1347             :      * have attempted to SubTransSetParent().
    1348             :      */
    1349        2730 :     for (i = 0; i < nsubxids; i++)
    1350        2688 :         SubTransSetParent(subxids[i], topxid);
    1351             : 
    1352             :     /* KnownAssignedXids isn't maintained yet, so we're done for now */
    1353          42 :     if (standbyState == STANDBY_INITIALIZED)
    1354           0 :         return;
    1355             : 
    1356             :     /*
    1357             :      * Uses same locking as transaction commit
    1358             :      */
    1359          42 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    1360             : 
    1361             :     /*
    1362             :      * Remove subxids from known-assigned-xacts.
    1363             :      */
    1364          42 :     KnownAssignedXidsRemoveTree(InvalidTransactionId, nsubxids, subxids);
    1365             : 
    1366             :     /*
    1367             :      * Advance lastOverflowedXid to be at least the last of these subxids.
    1368             :      */
    1369          42 :     if (TransactionIdPrecedes(procArray->lastOverflowedXid, max_xid))
    1370          42 :         procArray->lastOverflowedXid = max_xid;
    1371             : 
    1372          42 :     LWLockRelease(ProcArrayLock);
    1373             : }
    1374             : 
    1375             : /*
    1376             :  * TransactionIdIsInProgress -- is given transaction running in some backend
    1377             :  *
    1378             :  * Aside from some shortcuts such as checking RecentXmin and our own Xid,
    1379             :  * there are four possibilities for finding a running transaction:
    1380             :  *
    1381             :  * 1. The given Xid is a main transaction Id.  We will find this out cheaply
    1382             :  * by looking at ProcGlobal->xids.
    1383             :  *
    1384             :  * 2. The given Xid is one of the cached subxact Xids in the PGPROC array.
    1385             :  * We can find this out cheaply too.
    1386             :  *
    1387             :  * 3. In Hot Standby mode, we must search the KnownAssignedXids list to see
    1388             :  * if the Xid is running on the primary.
    1389             :  *
    1390             :  * 4. Search the SubTrans tree to find the Xid's topmost parent, and then see
    1391             :  * if that is running according to ProcGlobal->xids[] or KnownAssignedXids.
    1392             :  * This is the slowest way, but sadly it has to be done always if the others
    1393             :  * failed, unless we see that the cached subxact sets are complete (none have
    1394             :  * overflowed).
    1395             :  *
    1396             :  * ProcArrayLock has to be held while we do 1, 2, 3.  If we save the top Xids
    1397             :  * while doing 1 and 3, we can release the ProcArrayLock while we do 4.
    1398             :  * This buys back some concurrency (and we can't retrieve the main Xids from
    1399             :  * ProcGlobal->xids[] again anyway; see GetNewTransactionId).
    1400             :  */
    1401             : bool
    1402    21083092 : TransactionIdIsInProgress(TransactionId xid)
    1403             : {
    1404             :     static TransactionId *xids = NULL;
    1405             :     static TransactionId *other_xids;
    1406             :     XidCacheStatus *other_subxidstates;
    1407    21083092 :     int         nxids = 0;
    1408    21083092 :     ProcArrayStruct *arrayP = procArray;
    1409             :     TransactionId topxid;
    1410             :     TransactionId latestCompletedXid;
    1411             :     int         mypgxactoff;
    1412             :     int         numProcs;
    1413             :     int         j;
    1414             : 
    1415             :     /*
    1416             :      * Don't bother checking a transaction older than RecentXmin; it could not
    1417             :      * possibly still be running.  (Note: in particular, this guarantees that
    1418             :      * we reject InvalidTransactionId, FrozenTransactionId, etc as not
    1419             :      * running.)
    1420             :      */
    1421    21083092 :     if (TransactionIdPrecedes(xid, RecentXmin))
    1422             :     {
    1423             :         xc_by_recent_xmin_inc();
    1424     9259172 :         return false;
    1425             :     }
    1426             : 
    1427             :     /*
    1428             :      * We may have just checked the status of this transaction, so if it is
    1429             :      * already known to be completed, we can fall out without any access to
    1430             :      * shared memory.
    1431             :      */
    1432    11823920 :     if (TransactionIdEquals(cachedXidIsNotInProgress, xid))
    1433             :     {
    1434             :         xc_by_known_xact_inc();
    1435     2088812 :         return false;
    1436             :     }
    1437             : 
    1438             :     /*
    1439             :      * Also, we can handle our own transaction (and subtransactions) without
    1440             :      * any access to shared memory.
    1441             :      */
    1442     9735108 :     if (TransactionIdIsCurrentTransactionId(xid))
    1443             :     {
    1444             :         xc_by_my_xact_inc();
    1445      395974 :         return true;
    1446             :     }
    1447             : 
    1448             :     /*
    1449             :      * If first time through, get workspace to remember main XIDs in. We
    1450             :      * malloc it permanently to avoid repeated palloc/pfree overhead.
    1451             :      */
    1452     9339134 :     if (xids == NULL)
    1453             :     {
    1454             :         /*
    1455             :          * In hot standby mode, reserve enough space to hold all xids in the
    1456             :          * known-assigned list. If we later finish recovery, we no longer need
    1457             :          * the bigger array, but we don't bother to shrink it.
    1458             :          */
    1459        3066 :         int         maxxids = RecoveryInProgress() ? TOTAL_MAX_CACHED_SUBXIDS : arrayP->maxProcs;
    1460             : 
    1461        3066 :         xids = (TransactionId *) malloc(maxxids * sizeof(TransactionId));
    1462        3066 :         if (xids == NULL)
    1463           0 :             ereport(ERROR,
    1464             :                     (errcode(ERRCODE_OUT_OF_MEMORY),
    1465             :                      errmsg("out of memory")));
    1466             :     }
    1467             : 
    1468     9339134 :     other_xids = ProcGlobal->xids;
    1469     9339134 :     other_subxidstates = ProcGlobal->subxidStates;
    1470             : 
    1471     9339134 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    1472             : 
    1473             :     /*
    1474             :      * Now that we have the lock, we can check latestCompletedXid; if the
    1475             :      * target Xid is after that, it's surely still running.
    1476             :      */
    1477     9339134 :     latestCompletedXid =
    1478     9339134 :         XidFromFullTransactionId(TransamVariables->latestCompletedXid);
    1479     9339134 :     if (TransactionIdPrecedes(latestCompletedXid, xid))
    1480             :     {
    1481       71192 :         LWLockRelease(ProcArrayLock);
    1482             :         xc_by_latest_xid_inc();
    1483       71192 :         return true;
    1484             :     }
    1485             : 
    1486             :     /* No shortcuts, gotta grovel through the array */
    1487     9267942 :     mypgxactoff = MyProc->pgxactoff;
    1488     9267942 :     numProcs = arrayP->numProcs;
    1489     9527126 :     for (int pgxactoff = 0; pgxactoff < numProcs; pgxactoff++)
    1490             :     {
    1491             :         int         pgprocno;
    1492             :         PGPROC     *proc;
    1493             :         TransactionId pxid;
    1494             :         int         pxids;
    1495             : 
    1496             :         /* Ignore ourselves --- dealt with it above */
    1497     9504344 :         if (pgxactoff == mypgxactoff)
    1498       25378 :             continue;
    1499             : 
    1500             :         /* Fetch xid just once - see GetNewTransactionId */
    1501     9478966 :         pxid = UINT32_ACCESS_ONCE(other_xids[pgxactoff]);
    1502             : 
    1503     9478966 :         if (!TransactionIdIsValid(pxid))
    1504      158194 :             continue;
    1505             : 
    1506             :         /*
    1507             :          * Step 1: check the main Xid
    1508             :          */
    1509     9320772 :         if (TransactionIdEquals(pxid, xid))
    1510             :         {
    1511     9244632 :             LWLockRelease(ProcArrayLock);
    1512             :             xc_by_main_xid_inc();
    1513     9244632 :             return true;
    1514             :         }
    1515             : 
    1516             :         /*
    1517             :          * We can ignore main Xids that are younger than the target Xid, since
    1518             :          * the target could not possibly be their child.
    1519             :          */
    1520       76140 :         if (TransactionIdPrecedes(xid, pxid))
    1521       36928 :             continue;
    1522             : 
    1523             :         /*
    1524             :          * Step 2: check the cached child-Xids arrays
    1525             :          */
    1526       39212 :         pxids = other_subxidstates[pgxactoff].count;
    1527       39212 :         pg_read_barrier();      /* pairs with barrier in GetNewTransactionId() */
    1528       39212 :         pgprocno = arrayP->pgprocnos[pgxactoff];
    1529       39212 :         proc = &allProcs[pgprocno];
    1530      105916 :         for (j = pxids - 1; j >= 0; j--)
    1531             :         {
    1532             :             /* Fetch xid just once - see GetNewTransactionId */
    1533       67232 :             TransactionId cxid = UINT32_ACCESS_ONCE(proc->subxids.xids[j]);
    1534             : 
    1535       67232 :             if (TransactionIdEquals(cxid, xid))
    1536             :             {
    1537         528 :                 LWLockRelease(ProcArrayLock);
    1538             :                 xc_by_child_xid_inc();
    1539         528 :                 return true;
    1540             :             }
    1541             :         }
    1542             : 
    1543             :         /*
    1544             :          * Save the main Xid for step 4.  We only need to remember main Xids
    1545             :          * that have uncached children.  (Note: there is no race condition
    1546             :          * here because the overflowed flag cannot be cleared, only set, while
    1547             :          * we hold ProcArrayLock.  So we can't miss an Xid that we need to
    1548             :          * worry about.)
    1549             :          */
    1550       38684 :         if (other_subxidstates[pgxactoff].overflowed)
    1551         426 :             xids[nxids++] = pxid;
    1552             :     }
    1553             : 
    1554             :     /*
    1555             :      * Step 3: in hot standby mode, check the known-assigned-xids list.  XIDs
    1556             :      * in the list must be treated as running.
    1557             :      */
    1558       22782 :     if (RecoveryInProgress())
    1559             :     {
    1560             :         /* none of the PGPROC entries should have XIDs in hot standby mode */
    1561             :         Assert(nxids == 0);
    1562             : 
    1563           2 :         if (KnownAssignedXidExists(xid))
    1564             :         {
    1565           0 :             LWLockRelease(ProcArrayLock);
    1566             :             xc_by_known_assigned_inc();
    1567           0 :             return true;
    1568             :         }
    1569             : 
    1570             :         /*
    1571             :          * If the KnownAssignedXids overflowed, we have to check pg_subtrans
    1572             :          * too.  Fetch all xids from KnownAssignedXids that are lower than
    1573             :          * xid, since if xid is a subtransaction its parent will always have a
    1574             :          * lower value.  Note we will collect both main and subXIDs here, but
    1575             :          * there's no help for it.
    1576             :          */
    1577           2 :         if (TransactionIdPrecedesOrEquals(xid, procArray->lastOverflowedXid))
    1578           0 :             nxids = KnownAssignedXidsGet(xids, xid);
    1579             :     }
    1580             : 
    1581       22782 :     LWLockRelease(ProcArrayLock);
    1582             : 
    1583             :     /*
    1584             :      * If none of the relevant caches overflowed, we know the Xid is not
    1585             :      * running without even looking at pg_subtrans.
    1586             :      */
    1587       22782 :     if (nxids == 0)
    1588             :     {
    1589             :         xc_no_overflow_inc();
    1590       22356 :         cachedXidIsNotInProgress = xid;
    1591       22356 :         return false;
    1592             :     }
    1593             : 
    1594             :     /*
    1595             :      * Step 4: have to check pg_subtrans.
    1596             :      *
    1597             :      * At this point, we know it's either a subtransaction of one of the Xids
    1598             :      * in xids[], or it's not running.  If it's an already-failed
    1599             :      * subtransaction, we want to say "not running" even though its parent may
    1600             :      * still be running.  So first, check pg_xact to see if it's been aborted.
    1601             :      */
    1602             :     xc_slow_answer_inc();
    1603             : 
    1604         426 :     if (TransactionIdDidAbort(xid))
    1605             :     {
    1606           0 :         cachedXidIsNotInProgress = xid;
    1607           0 :         return false;
    1608             :     }
    1609             : 
    1610             :     /*
    1611             :      * It isn't aborted, so check whether the transaction tree it belongs to
    1612             :      * is still running (or, more precisely, whether it was running when we
    1613             :      * held ProcArrayLock).
    1614             :      */
    1615         426 :     topxid = SubTransGetTopmostTransaction(xid);
    1616             :     Assert(TransactionIdIsValid(topxid));
    1617         740 :     if (!TransactionIdEquals(topxid, xid) &&
    1618         314 :         pg_lfind32(topxid, xids, nxids))
    1619         314 :         return true;
    1620             : 
    1621         112 :     cachedXidIsNotInProgress = xid;
    1622         112 :     return false;
    1623             : }
    1624             : 
    1625             : 
    1626             : /*
    1627             :  * Determine XID horizons.
    1628             :  *
    1629             :  * This is used by wrapper functions like GetOldestNonRemovableTransactionId()
    1630             :  * (for VACUUM), GetReplicationHorizons() (for hot_standby_feedback), etc as
    1631             :  * well as "internally" by GlobalVisUpdate() (see comment above struct
    1632             :  * GlobalVisState).
    1633             :  *
    1634             :  * See the definition of ComputeXidHorizonsResult for the various computed
    1635             :  * horizons.
    1636             :  *
    1637             :  * For VACUUM separate horizons (used to decide which deleted tuples must
    1638             :  * be preserved), for shared and non-shared tables are computed.  For shared
    1639             :  * relations backends in all databases must be considered, but for non-shared
    1640             :  * relations that's not required, since only backends in my own database could
    1641             :  * ever see the tuples in them. Also, we can ignore concurrently running lazy
    1642             :  * VACUUMs because (a) they must be working on other tables, and (b) they
    1643             :  * don't need to do snapshot-based lookups.
    1644             :  *
    1645             :  * This also computes a horizon used to truncate pg_subtrans. For that
    1646             :  * backends in all databases have to be considered, and concurrently running
    1647             :  * lazy VACUUMs cannot be ignored, as they still may perform pg_subtrans
    1648             :  * accesses.
    1649             :  *
    1650             :  * Note: we include all currently running xids in the set of considered xids.
    1651             :  * This ensures that if a just-started xact has not yet set its snapshot,
    1652             :  * when it does set the snapshot it cannot set xmin less than what we compute.
    1653             :  * See notes in src/backend/access/transam/README.
    1654             :  *
    1655             :  * Note: despite the above, it's possible for the calculated values to move
    1656             :  * backwards on repeated calls. The calculated values are conservative, so
    1657             :  * that anything older is definitely not considered as running by anyone
    1658             :  * anymore, but the exact values calculated depend on a number of things. For
    1659             :  * example, if there are no transactions running in the current database, the
    1660             :  * horizon for normal tables will be latestCompletedXid. If a transaction
    1661             :  * begins after that, its xmin will include in-progress transactions in other
    1662             :  * databases that started earlier, so another call will return a lower value.
    1663             :  * Nonetheless it is safe to vacuum a table in the current database with the
    1664             :  * first result.  There are also replication-related effects: a walsender
    1665             :  * process can set its xmin based on transactions that are no longer running
    1666             :  * on the primary but are still being replayed on the standby, thus possibly
    1667             :  * making the values go backwards.  In this case there is a possibility that
    1668             :  * we lose data that the standby would like to have, but unless the standby
    1669             :  * uses a replication slot to make its xmin persistent there is little we can
    1670             :  * do about that --- data is only protected if the walsender runs continuously
    1671             :  * while queries are executed on the standby.  (The Hot Standby code deals
    1672             :  * with such cases by failing standby queries that needed to access
    1673             :  * already-removed data, so there's no integrity bug.)
    1674             :  *
    1675             :  * Note: the approximate horizons (see definition of GlobalVisState) are
    1676             :  * updated by the computations done here. That's currently required for
    1677             :  * correctness and a small optimization. Without doing so it's possible that
    1678             :  * heap vacuum's call to heap_page_prune_and_freeze() uses a more conservative
    1679             :  * horizon than later when deciding which tuples can be removed - which the
    1680             :  * code doesn't expect (breaking HOT).
    1681             :  */
    1682             : static void
    1683      420090 : ComputeXidHorizons(ComputeXidHorizonsResult *h)
    1684             : {
    1685      420090 :     ProcArrayStruct *arrayP = procArray;
    1686             :     TransactionId kaxmin;
    1687      420090 :     bool        in_recovery = RecoveryInProgress();
    1688      420090 :     TransactionId *other_xids = ProcGlobal->xids;
    1689             : 
    1690             :     /* inferred after ProcArrayLock is released */
    1691      420090 :     h->catalog_oldest_nonremovable = InvalidTransactionId;
    1692             : 
    1693      420090 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    1694             : 
    1695      420090 :     h->latest_completed = TransamVariables->latestCompletedXid;
    1696             : 
    1697             :     /*
    1698             :      * We initialize the MIN() calculation with latestCompletedXid + 1. This
    1699             :      * is a lower bound for the XIDs that might appear in the ProcArray later,
    1700             :      * and so protects us against overestimating the result due to future
    1701             :      * additions.
    1702             :      */
    1703             :     {
    1704             :         TransactionId initial;
    1705             : 
    1706      420090 :         initial = XidFromFullTransactionId(h->latest_completed);
    1707             :         Assert(TransactionIdIsValid(initial));
    1708      420090 :         TransactionIdAdvance(initial);
    1709             : 
    1710      420090 :         h->oldest_considered_running = initial;
    1711      420090 :         h->shared_oldest_nonremovable = initial;
    1712      420090 :         h->data_oldest_nonremovable = initial;
    1713             : 
    1714             :         /*
    1715             :          * Only modifications made by this backend affect the horizon for
    1716             :          * temporary relations. Instead of a check in each iteration of the
    1717             :          * loop over all PGPROCs it is cheaper to just initialize to the
    1718             :          * current top-level xid any.
    1719             :          *
    1720             :          * Without an assigned xid we could use a horizon as aggressive as
    1721             :          * GetNewTransactionId(), but we can get away with the much cheaper
    1722             :          * latestCompletedXid + 1: If this backend has no xid there, by
    1723             :          * definition, can't be any newer changes in the temp table than
    1724             :          * latestCompletedXid.
    1725             :          */
    1726      420090 :         if (TransactionIdIsValid(MyProc->xid))
    1727       66824 :             h->temp_oldest_nonremovable = MyProc->xid;
    1728             :         else
    1729      353266 :             h->temp_oldest_nonremovable = initial;
    1730             :     }
    1731             : 
    1732             :     /*
    1733             :      * Fetch slot horizons while ProcArrayLock is held - the
    1734             :      * LWLockAcquire/LWLockRelease are a barrier, ensuring this happens inside
    1735             :      * the lock.
    1736             :      */
    1737      420090 :     h->slot_xmin = procArray->replication_slot_xmin;
    1738      420090 :     h->slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
    1739             : 
    1740     2646408 :     for (int index = 0; index < arrayP->numProcs; index++)
    1741             :     {
    1742     2226318 :         int         pgprocno = arrayP->pgprocnos[index];
    1743     2226318 :         PGPROC     *proc = &allProcs[pgprocno];
    1744     2226318 :         int8        statusFlags = ProcGlobal->statusFlags[index];
    1745             :         TransactionId xid;
    1746             :         TransactionId xmin;
    1747             : 
    1748             :         /* Fetch xid just once - see GetNewTransactionId */
    1749     2226318 :         xid = UINT32_ACCESS_ONCE(other_xids[index]);
    1750     2226318 :         xmin = UINT32_ACCESS_ONCE(proc->xmin);
    1751             : 
    1752             :         /*
    1753             :          * Consider both the transaction's Xmin, and its Xid.
    1754             :          *
    1755             :          * We must check both because a transaction might have an Xmin but not
    1756             :          * (yet) an Xid; conversely, if it has an Xid, that could determine
    1757             :          * some not-yet-set Xmin.
    1758             :          */
    1759     2226318 :         xmin = TransactionIdOlder(xmin, xid);
    1760             : 
    1761             :         /* if neither is set, this proc doesn't influence the horizon */
    1762     2226318 :         if (!TransactionIdIsValid(xmin))
    1763      924558 :             continue;
    1764             : 
    1765             :         /*
    1766             :          * Don't ignore any procs when determining which transactions might be
    1767             :          * considered running.  While slots should ensure logical decoding
    1768             :          * backends are protected even without this check, it can't hurt to
    1769             :          * include them here as well..
    1770             :          */
    1771     1301760 :         h->oldest_considered_running =
    1772     1301760 :             TransactionIdOlder(h->oldest_considered_running, xmin);
    1773             : 
    1774             :         /*
    1775             :          * Skip over backends either vacuuming (which is ok with rows being
    1776             :          * removed, as long as pg_subtrans is not truncated) or doing logical
    1777             :          * decoding (which manages xmin separately, check below).
    1778             :          */
    1779     1301760 :         if (statusFlags & (PROC_IN_VACUUM | PROC_IN_LOGICAL_DECODING))
    1780      491674 :             continue;
    1781             : 
    1782             :         /* shared tables need to take backends in all databases into account */
    1783      810086 :         h->shared_oldest_nonremovable =
    1784      810086 :             TransactionIdOlder(h->shared_oldest_nonremovable, xmin);
    1785             : 
    1786             :         /*
    1787             :          * Normally sessions in other databases are ignored for anything but
    1788             :          * the shared horizon.
    1789             :          *
    1790             :          * However, include them when MyDatabaseId is not (yet) set.  A
    1791             :          * backend in the process of starting up must not compute a "too
    1792             :          * aggressive" horizon, otherwise we could end up using it to prune
    1793             :          * still-needed data away.  If the current backend never connects to a
    1794             :          * database this is harmless, because data_oldest_nonremovable will
    1795             :          * never be utilized.
    1796             :          *
    1797             :          * Also, sessions marked with PROC_AFFECTS_ALL_HORIZONS should always
    1798             :          * be included.  (This flag is used for hot standby feedback, which
    1799             :          * can't be tied to a specific database.)
    1800             :          *
    1801             :          * Also, while in recovery we cannot compute an accurate per-database
    1802             :          * horizon, as all xids are managed via the KnownAssignedXids
    1803             :          * machinery.
    1804             :          */
    1805      810086 :         if (proc->databaseId == MyDatabaseId ||
    1806       34012 :             MyDatabaseId == InvalidOid ||
    1807       21400 :             (statusFlags & PROC_AFFECTS_ALL_HORIZONS) ||
    1808             :             in_recovery)
    1809             :         {
    1810      788690 :             h->data_oldest_nonremovable =
    1811      788690 :                 TransactionIdOlder(h->data_oldest_nonremovable, xmin);
    1812             :         }
    1813             :     }
    1814             : 
    1815             :     /*
    1816             :      * If in recovery fetch oldest xid in KnownAssignedXids, will be applied
    1817             :      * after lock is released.
    1818             :      */
    1819      420090 :     if (in_recovery)
    1820         700 :         kaxmin = KnownAssignedXidsGetOldestXmin();
    1821             : 
    1822             :     /*
    1823             :      * No other information from shared state is needed, release the lock
    1824             :      * immediately. The rest of the computations can be done without a lock.
    1825             :      */
    1826      420090 :     LWLockRelease(ProcArrayLock);
    1827             : 
    1828      420090 :     if (in_recovery)
    1829             :     {
    1830         700 :         h->oldest_considered_running =
    1831         700 :             TransactionIdOlder(h->oldest_considered_running, kaxmin);
    1832         700 :         h->shared_oldest_nonremovable =
    1833         700 :             TransactionIdOlder(h->shared_oldest_nonremovable, kaxmin);
    1834         700 :         h->data_oldest_nonremovable =
    1835         700 :             TransactionIdOlder(h->data_oldest_nonremovable, kaxmin);
    1836             :         /* temp relations cannot be accessed in recovery */
    1837             :     }
    1838             : 
    1839             :     Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
    1840             :                                          h->shared_oldest_nonremovable));
    1841             :     Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
    1842             :                                          h->data_oldest_nonremovable));
    1843             : 
    1844             :     /*
    1845             :      * Check whether there are replication slots requiring an older xmin.
    1846             :      */
    1847      420090 :     h->shared_oldest_nonremovable =
    1848      420090 :         TransactionIdOlder(h->shared_oldest_nonremovable, h->slot_xmin);
    1849      420090 :     h->data_oldest_nonremovable =
    1850      420090 :         TransactionIdOlder(h->data_oldest_nonremovable, h->slot_xmin);
    1851             : 
    1852             :     /*
    1853             :      * The only difference between catalog / data horizons is that the slot's
    1854             :      * catalog xmin is applied to the catalog one (so catalogs can be accessed
    1855             :      * for logical decoding). Initialize with data horizon, and then back up
    1856             :      * further if necessary. Have to back up the shared horizon as well, since
    1857             :      * that also can contain catalogs.
    1858             :      */
    1859      420090 :     h->shared_oldest_nonremovable_raw = h->shared_oldest_nonremovable;
    1860      420090 :     h->shared_oldest_nonremovable =
    1861      420090 :         TransactionIdOlder(h->shared_oldest_nonremovable,
    1862             :                            h->slot_catalog_xmin);
    1863      420090 :     h->catalog_oldest_nonremovable = h->data_oldest_nonremovable;
    1864      420090 :     h->catalog_oldest_nonremovable =
    1865      420090 :         TransactionIdOlder(h->catalog_oldest_nonremovable,
    1866             :                            h->slot_catalog_xmin);
    1867             : 
    1868             :     /*
    1869             :      * It's possible that slots backed up the horizons further than
    1870             :      * oldest_considered_running. Fix.
    1871             :      */
    1872      420090 :     h->oldest_considered_running =
    1873      420090 :         TransactionIdOlder(h->oldest_considered_running,
    1874             :                            h->shared_oldest_nonremovable);
    1875      420090 :     h->oldest_considered_running =
    1876      420090 :         TransactionIdOlder(h->oldest_considered_running,
    1877             :                            h->catalog_oldest_nonremovable);
    1878      420090 :     h->oldest_considered_running =
    1879      420090 :         TransactionIdOlder(h->oldest_considered_running,
    1880             :                            h->data_oldest_nonremovable);
    1881             : 
    1882             :     /*
    1883             :      * shared horizons have to be at least as old as the oldest visible in
    1884             :      * current db
    1885             :      */
    1886             :     Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
    1887             :                                          h->data_oldest_nonremovable));
    1888             :     Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
    1889             :                                          h->catalog_oldest_nonremovable));
    1890             : 
    1891             :     /*
    1892             :      * Horizons need to ensure that pg_subtrans access is still possible for
    1893             :      * the relevant backends.
    1894             :      */
    1895             :     Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
    1896             :                                          h->shared_oldest_nonremovable));
    1897             :     Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
    1898             :                                          h->catalog_oldest_nonremovable));
    1899             :     Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
    1900             :                                          h->data_oldest_nonremovable));
    1901             :     Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
    1902             :                                          h->temp_oldest_nonremovable));
    1903             :     Assert(!TransactionIdIsValid(h->slot_xmin) ||
    1904             :            TransactionIdPrecedesOrEquals(h->oldest_considered_running,
    1905             :                                          h->slot_xmin));
    1906             :     Assert(!TransactionIdIsValid(h->slot_catalog_xmin) ||
    1907             :            TransactionIdPrecedesOrEquals(h->oldest_considered_running,
    1908             :                                          h->slot_catalog_xmin));
    1909             : 
    1910             :     /* update approximate horizons with the computed horizons */
    1911      420090 :     GlobalVisUpdateApply(h);
    1912      420090 : }
    1913             : 
    1914             : /*
    1915             :  * Determine what kind of visibility horizon needs to be used for a
    1916             :  * relation. If rel is NULL, the most conservative horizon is used.
    1917             :  */
    1918             : static inline GlobalVisHorizonKind
    1919    28875322 : GlobalVisHorizonKindForRel(Relation rel)
    1920             : {
    1921             :     /*
    1922             :      * Other relkinds currently don't contain xids, nor always the necessary
    1923             :      * logical decoding markers.
    1924             :      */
    1925             :     Assert(!rel ||
    1926             :            rel->rd_rel->relkind == RELKIND_RELATION ||
    1927             :            rel->rd_rel->relkind == RELKIND_MATVIEW ||
    1928             :            rel->rd_rel->relkind == RELKIND_TOASTVALUE);
    1929             : 
    1930    28875322 :     if (rel == NULL || rel->rd_rel->relisshared || RecoveryInProgress())
    1931      223600 :         return VISHORIZON_SHARED;
    1932    28651722 :     else if (IsCatalogRelation(rel) ||
    1933    22754000 :              RelationIsAccessibleInLogicalDecoding(rel))
    1934     5897730 :         return VISHORIZON_CATALOG;
    1935    22753992 :     else if (!RELATION_IS_LOCAL(rel))
    1936    22645652 :         return VISHORIZON_DATA;
    1937             :     else
    1938      108340 :         return VISHORIZON_TEMP;
    1939             : }
    1940             : 
    1941             : /*
    1942             :  * Return the oldest XID for which deleted tuples must be preserved in the
    1943             :  * passed table.
    1944             :  *
    1945             :  * If rel is not NULL the horizon may be considerably more recent than
    1946             :  * otherwise (i.e. fewer tuples will be removable). In the NULL case a horizon
    1947             :  * that is correct (but not optimal) for all relations will be returned.
    1948             :  *
    1949             :  * This is used by VACUUM to decide which deleted tuples must be preserved in
    1950             :  * the passed in table.
    1951             :  */
    1952             : TransactionId
    1953      306170 : GetOldestNonRemovableTransactionId(Relation rel)
    1954             : {
    1955             :     ComputeXidHorizonsResult horizons;
    1956             : 
    1957      306170 :     ComputeXidHorizons(&horizons);
    1958             : 
    1959      306170 :     switch (GlobalVisHorizonKindForRel(rel))
    1960             :     {
    1961       47364 :         case VISHORIZON_SHARED:
    1962       47364 :             return horizons.shared_oldest_nonremovable;
    1963      189388 :         case VISHORIZON_CATALOG:
    1964      189388 :             return horizons.catalog_oldest_nonremovable;
    1965       44966 :         case VISHORIZON_DATA:
    1966       44966 :             return horizons.data_oldest_nonremovable;
    1967       24452 :         case VISHORIZON_TEMP:
    1968       24452 :             return horizons.temp_oldest_nonremovable;
    1969             :     }
    1970             : 
    1971             :     /* just to prevent compiler warnings */
    1972           0 :     return InvalidTransactionId;
    1973             : }
    1974             : 
    1975             : /*
    1976             :  * Return the oldest transaction id any currently running backend might still
    1977             :  * consider running. This should not be used for visibility / pruning
    1978             :  * determinations (see GetOldestNonRemovableTransactionId()), but for
    1979             :  * decisions like up to where pg_subtrans can be truncated.
    1980             :  */
    1981             : TransactionId
    1982        3330 : GetOldestTransactionIdConsideredRunning(void)
    1983             : {
    1984             :     ComputeXidHorizonsResult horizons;
    1985             : 
    1986        3330 :     ComputeXidHorizons(&horizons);
    1987             : 
    1988        3330 :     return horizons.oldest_considered_running;
    1989             : }
    1990             : 
    1991             : /*
    1992             :  * Return the visibility horizons for a hot standby feedback message.
    1993             :  */
    1994             : void
    1995          94 : GetReplicationHorizons(TransactionId *xmin, TransactionId *catalog_xmin)
    1996             : {
    1997             :     ComputeXidHorizonsResult horizons;
    1998             : 
    1999          94 :     ComputeXidHorizons(&horizons);
    2000             : 
    2001             :     /*
    2002             :      * Don't want to use shared_oldest_nonremovable here, as that contains the
    2003             :      * effect of replication slot's catalog_xmin. We want to send a separate
    2004             :      * feedback for the catalog horizon, so the primary can remove data table
    2005             :      * contents more aggressively.
    2006             :      */
    2007          94 :     *xmin = horizons.shared_oldest_nonremovable_raw;
    2008          94 :     *catalog_xmin = horizons.slot_catalog_xmin;
    2009          94 : }
    2010             : 
    2011             : /*
    2012             :  * GetMaxSnapshotXidCount -- get max size for snapshot XID array
    2013             :  *
    2014             :  * We have to export this for use by snapmgr.c.
    2015             :  */
    2016             : int
    2017       64942 : GetMaxSnapshotXidCount(void)
    2018             : {
    2019       64942 :     return procArray->maxProcs;
    2020             : }
    2021             : 
    2022             : /*
    2023             :  * GetMaxSnapshotSubxidCount -- get max size for snapshot sub-XID array
    2024             :  *
    2025             :  * We have to export this for use by snapmgr.c.
    2026             :  */
    2027             : int
    2028       64572 : GetMaxSnapshotSubxidCount(void)
    2029             : {
    2030       64572 :     return TOTAL_MAX_CACHED_SUBXIDS;
    2031             : }
    2032             : 
    2033             : /*
    2034             :  * Helper function for GetSnapshotData() that checks if the bulk of the
    2035             :  * visibility information in the snapshot is still valid. If so, it updates
    2036             :  * the fields that need to change and returns true. Otherwise it returns
    2037             :  * false.
    2038             :  *
    2039             :  * This very likely can be evolved to not need ProcArrayLock held (at very
    2040             :  * least in the case we already hold a snapshot), but that's for another day.
    2041             :  */
    2042             : static bool
    2043     4322148 : GetSnapshotDataReuse(Snapshot snapshot)
    2044             : {
    2045             :     uint64      curXactCompletionCount;
    2046             : 
    2047             :     Assert(LWLockHeldByMe(ProcArrayLock));
    2048             : 
    2049     4322148 :     if (unlikely(snapshot->snapXactCompletionCount == 0))
    2050       64528 :         return false;
    2051             : 
    2052     4257620 :     curXactCompletionCount = TransamVariables->xactCompletionCount;
    2053     4257620 :     if (curXactCompletionCount != snapshot->snapXactCompletionCount)
    2054      699994 :         return false;
    2055             : 
    2056             :     /*
    2057             :      * If the current xactCompletionCount is still the same as it was at the
    2058             :      * time the snapshot was built, we can be sure that rebuilding the
    2059             :      * contents of the snapshot the hard way would result in the same snapshot
    2060             :      * contents:
    2061             :      *
    2062             :      * As explained in transam/README, the set of xids considered running by
    2063             :      * GetSnapshotData() cannot change while ProcArrayLock is held. Snapshot
    2064             :      * contents only depend on transactions with xids and xactCompletionCount
    2065             :      * is incremented whenever a transaction with an xid finishes (while
    2066             :      * holding ProcArrayLock exclusively). Thus the xactCompletionCount check
    2067             :      * ensures we would detect if the snapshot would have changed.
    2068             :      *
    2069             :      * As the snapshot contents are the same as it was before, it is safe to
    2070             :      * re-enter the snapshot's xmin into the PGPROC array. None of the rows
    2071             :      * visible under the snapshot could already have been removed (that'd
    2072             :      * require the set of running transactions to change) and it fulfills the
    2073             :      * requirement that concurrent GetSnapshotData() calls yield the same
    2074             :      * xmin.
    2075             :      */
    2076     3557626 :     if (!TransactionIdIsValid(MyProc->xmin))
    2077     1410618 :         MyProc->xmin = TransactionXmin = snapshot->xmin;
    2078             : 
    2079     3557626 :     RecentXmin = snapshot->xmin;
    2080             :     Assert(TransactionIdPrecedesOrEquals(TransactionXmin, RecentXmin));
    2081             : 
    2082     3557626 :     snapshot->curcid = GetCurrentCommandId(false);
    2083     3557626 :     snapshot->active_count = 0;
    2084     3557626 :     snapshot->regd_count = 0;
    2085     3557626 :     snapshot->copied = false;
    2086             : 
    2087     3557626 :     return true;
    2088             : }
    2089             : 
    2090             : /*
    2091             :  * GetSnapshotData -- returns information about running transactions.
    2092             :  *
    2093             :  * The returned snapshot includes xmin (lowest still-running xact ID),
    2094             :  * xmax (highest completed xact ID + 1), and a list of running xact IDs
    2095             :  * in the range xmin <= xid < xmax.  It is used as follows:
    2096             :  *      All xact IDs < xmin are considered finished.
    2097             :  *      All xact IDs >= xmax are considered still running.
    2098             :  *      For an xact ID xmin <= xid < xmax, consult list to see whether
    2099             :  *      it is considered running or not.
    2100             :  * This ensures that the set of transactions seen as "running" by the
    2101             :  * current xact will not change after it takes the snapshot.
    2102             :  *
    2103             :  * All running top-level XIDs are included in the snapshot, except for lazy
    2104             :  * VACUUM processes.  We also try to include running subtransaction XIDs,
    2105             :  * but since PGPROC has only a limited cache area for subxact XIDs, full
    2106             :  * information may not be available.  If we find any overflowed subxid arrays,
    2107             :  * we have to mark the snapshot's subxid data as overflowed, and extra work
    2108             :  * *may* need to be done to determine what's running (see XidInMVCCSnapshot()).
    2109             :  *
    2110             :  * We also update the following backend-global variables:
    2111             :  *      TransactionXmin: the oldest xmin of any snapshot in use in the
    2112             :  *          current transaction (this is the same as MyProc->xmin).
    2113             :  *      RecentXmin: the xmin computed for the most recent snapshot.  XIDs
    2114             :  *          older than this are known not running any more.
    2115             :  *
    2116             :  * And try to advance the bounds of GlobalVis{Shared,Catalog,Data,Temp}Rels
    2117             :  * for the benefit of the GlobalVisTest* family of functions.
    2118             :  *
    2119             :  * Note: this function should probably not be called with an argument that's
    2120             :  * not statically allocated (see xip allocation below).
    2121             :  */
    2122             : Snapshot
    2123     4322148 : GetSnapshotData(Snapshot snapshot)
    2124             : {
    2125     4322148 :     ProcArrayStruct *arrayP = procArray;
    2126     4322148 :     TransactionId *other_xids = ProcGlobal->xids;
    2127             :     TransactionId xmin;
    2128             :     TransactionId xmax;
    2129     4322148 :     int         count = 0;
    2130     4322148 :     int         subcount = 0;
    2131     4322148 :     bool        suboverflowed = false;
    2132             :     FullTransactionId latest_completed;
    2133             :     TransactionId oldestxid;
    2134             :     int         mypgxactoff;
    2135             :     TransactionId myxid;
    2136             :     uint64      curXactCompletionCount;
    2137             : 
    2138     4322148 :     TransactionId replication_slot_xmin = InvalidTransactionId;
    2139     4322148 :     TransactionId replication_slot_catalog_xmin = InvalidTransactionId;
    2140             : 
    2141             :     Assert(snapshot != NULL);
    2142             : 
    2143             :     /*
    2144             :      * Allocating space for maxProcs xids is usually overkill; numProcs would
    2145             :      * be sufficient.  But it seems better to do the malloc while not holding
    2146             :      * the lock, so we can't look at numProcs.  Likewise, we allocate much
    2147             :      * more subxip storage than is probably needed.
    2148             :      *
    2149             :      * This does open a possibility for avoiding repeated malloc/free: since
    2150             :      * maxProcs does not change at runtime, we can simply reuse the previous
    2151             :      * xip arrays if any.  (This relies on the fact that all callers pass
    2152             :      * static SnapshotData structs.)
    2153             :      */
    2154     4322148 :     if (snapshot->xip == NULL)
    2155             :     {
    2156             :         /*
    2157             :          * First call for this snapshot. Snapshot is same size whether or not
    2158             :          * we are in recovery, see later comments.
    2159             :          */
    2160       64516 :         snapshot->xip = (TransactionId *)
    2161       64516 :             malloc(GetMaxSnapshotXidCount() * sizeof(TransactionId));
    2162       64516 :         if (snapshot->xip == NULL)
    2163           0 :             ereport(ERROR,
    2164             :                     (errcode(ERRCODE_OUT_OF_MEMORY),
    2165             :                      errmsg("out of memory")));
    2166             :         Assert(snapshot->subxip == NULL);
    2167       64516 :         snapshot->subxip = (TransactionId *)
    2168       64516 :             malloc(GetMaxSnapshotSubxidCount() * sizeof(TransactionId));
    2169       64516 :         if (snapshot->subxip == NULL)
    2170           0 :             ereport(ERROR,
    2171             :                     (errcode(ERRCODE_OUT_OF_MEMORY),
    2172             :                      errmsg("out of memory")));
    2173             :     }
    2174             : 
    2175             :     /*
    2176             :      * It is sufficient to get shared lock on ProcArrayLock, even if we are
    2177             :      * going to set MyProc->xmin.
    2178             :      */
    2179     4322148 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    2180             : 
    2181     4322148 :     if (GetSnapshotDataReuse(snapshot))
    2182             :     {
    2183     3557626 :         LWLockRelease(ProcArrayLock);
    2184     3557626 :         return snapshot;
    2185             :     }
    2186             : 
    2187      764522 :     latest_completed = TransamVariables->latestCompletedXid;
    2188      764522 :     mypgxactoff = MyProc->pgxactoff;
    2189      764522 :     myxid = other_xids[mypgxactoff];
    2190             :     Assert(myxid == MyProc->xid);
    2191             : 
    2192      764522 :     oldestxid = TransamVariables->oldestXid;
    2193      764522 :     curXactCompletionCount = TransamVariables->xactCompletionCount;
    2194             : 
    2195             :     /* xmax is always latestCompletedXid + 1 */
    2196      764522 :     xmax = XidFromFullTransactionId(latest_completed);
    2197      764522 :     TransactionIdAdvance(xmax);
    2198             :     Assert(TransactionIdIsNormal(xmax));
    2199             : 
    2200             :     /* initialize xmin calculation with xmax */
    2201      764522 :     xmin = xmax;
    2202             : 
    2203             :     /* take own xid into account, saves a check inside the loop */
    2204      764522 :     if (TransactionIdIsNormal(myxid) && NormalTransactionIdPrecedes(myxid, xmin))
    2205       46838 :         xmin = myxid;
    2206             : 
    2207      764522 :     snapshot->takenDuringRecovery = RecoveryInProgress();
    2208             : 
    2209      764522 :     if (!snapshot->takenDuringRecovery)
    2210             :     {
    2211      762446 :         int         numProcs = arrayP->numProcs;
    2212      762446 :         TransactionId *xip = snapshot->xip;
    2213      762446 :         int        *pgprocnos = arrayP->pgprocnos;
    2214      762446 :         XidCacheStatus *subxidStates = ProcGlobal->subxidStates;
    2215      762446 :         uint8      *allStatusFlags = ProcGlobal->statusFlags;
    2216             : 
    2217             :         /*
    2218             :          * First collect set of pgxactoff/xids that need to be included in the
    2219             :          * snapshot.
    2220             :          */
    2221     6283714 :         for (int pgxactoff = 0; pgxactoff < numProcs; pgxactoff++)
    2222             :         {
    2223             :             /* Fetch xid just once - see GetNewTransactionId */
    2224     5521268 :             TransactionId xid = UINT32_ACCESS_ONCE(other_xids[pgxactoff]);
    2225             :             uint8       statusFlags;
    2226             : 
    2227             :             Assert(allProcs[arrayP->pgprocnos[pgxactoff]].pgxactoff == pgxactoff);
    2228             : 
    2229             :             /*
    2230             :              * If the transaction has no XID assigned, we can skip it; it
    2231             :              * won't have sub-XIDs either.
    2232             :              */
    2233     5521268 :             if (likely(xid == InvalidTransactionId))
    2234     4274230 :                 continue;
    2235             : 
    2236             :             /*
    2237             :              * We don't include our own XIDs (if any) in the snapshot. It
    2238             :              * needs to be included in the xmin computation, but we did so
    2239             :              * outside the loop.
    2240             :              */
    2241     1247038 :             if (pgxactoff == mypgxactoff)
    2242       93226 :                 continue;
    2243             : 
    2244             :             /*
    2245             :              * The only way we are able to get here with a non-normal xid is
    2246             :              * during bootstrap - with this backend using
    2247             :              * BootstrapTransactionId. But the above test should filter that
    2248             :              * out.
    2249             :              */
    2250             :             Assert(TransactionIdIsNormal(xid));
    2251             : 
    2252             :             /*
    2253             :              * If the XID is >= xmax, we can skip it; such transactions will
    2254             :              * be treated as running anyway (and any sub-XIDs will also be >=
    2255             :              * xmax).
    2256             :              */
    2257     1153812 :             if (!NormalTransactionIdPrecedes(xid, xmax))
    2258      248554 :                 continue;
    2259             : 
    2260             :             /*
    2261             :              * Skip over backends doing logical decoding which manages xmin
    2262             :              * separately (check below) and ones running LAZY VACUUM.
    2263             :              */
    2264      905258 :             statusFlags = allStatusFlags[pgxactoff];
    2265      905258 :             if (statusFlags & (PROC_IN_LOGICAL_DECODING | PROC_IN_VACUUM))
    2266         116 :                 continue;
    2267             : 
    2268      905142 :             if (NormalTransactionIdPrecedes(xid, xmin))
    2269      476964 :                 xmin = xid;
    2270             : 
    2271             :             /* Add XID to snapshot. */
    2272      905142 :             xip[count++] = xid;
    2273             : 
    2274             :             /*
    2275             :              * Save subtransaction XIDs if possible (if we've already
    2276             :              * overflowed, there's no point).  Note that the subxact XIDs must
    2277             :              * be later than their parent, so no need to check them against
    2278             :              * xmin.  We could filter against xmax, but it seems better not to
    2279             :              * do that much work while holding the ProcArrayLock.
    2280             :              *
    2281             :              * The other backend can add more subxids concurrently, but cannot
    2282             :              * remove any.  Hence it's important to fetch nxids just once.
    2283             :              * Should be safe to use memcpy, though.  (We needn't worry about
    2284             :              * missing any xids added concurrently, because they must postdate
    2285             :              * xmax.)
    2286             :              *
    2287             :              * Again, our own XIDs are not included in the snapshot.
    2288             :              */
    2289      905142 :             if (!suboverflowed)
    2290             :             {
    2291             : 
    2292      905134 :                 if (subxidStates[pgxactoff].overflowed)
    2293        1382 :                     suboverflowed = true;
    2294             :                 else
    2295             :                 {
    2296      903752 :                     int         nsubxids = subxidStates[pgxactoff].count;
    2297             : 
    2298      903752 :                     if (nsubxids > 0)
    2299             :                     {
    2300        9630 :                         int         pgprocno = pgprocnos[pgxactoff];
    2301        9630 :                         PGPROC     *proc = &allProcs[pgprocno];
    2302             : 
    2303        9630 :                         pg_read_barrier();  /* pairs with GetNewTransactionId */
    2304             : 
    2305        9630 :                         memcpy(snapshot->subxip + subcount,
    2306        9630 :                                proc->subxids.xids,
    2307             :                                nsubxids * sizeof(TransactionId));
    2308        9630 :                         subcount += nsubxids;
    2309             :                     }
    2310             :                 }
    2311             :             }
    2312             :         }
    2313             :     }
    2314             :     else
    2315             :     {
    2316             :         /*
    2317             :          * We're in hot standby, so get XIDs from KnownAssignedXids.
    2318             :          *
    2319             :          * We store all xids directly into subxip[]. Here's why:
    2320             :          *
    2321             :          * In recovery we don't know which xids are top-level and which are
    2322             :          * subxacts, a design choice that greatly simplifies xid processing.
    2323             :          *
    2324             :          * It seems like we would want to try to put xids into xip[] only, but
    2325             :          * that is fairly small. We would either need to make that bigger or
    2326             :          * to increase the rate at which we WAL-log xid assignment; neither is
    2327             :          * an appealing choice.
    2328             :          *
    2329             :          * We could try to store xids into xip[] first and then into subxip[]
    2330             :          * if there are too many xids. That only works if the snapshot doesn't
    2331             :          * overflow because we do not search subxip[] in that case. A simpler
    2332             :          * way is to just store all xids in the subxip array because this is
    2333             :          * by far the bigger array. We just leave the xip array empty.
    2334             :          *
    2335             :          * Either way we need to change the way XidInMVCCSnapshot() works
    2336             :          * depending upon when the snapshot was taken, or change normal
    2337             :          * snapshot processing so it matches.
    2338             :          *
    2339             :          * Note: It is possible for recovery to end before we finish taking
    2340             :          * the snapshot, and for newly assigned transaction ids to be added to
    2341             :          * the ProcArray.  xmax cannot change while we hold ProcArrayLock, so
    2342             :          * those newly added transaction ids would be filtered away, so we
    2343             :          * need not be concerned about them.
    2344             :          */
    2345        2076 :         subcount = KnownAssignedXidsGetAndSetXmin(snapshot->subxip, &xmin,
    2346             :                                                   xmax);
    2347             : 
    2348        2076 :         if (TransactionIdPrecedesOrEquals(xmin, procArray->lastOverflowedXid))
    2349          12 :             suboverflowed = true;
    2350             :     }
    2351             : 
    2352             : 
    2353             :     /*
    2354             :      * Fetch into local variable while ProcArrayLock is held - the
    2355             :      * LWLockRelease below is a barrier, ensuring this happens inside the
    2356             :      * lock.
    2357             :      */
    2358      764522 :     replication_slot_xmin = procArray->replication_slot_xmin;
    2359      764522 :     replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
    2360             : 
    2361      764522 :     if (!TransactionIdIsValid(MyProc->xmin))
    2362      419996 :         MyProc->xmin = TransactionXmin = xmin;
    2363             : 
    2364      764522 :     LWLockRelease(ProcArrayLock);
    2365             : 
    2366             :     /* maintain state for GlobalVis* */
    2367             :     {
    2368             :         TransactionId def_vis_xid;
    2369             :         TransactionId def_vis_xid_data;
    2370             :         FullTransactionId def_vis_fxid;
    2371             :         FullTransactionId def_vis_fxid_data;
    2372             :         FullTransactionId oldestfxid;
    2373             : 
    2374             :         /*
    2375             :          * Converting oldestXid is only safe when xid horizon cannot advance,
    2376             :          * i.e. holding locks. While we don't hold the lock anymore, all the
    2377             :          * necessary data has been gathered with lock held.
    2378             :          */
    2379      764522 :         oldestfxid = FullXidRelativeTo(latest_completed, oldestxid);
    2380             : 
    2381             :         /* Check whether there's a replication slot requiring an older xmin. */
    2382             :         def_vis_xid_data =
    2383      764522 :             TransactionIdOlder(xmin, replication_slot_xmin);
    2384             : 
    2385             :         /*
    2386             :          * Rows in non-shared, non-catalog tables possibly could be vacuumed
    2387             :          * if older than this xid.
    2388             :          */
    2389      764522 :         def_vis_xid = def_vis_xid_data;
    2390             : 
    2391             :         /*
    2392             :          * Check whether there's a replication slot requiring an older catalog
    2393             :          * xmin.
    2394             :          */
    2395             :         def_vis_xid =
    2396      764522 :             TransactionIdOlder(replication_slot_catalog_xmin, def_vis_xid);
    2397             : 
    2398      764522 :         def_vis_fxid = FullXidRelativeTo(latest_completed, def_vis_xid);
    2399      764522 :         def_vis_fxid_data = FullXidRelativeTo(latest_completed, def_vis_xid_data);
    2400             : 
    2401             :         /*
    2402             :          * Check if we can increase upper bound. As a previous
    2403             :          * GlobalVisUpdate() might have computed more aggressive values, don't
    2404             :          * overwrite them if so.
    2405             :          */
    2406             :         GlobalVisSharedRels.definitely_needed =
    2407      764522 :             FullTransactionIdNewer(def_vis_fxid,
    2408             :                                    GlobalVisSharedRels.definitely_needed);
    2409             :         GlobalVisCatalogRels.definitely_needed =
    2410      764522 :             FullTransactionIdNewer(def_vis_fxid,
    2411             :                                    GlobalVisCatalogRels.definitely_needed);
    2412             :         GlobalVisDataRels.definitely_needed =
    2413      764522 :             FullTransactionIdNewer(def_vis_fxid_data,
    2414             :                                    GlobalVisDataRels.definitely_needed);
    2415             :         /* See temp_oldest_nonremovable computation in ComputeXidHorizons() */
    2416      764522 :         if (TransactionIdIsNormal(myxid))
    2417             :             GlobalVisTempRels.definitely_needed =
    2418       93022 :                 FullXidRelativeTo(latest_completed, myxid);
    2419             :         else
    2420             :         {
    2421      671500 :             GlobalVisTempRels.definitely_needed = latest_completed;
    2422      671500 :             FullTransactionIdAdvance(&GlobalVisTempRels.definitely_needed);
    2423             :         }
    2424             : 
    2425             :         /*
    2426             :          * Check if we know that we can initialize or increase the lower
    2427             :          * bound. Currently the only cheap way to do so is to use
    2428             :          * TransamVariables->oldestXid as input.
    2429             :          *
    2430             :          * We should definitely be able to do better. We could e.g. put a
    2431             :          * global lower bound value into TransamVariables.
    2432             :          */
    2433             :         GlobalVisSharedRels.maybe_needed =
    2434      764522 :             FullTransactionIdNewer(GlobalVisSharedRels.maybe_needed,
    2435             :                                    oldestfxid);
    2436             :         GlobalVisCatalogRels.maybe_needed =
    2437      764522 :             FullTransactionIdNewer(GlobalVisCatalogRels.maybe_needed,
    2438             :                                    oldestfxid);
    2439             :         GlobalVisDataRels.maybe_needed =
    2440      764522 :             FullTransactionIdNewer(GlobalVisDataRels.maybe_needed,
    2441             :                                    oldestfxid);
    2442             :         /* accurate value known */
    2443      764522 :         GlobalVisTempRels.maybe_needed = GlobalVisTempRels.definitely_needed;
    2444             :     }
    2445             : 
    2446      764522 :     RecentXmin = xmin;
    2447             :     Assert(TransactionIdPrecedesOrEquals(TransactionXmin, RecentXmin));
    2448             : 
    2449      764522 :     snapshot->xmin = xmin;
    2450      764522 :     snapshot->xmax = xmax;
    2451      764522 :     snapshot->xcnt = count;
    2452      764522 :     snapshot->subxcnt = subcount;
    2453      764522 :     snapshot->suboverflowed = suboverflowed;
    2454      764522 :     snapshot->snapXactCompletionCount = curXactCompletionCount;
    2455             : 
    2456      764522 :     snapshot->curcid = GetCurrentCommandId(false);
    2457             : 
    2458             :     /*
    2459             :      * This is a new snapshot, so set both refcounts are zero, and mark it as
    2460             :      * not copied in persistent memory.
    2461             :      */
    2462      764522 :     snapshot->active_count = 0;
    2463      764522 :     snapshot->regd_count = 0;
    2464      764522 :     snapshot->copied = false;
    2465             : 
    2466      764522 :     return snapshot;
    2467             : }
    2468             : 
    2469             : /*
    2470             :  * ProcArrayInstallImportedXmin -- install imported xmin into MyProc->xmin
    2471             :  *
    2472             :  * This is called when installing a snapshot imported from another
    2473             :  * transaction.  To ensure that OldestXmin doesn't go backwards, we must
    2474             :  * check that the source transaction is still running, and we'd better do
    2475             :  * that atomically with installing the new xmin.
    2476             :  *
    2477             :  * Returns true if successful, false if source xact is no longer running.
    2478             :  */
    2479             : bool
    2480          36 : ProcArrayInstallImportedXmin(TransactionId xmin,
    2481             :                              VirtualTransactionId *sourcevxid)
    2482             : {
    2483          36 :     bool        result = false;
    2484          36 :     ProcArrayStruct *arrayP = procArray;
    2485             :     int         index;
    2486             : 
    2487             :     Assert(TransactionIdIsNormal(xmin));
    2488          36 :     if (!sourcevxid)
    2489           0 :         return false;
    2490             : 
    2491             :     /* Get lock so source xact can't end while we're doing this */
    2492          36 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    2493             : 
    2494             :     /*
    2495             :      * Find the PGPROC entry of the source transaction. (This could use
    2496             :      * GetPGProcByNumber(), unless it's a prepared xact.  But this isn't
    2497             :      * performance critical.)
    2498             :      */
    2499          36 :     for (index = 0; index < arrayP->numProcs; index++)
    2500             :     {
    2501          36 :         int         pgprocno = arrayP->pgprocnos[index];
    2502          36 :         PGPROC     *proc = &allProcs[pgprocno];
    2503          36 :         int         statusFlags = ProcGlobal->statusFlags[index];
    2504             :         TransactionId xid;
    2505             : 
    2506             :         /* Ignore procs running LAZY VACUUM */
    2507          36 :         if (statusFlags & PROC_IN_VACUUM)
    2508           0 :             continue;
    2509             : 
    2510             :         /* We are only interested in the specific virtual transaction. */
    2511          36 :         if (proc->vxid.procNumber != sourcevxid->procNumber)
    2512           0 :             continue;
    2513          36 :         if (proc->vxid.lxid != sourcevxid->localTransactionId)
    2514           0 :             continue;
    2515             : 
    2516             :         /*
    2517             :          * We check the transaction's database ID for paranoia's sake: if it's
    2518             :          * in another DB then its xmin does not cover us.  Caller should have
    2519             :          * detected this already, so we just treat any funny cases as
    2520             :          * "transaction not found".
    2521             :          */
    2522          36 :         if (proc->databaseId != MyDatabaseId)
    2523           0 :             continue;
    2524             : 
    2525             :         /*
    2526             :          * Likewise, let's just make real sure its xmin does cover us.
    2527             :          */
    2528          36 :         xid = UINT32_ACCESS_ONCE(proc->xmin);
    2529          36 :         if (!TransactionIdIsNormal(xid) ||
    2530          36 :             !TransactionIdPrecedesOrEquals(xid, xmin))
    2531           0 :             continue;
    2532             : 
    2533             :         /*
    2534             :          * We're good.  Install the new xmin.  As in GetSnapshotData, set
    2535             :          * TransactionXmin too.  (Note that because snapmgr.c called
    2536             :          * GetSnapshotData first, we'll be overwriting a valid xmin here, so
    2537             :          * we don't check that.)
    2538             :          */
    2539          36 :         MyProc->xmin = TransactionXmin = xmin;
    2540             : 
    2541          36 :         result = true;
    2542          36 :         break;
    2543             :     }
    2544             : 
    2545          36 :     LWLockRelease(ProcArrayLock);
    2546             : 
    2547          36 :     return result;
    2548             : }
    2549             : 
    2550             : /*
    2551             :  * ProcArrayInstallRestoredXmin -- install restored xmin into MyProc->xmin
    2552             :  *
    2553             :  * This is like ProcArrayInstallImportedXmin, but we have a pointer to the
    2554             :  * PGPROC of the transaction from which we imported the snapshot, rather than
    2555             :  * an XID.
    2556             :  *
    2557             :  * Note that this function also copies statusFlags from the source `proc` in
    2558             :  * order to avoid the case where MyProc's xmin needs to be skipped for
    2559             :  * computing xid horizon.
    2560             :  *
    2561             :  * Returns true if successful, false if source xact is no longer running.
    2562             :  */
    2563             : bool
    2564        3122 : ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
    2565             : {
    2566        3122 :     bool        result = false;
    2567             :     TransactionId xid;
    2568             : 
    2569             :     Assert(TransactionIdIsNormal(xmin));
    2570             :     Assert(proc != NULL);
    2571             : 
    2572             :     /*
    2573             :      * Get an exclusive lock so that we can copy statusFlags from source proc.
    2574             :      */
    2575        3122 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    2576             : 
    2577             :     /*
    2578             :      * Be certain that the referenced PGPROC has an advertised xmin which is
    2579             :      * no later than the one we're installing, so that the system-wide xmin
    2580             :      * can't go backwards.  Also, make sure it's running in the same database,
    2581             :      * so that the per-database xmin cannot go backwards.
    2582             :      */
    2583        3122 :     xid = UINT32_ACCESS_ONCE(proc->xmin);
    2584        3122 :     if (proc->databaseId == MyDatabaseId &&
    2585        3122 :         TransactionIdIsNormal(xid) &&
    2586        3122 :         TransactionIdPrecedesOrEquals(xid, xmin))
    2587             :     {
    2588             :         /*
    2589             :          * Install xmin and propagate the statusFlags that affect how the
    2590             :          * value is interpreted by vacuum.
    2591             :          */
    2592        3122 :         MyProc->xmin = TransactionXmin = xmin;
    2593        3122 :         MyProc->statusFlags = (MyProc->statusFlags & ~PROC_XMIN_FLAGS) |
    2594        3122 :             (proc->statusFlags & PROC_XMIN_FLAGS);
    2595        3122 :         ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
    2596             : 
    2597        3122 :         result = true;
    2598             :     }
    2599             : 
    2600        3122 :     LWLockRelease(ProcArrayLock);
    2601             : 
    2602        3122 :     return result;
    2603             : }
    2604             : 
    2605             : /*
    2606             :  * GetRunningTransactionData -- returns information about running transactions.
    2607             :  *
    2608             :  * Similar to GetSnapshotData but returns more information. We include
    2609             :  * all PGPROCs with an assigned TransactionId, even VACUUM processes and
    2610             :  * prepared transactions.
    2611             :  *
    2612             :  * We acquire XidGenLock and ProcArrayLock, but the caller is responsible for
    2613             :  * releasing them. Acquiring XidGenLock ensures that no new XIDs enter the proc
    2614             :  * array until the caller has WAL-logged this snapshot, and releases the
    2615             :  * lock. Acquiring ProcArrayLock ensures that no transactions commit until the
    2616             :  * lock is released.
    2617             :  *
    2618             :  * The returned data structure is statically allocated; caller should not
    2619             :  * modify it, and must not assume it is valid past the next call.
    2620             :  *
    2621             :  * This is never executed during recovery so there is no need to look at
    2622             :  * KnownAssignedXids.
    2623             :  *
    2624             :  * Dummy PGPROCs from prepared transaction are included, meaning that this
    2625             :  * may return entries with duplicated TransactionId values coming from
    2626             :  * transaction finishing to prepare.  Nothing is done about duplicated
    2627             :  * entries here to not hold on ProcArrayLock more than necessary.
    2628             :  *
    2629             :  * We don't worry about updating other counters, we want to keep this as
    2630             :  * simple as possible and leave GetSnapshotData() as the primary code for
    2631             :  * that bookkeeping.
    2632             :  *
    2633             :  * Note that if any transaction has overflowed its cached subtransactions
    2634             :  * then there is no real need include any subtransactions.
    2635             :  */
    2636             : RunningTransactions
    2637        2746 : GetRunningTransactionData(void)
    2638             : {
    2639             :     /* result workspace */
    2640             :     static RunningTransactionsData CurrentRunningXactsData;
    2641             : 
    2642        2746 :     ProcArrayStruct *arrayP = procArray;
    2643        2746 :     TransactionId *other_xids = ProcGlobal->xids;
    2644        2746 :     RunningTransactions CurrentRunningXacts = &CurrentRunningXactsData;
    2645             :     TransactionId latestCompletedXid;
    2646             :     TransactionId oldestRunningXid;
    2647             :     TransactionId oldestDatabaseRunningXid;
    2648             :     TransactionId *xids;
    2649             :     int         index;
    2650             :     int         count;
    2651             :     int         subcount;
    2652             :     bool        suboverflowed;
    2653             : 
    2654             :     Assert(!RecoveryInProgress());
    2655             : 
    2656             :     /*
    2657             :      * Allocating space for maxProcs xids is usually overkill; numProcs would
    2658             :      * be sufficient.  But it seems better to do the malloc while not holding
    2659             :      * the lock, so we can't look at numProcs.  Likewise, we allocate much
    2660             :      * more subxip storage than is probably needed.
    2661             :      *
    2662             :      * Should only be allocated in bgwriter, since only ever executed during
    2663             :      * checkpoints.
    2664             :      */
    2665        2746 :     if (CurrentRunningXacts->xids == NULL)
    2666             :     {
    2667             :         /*
    2668             :          * First call
    2669             :          */
    2670        1052 :         CurrentRunningXacts->xids = (TransactionId *)
    2671        1052 :             malloc(TOTAL_MAX_CACHED_SUBXIDS * sizeof(TransactionId));
    2672        1052 :         if (CurrentRunningXacts->xids == NULL)
    2673           0 :             ereport(ERROR,
    2674             :                     (errcode(ERRCODE_OUT_OF_MEMORY),
    2675             :                      errmsg("out of memory")));
    2676             :     }
    2677             : 
    2678        2746 :     xids = CurrentRunningXacts->xids;
    2679             : 
    2680        2746 :     count = subcount = 0;
    2681        2746 :     suboverflowed = false;
    2682             : 
    2683             :     /*
    2684             :      * Ensure that no xids enter or leave the procarray while we obtain
    2685             :      * snapshot.
    2686             :      */
    2687        2746 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    2688        2746 :     LWLockAcquire(XidGenLock, LW_SHARED);
    2689             : 
    2690        2746 :     latestCompletedXid =
    2691        2746 :         XidFromFullTransactionId(TransamVariables->latestCompletedXid);
    2692        2746 :     oldestDatabaseRunningXid = oldestRunningXid =
    2693        2746 :         XidFromFullTransactionId(TransamVariables->nextXid);
    2694             : 
    2695             :     /*
    2696             :      * Spin over procArray collecting all xids
    2697             :      */
    2698       13264 :     for (index = 0; index < arrayP->numProcs; index++)
    2699             :     {
    2700             :         TransactionId xid;
    2701             : 
    2702             :         /* Fetch xid just once - see GetNewTransactionId */
    2703       10518 :         xid = UINT32_ACCESS_ONCE(other_xids[index]);
    2704             : 
    2705             :         /*
    2706             :          * We don't need to store transactions that don't have a TransactionId
    2707             :          * yet because they will not show as running on a standby server.
    2708             :          */
    2709       10518 :         if (!TransactionIdIsValid(xid))
    2710        8858 :             continue;
    2711             : 
    2712             :         /*
    2713             :          * Be careful not to exclude any xids before calculating the values of
    2714             :          * oldestRunningXid and suboverflowed, since these are used to clean
    2715             :          * up transaction information held on standbys.
    2716             :          */
    2717        1660 :         if (TransactionIdPrecedes(xid, oldestRunningXid))
    2718        1192 :             oldestRunningXid = xid;
    2719             : 
    2720             :         /*
    2721             :          * Also, update the oldest running xid within the current database. As
    2722             :          * fetching pgprocno and PGPROC could cause cache misses, we do cheap
    2723             :          * TransactionId comparison first.
    2724             :          */
    2725        1660 :         if (TransactionIdPrecedes(xid, oldestDatabaseRunningXid))
    2726             :         {
    2727        1660 :             int         pgprocno = arrayP->pgprocnos[index];
    2728        1660 :             PGPROC     *proc = &allProcs[pgprocno];
    2729             : 
    2730        1660 :             if (proc->databaseId == MyDatabaseId)
    2731         426 :                 oldestDatabaseRunningXid = xid;
    2732             :         }
    2733             : 
    2734        1660 :         if (ProcGlobal->subxidStates[index].overflowed)
    2735           4 :             suboverflowed = true;
    2736             : 
    2737             :         /*
    2738             :          * If we wished to exclude xids this would be the right place for it.
    2739             :          * Procs with the PROC_IN_VACUUM flag set don't usually assign xids,
    2740             :          * but they do during truncation at the end when they get the lock and
    2741             :          * truncate, so it is not much of a problem to include them if they
    2742             :          * are seen and it is cleaner to include them.
    2743             :          */
    2744             : 
    2745        1660 :         xids[count++] = xid;
    2746             :     }
    2747             : 
    2748             :     /*
    2749             :      * Spin over procArray collecting all subxids, but only if there hasn't
    2750             :      * been a suboverflow.
    2751             :      */
    2752        2746 :     if (!suboverflowed)
    2753             :     {
    2754        2742 :         XidCacheStatus *other_subxidstates = ProcGlobal->subxidStates;
    2755             : 
    2756       13248 :         for (index = 0; index < arrayP->numProcs; index++)
    2757             :         {
    2758       10506 :             int         pgprocno = arrayP->pgprocnos[index];
    2759       10506 :             PGPROC     *proc = &allProcs[pgprocno];
    2760             :             int         nsubxids;
    2761             : 
    2762             :             /*
    2763             :              * Save subtransaction XIDs. Other backends can't add or remove
    2764             :              * entries while we're holding XidGenLock.
    2765             :              */
    2766       10506 :             nsubxids = other_subxidstates[index].count;
    2767       10506 :             if (nsubxids > 0)
    2768             :             {
    2769             :                 /* barrier not really required, as XidGenLock is held, but ... */
    2770          28 :                 pg_read_barrier();  /* pairs with GetNewTransactionId */
    2771             : 
    2772          28 :                 memcpy(&xids[count], proc->subxids.xids,
    2773             :                        nsubxids * sizeof(TransactionId));
    2774          28 :                 count += nsubxids;
    2775          28 :                 subcount += nsubxids;
    2776             : 
    2777             :                 /*
    2778             :                  * Top-level XID of a transaction is always less than any of
    2779             :                  * its subxids, so we don't need to check if any of the
    2780             :                  * subxids are smaller than oldestRunningXid
    2781             :                  */
    2782             :             }
    2783             :         }
    2784             :     }
    2785             : 
    2786             :     /*
    2787             :      * It's important *not* to include the limits set by slots here because
    2788             :      * snapbuild.c uses oldestRunningXid to manage its xmin horizon. If those
    2789             :      * were to be included here the initial value could never increase because
    2790             :      * of a circular dependency where slots only increase their limits when
    2791             :      * running xacts increases oldestRunningXid and running xacts only
    2792             :      * increases if slots do.
    2793             :      */
    2794             : 
    2795        2746 :     CurrentRunningXacts->xcnt = count - subcount;
    2796        2746 :     CurrentRunningXacts->subxcnt = subcount;
    2797        2746 :     CurrentRunningXacts->subxid_status = suboverflowed ? SUBXIDS_IN_SUBTRANS : SUBXIDS_IN_ARRAY;
    2798        2746 :     CurrentRunningXacts->nextXid = XidFromFullTransactionId(TransamVariables->nextXid);
    2799        2746 :     CurrentRunningXacts->oldestRunningXid = oldestRunningXid;
    2800        2746 :     CurrentRunningXacts->oldestDatabaseRunningXid = oldestDatabaseRunningXid;
    2801        2746 :     CurrentRunningXacts->latestCompletedXid = latestCompletedXid;
    2802             : 
    2803             :     Assert(TransactionIdIsValid(CurrentRunningXacts->nextXid));
    2804             :     Assert(TransactionIdIsValid(CurrentRunningXacts->oldestRunningXid));
    2805             :     Assert(TransactionIdIsNormal(CurrentRunningXacts->latestCompletedXid));
    2806             : 
    2807             :     /* We don't release the locks here, the caller is responsible for that */
    2808             : 
    2809        2746 :     return CurrentRunningXacts;
    2810             : }
    2811             : 
    2812             : /*
    2813             :  * GetOldestActiveTransactionId()
    2814             :  *
    2815             :  * Similar to GetSnapshotData but returns just oldestActiveXid. We include
    2816             :  * all PGPROCs with an assigned TransactionId, even VACUUM processes.
    2817             :  *
    2818             :  * If allDbs is true, we look at all databases, though there is no need to
    2819             :  * include WALSender since this has no effect on hot standby conflicts. If
    2820             :  * allDbs is false, skip processes attached to other databases.
    2821             :  *
    2822             :  * This is never executed during recovery so there is no need to look at
    2823             :  * KnownAssignedXids.
    2824             :  *
    2825             :  * We don't worry about updating other counters, we want to keep this as
    2826             :  * simple as possible and leave GetSnapshotData() as the primary code for
    2827             :  * that bookkeeping.
    2828             :  *
    2829             :  * inCommitOnly indicates getting the oldestActiveXid among the transactions
    2830             :  * in the commit critical section.
    2831             :  */
    2832             : TransactionId
    2833        4396 : GetOldestActiveTransactionId(bool inCommitOnly, bool allDbs)
    2834             : {
    2835        4396 :     ProcArrayStruct *arrayP = procArray;
    2836        4396 :     TransactionId *other_xids = ProcGlobal->xids;
    2837             :     TransactionId oldestRunningXid;
    2838             :     int         index;
    2839             : 
    2840             :     Assert(!RecoveryInProgress());
    2841             : 
    2842             :     /*
    2843             :      * Read nextXid, as the upper bound of what's still active.
    2844             :      *
    2845             :      * Reading a TransactionId is atomic, but we must grab the lock to make
    2846             :      * sure that all XIDs < nextXid are already present in the proc array (or
    2847             :      * have already completed), when we spin over it.
    2848             :      */
    2849        4396 :     LWLockAcquire(XidGenLock, LW_SHARED);
    2850        4396 :     oldestRunningXid = XidFromFullTransactionId(TransamVariables->nextXid);
    2851        4396 :     LWLockRelease(XidGenLock);
    2852             : 
    2853             :     /*
    2854             :      * Spin over procArray collecting all xids and subxids.
    2855             :      */
    2856        4396 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    2857       21344 :     for (index = 0; index < arrayP->numProcs; index++)
    2858             :     {
    2859             :         TransactionId xid;
    2860       16948 :         int         pgprocno = arrayP->pgprocnos[index];
    2861       16948 :         PGPROC     *proc = &allProcs[pgprocno];
    2862             : 
    2863             :         /* Fetch xid just once - see GetNewTransactionId */
    2864       16948 :         xid = UINT32_ACCESS_ONCE(other_xids[index]);
    2865             : 
    2866       16948 :         if (!TransactionIdIsNormal(xid))
    2867       12718 :             continue;
    2868             : 
    2869        4230 :         if (inCommitOnly &&
    2870        2592 :             (proc->delayChkptFlags & DELAY_CHKPT_IN_COMMIT) == 0)
    2871           2 :             continue;
    2872             : 
    2873        4228 :         if (!allDbs && proc->databaseId != MyDatabaseId)
    2874           0 :             continue;
    2875             : 
    2876        4228 :         if (TransactionIdPrecedes(xid, oldestRunningXid))
    2877        3724 :             oldestRunningXid = xid;
    2878             : 
    2879             :         /*
    2880             :          * Top-level XID of a transaction is always less than any of its
    2881             :          * subxids, so we don't need to check if any of the subxids are
    2882             :          * smaller than oldestRunningXid
    2883             :          */
    2884             :     }
    2885        4396 :     LWLockRelease(ProcArrayLock);
    2886             : 
    2887        4396 :     return oldestRunningXid;
    2888             : }
    2889             : 
    2890             : /*
    2891             :  * GetOldestSafeDecodingTransactionId -- lowest xid not affected by vacuum
    2892             :  *
    2893             :  * Returns the oldest xid that we can guarantee not to have been affected by
    2894             :  * vacuum, i.e. no rows >= that xid have been vacuumed away unless the
    2895             :  * transaction aborted. Note that the value can (and most of the time will) be
    2896             :  * much more conservative than what really has been affected by vacuum, but we
    2897             :  * currently don't have better data available.
    2898             :  *
    2899             :  * This is useful to initialize the cutoff xid after which a new changeset
    2900             :  * extraction replication slot can start decoding changes.
    2901             :  *
    2902             :  * Must be called with ProcArrayLock held either shared or exclusively,
    2903             :  * although most callers will want to use exclusive mode since it is expected
    2904             :  * that the caller will immediately use the xid to peg the xmin horizon.
    2905             :  */
    2906             : TransactionId
    2907        1314 : GetOldestSafeDecodingTransactionId(bool catalogOnly)
    2908             : {
    2909        1314 :     ProcArrayStruct *arrayP = procArray;
    2910             :     TransactionId oldestSafeXid;
    2911             :     int         index;
    2912        1314 :     bool        recovery_in_progress = RecoveryInProgress();
    2913             : 
    2914             :     Assert(LWLockHeldByMe(ProcArrayLock));
    2915             : 
    2916             :     /*
    2917             :      * Acquire XidGenLock, so no transactions can acquire an xid while we're
    2918             :      * running. If no transaction with xid were running concurrently a new xid
    2919             :      * could influence the RecentXmin et al.
    2920             :      *
    2921             :      * We initialize the computation to nextXid since that's guaranteed to be
    2922             :      * a safe, albeit pessimal, value.
    2923             :      */
    2924        1314 :     LWLockAcquire(XidGenLock, LW_SHARED);
    2925        1314 :     oldestSafeXid = XidFromFullTransactionId(TransamVariables->nextXid);
    2926             : 
    2927             :     /*
    2928             :      * If there's already a slot pegging the xmin horizon, we can start with
    2929             :      * that value, it's guaranteed to be safe since it's computed by this
    2930             :      * routine initially and has been enforced since.  We can always use the
    2931             :      * slot's general xmin horizon, but the catalog horizon is only usable
    2932             :      * when only catalog data is going to be looked at.
    2933             :      */
    2934        1734 :     if (TransactionIdIsValid(procArray->replication_slot_xmin) &&
    2935         420 :         TransactionIdPrecedes(procArray->replication_slot_xmin,
    2936             :                               oldestSafeXid))
    2937          24 :         oldestSafeXid = procArray->replication_slot_xmin;
    2938             : 
    2939        1314 :     if (catalogOnly &&
    2940         662 :         TransactionIdIsValid(procArray->replication_slot_catalog_xmin) &&
    2941         134 :         TransactionIdPrecedes(procArray->replication_slot_catalog_xmin,
    2942             :                               oldestSafeXid))
    2943          52 :         oldestSafeXid = procArray->replication_slot_catalog_xmin;
    2944             : 
    2945             :     /*
    2946             :      * If we're not in recovery, we walk over the procarray and collect the
    2947             :      * lowest xid. Since we're called with ProcArrayLock held and have
    2948             :      * acquired XidGenLock, no entries can vanish concurrently, since
    2949             :      * ProcGlobal->xids[i] is only set with XidGenLock held and only cleared
    2950             :      * with ProcArrayLock held.
    2951             :      *
    2952             :      * In recovery we can't lower the safe value besides what we've computed
    2953             :      * above, so we'll have to wait a bit longer there. We unfortunately can
    2954             :      * *not* use KnownAssignedXidsGetOldestXmin() since the KnownAssignedXids
    2955             :      * machinery can miss values and return an older value than is safe.
    2956             :      */
    2957        1314 :     if (!recovery_in_progress)
    2958             :     {
    2959        1258 :         TransactionId *other_xids = ProcGlobal->xids;
    2960             : 
    2961             :         /*
    2962             :          * Spin over procArray collecting min(ProcGlobal->xids[i])
    2963             :          */
    2964        6434 :         for (index = 0; index < arrayP->numProcs; index++)
    2965             :         {
    2966             :             TransactionId xid;
    2967             : 
    2968             :             /* Fetch xid just once - see GetNewTransactionId */
    2969        5176 :             xid = UINT32_ACCESS_ONCE(other_xids[index]);
    2970             : 
    2971        5176 :             if (!TransactionIdIsNormal(xid))
    2972        5156 :                 continue;
    2973             : 
    2974          20 :             if (TransactionIdPrecedes(xid, oldestSafeXid))
    2975          16 :                 oldestSafeXid = xid;
    2976             :         }
    2977             :     }
    2978             : 
    2979        1314 :     LWLockRelease(XidGenLock);
    2980             : 
    2981        1314 :     return oldestSafeXid;
    2982             : }
    2983             : 
    2984             : /*
    2985             :  * GetVirtualXIDsDelayingChkpt -- Get the VXIDs of transactions that are
    2986             :  * delaying checkpoint because they have critical actions in progress.
    2987             :  *
    2988             :  * Constructs an array of VXIDs of transactions that are currently in commit
    2989             :  * critical sections, as shown by having specified delayChkptFlags bits set
    2990             :  * in their PGPROC.
    2991             :  *
    2992             :  * Returns a palloc'd array that should be freed by the caller.
    2993             :  * *nvxids is the number of valid entries.
    2994             :  *
    2995             :  * Note that because backends set or clear delayChkptFlags without holding any
    2996             :  * lock, the result is somewhat indeterminate, but we don't really care.  Even
    2997             :  * in a multiprocessor with delayed writes to shared memory, it should be
    2998             :  * certain that setting of delayChkptFlags will propagate to shared memory
    2999             :  * when the backend takes a lock, so we cannot fail to see a virtual xact as
    3000             :  * delayChkptFlags if it's already inserted its commit record.  Whether it
    3001             :  * takes a little while for clearing of delayChkptFlags to propagate is
    3002             :  * unimportant for correctness.
    3003             :  */
    3004             : VirtualTransactionId *
    3005        6004 : GetVirtualXIDsDelayingChkpt(int *nvxids, int type)
    3006             : {
    3007             :     VirtualTransactionId *vxids;
    3008        6004 :     ProcArrayStruct *arrayP = procArray;
    3009        6004 :     int         count = 0;
    3010             :     int         index;
    3011             : 
    3012             :     Assert(type != 0);
    3013             : 
    3014             :     /* allocate what's certainly enough result space */
    3015             :     vxids = (VirtualTransactionId *)
    3016        6004 :         palloc(sizeof(VirtualTransactionId) * arrayP->maxProcs);
    3017             : 
    3018        6004 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3019             : 
    3020       19984 :     for (index = 0; index < arrayP->numProcs; index++)
    3021             :     {
    3022       13980 :         int         pgprocno = arrayP->pgprocnos[index];
    3023       13980 :         PGPROC     *proc = &allProcs[pgprocno];
    3024             : 
    3025       13980 :         if ((proc->delayChkptFlags & type) != 0)
    3026             :         {
    3027             :             VirtualTransactionId vxid;
    3028             : 
    3029          92 :             GET_VXID_FROM_PGPROC(vxid, *proc);
    3030          92 :             if (VirtualTransactionIdIsValid(vxid))
    3031          92 :                 vxids[count++] = vxid;
    3032             :         }
    3033             :     }
    3034             : 
    3035        6004 :     LWLockRelease(ProcArrayLock);
    3036             : 
    3037        6004 :     *nvxids = count;
    3038        6004 :     return vxids;
    3039             : }
    3040             : 
    3041             : /*
    3042             :  * HaveVirtualXIDsDelayingChkpt -- Are any of the specified VXIDs delaying?
    3043             :  *
    3044             :  * This is used with the results of GetVirtualXIDsDelayingChkpt to see if any
    3045             :  * of the specified VXIDs are still in critical sections of code.
    3046             :  *
    3047             :  * Note: this is O(N^2) in the number of vxacts that are/were delaying, but
    3048             :  * those numbers should be small enough for it not to be a problem.
    3049             :  */
    3050             : bool
    3051          96 : HaveVirtualXIDsDelayingChkpt(VirtualTransactionId *vxids, int nvxids, int type)
    3052             : {
    3053          96 :     bool        result = false;
    3054          96 :     ProcArrayStruct *arrayP = procArray;
    3055             :     int         index;
    3056             : 
    3057             :     Assert(type != 0);
    3058             : 
    3059          96 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3060             : 
    3061         910 :     for (index = 0; index < arrayP->numProcs; index++)
    3062             :     {
    3063         832 :         int         pgprocno = arrayP->pgprocnos[index];
    3064         832 :         PGPROC     *proc = &allProcs[pgprocno];
    3065             :         VirtualTransactionId vxid;
    3066             : 
    3067         832 :         GET_VXID_FROM_PGPROC(vxid, *proc);
    3068             : 
    3069         832 :         if ((proc->delayChkptFlags & type) != 0 &&
    3070          50 :             VirtualTransactionIdIsValid(vxid))
    3071             :         {
    3072             :             int         i;
    3073             : 
    3074          90 :             for (i = 0; i < nvxids; i++)
    3075             :             {
    3076          58 :                 if (VirtualTransactionIdEquals(vxid, vxids[i]))
    3077             :                 {
    3078          18 :                     result = true;
    3079          18 :                     break;
    3080             :                 }
    3081             :             }
    3082          50 :             if (result)
    3083          18 :                 break;
    3084             :         }
    3085             :     }
    3086             : 
    3087          96 :     LWLockRelease(ProcArrayLock);
    3088             : 
    3089          96 :     return result;
    3090             : }
    3091             : 
    3092             : /*
    3093             :  * ProcNumberGetProc -- get a backend's PGPROC given its proc number
    3094             :  *
    3095             :  * The result may be out of date arbitrarily quickly, so the caller
    3096             :  * must be careful about how this information is used.  NULL is
    3097             :  * returned if the backend is not active.
    3098             :  */
    3099             : PGPROC *
    3100        1098 : ProcNumberGetProc(ProcNumber procNumber)
    3101             : {
    3102             :     PGPROC     *result;
    3103             : 
    3104        1098 :     if (procNumber < 0 || procNumber >= ProcGlobal->allProcCount)
    3105           2 :         return NULL;
    3106        1096 :     result = GetPGProcByNumber(procNumber);
    3107             : 
    3108        1096 :     if (result->pid == 0)
    3109           6 :         return NULL;
    3110             : 
    3111        1090 :     return result;
    3112             : }
    3113             : 
    3114             : /*
    3115             :  * ProcNumberGetTransactionIds -- get a backend's transaction status
    3116             :  *
    3117             :  * Get the xid, xmin, nsubxid and overflow status of the backend.  The
    3118             :  * result may be out of date arbitrarily quickly, so the caller must be
    3119             :  * careful about how this information is used.
    3120             :  */
    3121             : void
    3122       20792 : ProcNumberGetTransactionIds(ProcNumber procNumber, TransactionId *xid,
    3123             :                             TransactionId *xmin, int *nsubxid, bool *overflowed)
    3124             : {
    3125             :     PGPROC     *proc;
    3126             : 
    3127       20792 :     *xid = InvalidTransactionId;
    3128       20792 :     *xmin = InvalidTransactionId;
    3129       20792 :     *nsubxid = 0;
    3130       20792 :     *overflowed = false;
    3131             : 
    3132       20792 :     if (procNumber < 0 || procNumber >= ProcGlobal->allProcCount)
    3133           0 :         return;
    3134       20792 :     proc = GetPGProcByNumber(procNumber);
    3135             : 
    3136             :     /* Need to lock out additions/removals of backends */
    3137       20792 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3138             : 
    3139       20792 :     if (proc->pid != 0)
    3140             :     {
    3141       20792 :         *xid = proc->xid;
    3142       20792 :         *xmin = proc->xmin;
    3143       20792 :         *nsubxid = proc->subxidStatus.count;
    3144       20792 :         *overflowed = proc->subxidStatus.overflowed;
    3145             :     }
    3146             : 
    3147       20792 :     LWLockRelease(ProcArrayLock);
    3148             : }
    3149             : 
    3150             : /*
    3151             :  * BackendPidGetProc -- get a backend's PGPROC given its PID
    3152             :  *
    3153             :  * Returns NULL if not found.  Note that it is up to the caller to be
    3154             :  * sure that the question remains meaningful for long enough for the
    3155             :  * answer to be used ...
    3156             :  */
    3157             : PGPROC *
    3158       22782 : BackendPidGetProc(int pid)
    3159             : {
    3160             :     PGPROC     *result;
    3161             : 
    3162       22782 :     if (pid == 0)               /* never match dummy PGPROCs */
    3163           6 :         return NULL;
    3164             : 
    3165       22776 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3166             : 
    3167       22776 :     result = BackendPidGetProcWithLock(pid);
    3168             : 
    3169       22776 :     LWLockRelease(ProcArrayLock);
    3170             : 
    3171       22776 :     return result;
    3172             : }
    3173             : 
    3174             : /*
    3175             :  * BackendPidGetProcWithLock -- get a backend's PGPROC given its PID
    3176             :  *
    3177             :  * Same as above, except caller must be holding ProcArrayLock.  The found
    3178             :  * entry, if any, can be assumed to be valid as long as the lock remains held.
    3179             :  */
    3180             : PGPROC *
    3181       26158 : BackendPidGetProcWithLock(int pid)
    3182             : {
    3183       26158 :     PGPROC     *result = NULL;
    3184       26158 :     ProcArrayStruct *arrayP = procArray;
    3185             :     int         index;
    3186             : 
    3187       26158 :     if (pid == 0)               /* never match dummy PGPROCs */
    3188           0 :         return NULL;
    3189             : 
    3190      102850 :     for (index = 0; index < arrayP->numProcs; index++)
    3191             :     {
    3192       91372 :         PGPROC     *proc = &allProcs[arrayP->pgprocnos[index]];
    3193             : 
    3194       91372 :         if (proc->pid == pid)
    3195             :         {
    3196       14680 :             result = proc;
    3197       14680 :             break;
    3198             :         }
    3199             :     }
    3200             : 
    3201       26158 :     return result;
    3202             : }
    3203             : 
    3204             : /*
    3205             :  * BackendXidGetPid -- get a backend's pid given its XID
    3206             :  *
    3207             :  * Returns 0 if not found or it's a prepared transaction.  Note that
    3208             :  * it is up to the caller to be sure that the question remains
    3209             :  * meaningful for long enough for the answer to be used ...
    3210             :  *
    3211             :  * Only main transaction Ids are considered.  This function is mainly
    3212             :  * useful for determining what backend owns a lock.
    3213             :  *
    3214             :  * Beware that not every xact has an XID assigned.  However, as long as you
    3215             :  * only call this using an XID found on disk, you're safe.
    3216             :  */
    3217             : int
    3218          60 : BackendXidGetPid(TransactionId xid)
    3219             : {
    3220          60 :     int         result = 0;
    3221          60 :     ProcArrayStruct *arrayP = procArray;
    3222          60 :     TransactionId *other_xids = ProcGlobal->xids;
    3223             :     int         index;
    3224             : 
    3225          60 :     if (xid == InvalidTransactionId)    /* never match invalid xid */
    3226           0 :         return 0;
    3227             : 
    3228          60 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3229             : 
    3230         184 :     for (index = 0; index < arrayP->numProcs; index++)
    3231             :     {
    3232         168 :         if (other_xids[index] == xid)
    3233             :         {
    3234          44 :             int         pgprocno = arrayP->pgprocnos[index];
    3235          44 :             PGPROC     *proc = &allProcs[pgprocno];
    3236             : 
    3237          44 :             result = proc->pid;
    3238          44 :             break;
    3239             :         }
    3240             :     }
    3241             : 
    3242          60 :     LWLockRelease(ProcArrayLock);
    3243             : 
    3244          60 :     return result;
    3245             : }
    3246             : 
    3247             : /*
    3248             :  * IsBackendPid -- is a given pid a running backend
    3249             :  *
    3250             :  * This is not called by the backend, but is called by external modules.
    3251             :  */
    3252             : bool
    3253           4 : IsBackendPid(int pid)
    3254             : {
    3255           4 :     return (BackendPidGetProc(pid) != NULL);
    3256             : }
    3257             : 
    3258             : 
    3259             : /*
    3260             :  * GetCurrentVirtualXIDs -- returns an array of currently active VXIDs.
    3261             :  *
    3262             :  * The array is palloc'd. The number of valid entries is returned into *nvxids.
    3263             :  *
    3264             :  * The arguments allow filtering the set of VXIDs returned.  Our own process
    3265             :  * is always skipped.  In addition:
    3266             :  *  If limitXmin is not InvalidTransactionId, skip processes with
    3267             :  *      xmin > limitXmin.
    3268             :  *  If excludeXmin0 is true, skip processes with xmin = 0.
    3269             :  *  If allDbs is false, skip processes attached to other databases.
    3270             :  *  If excludeVacuum isn't zero, skip processes for which
    3271             :  *      (statusFlags & excludeVacuum) is not zero.
    3272             :  *
    3273             :  * Note: the purpose of the limitXmin and excludeXmin0 parameters is to
    3274             :  * allow skipping backends whose oldest live snapshot is no older than
    3275             :  * some snapshot we have.  Since we examine the procarray with only shared
    3276             :  * lock, there are race conditions: a backend could set its xmin just after
    3277             :  * we look.  Indeed, on multiprocessors with weak memory ordering, the
    3278             :  * other backend could have set its xmin *before* we look.  We know however
    3279             :  * that such a backend must have held shared ProcArrayLock overlapping our
    3280             :  * own hold of ProcArrayLock, else we would see its xmin update.  Therefore,
    3281             :  * any snapshot the other backend is taking concurrently with our scan cannot
    3282             :  * consider any transactions as still running that we think are committed
    3283             :  * (since backends must hold ProcArrayLock exclusive to commit).
    3284             :  */
    3285             : VirtualTransactionId *
    3286         752 : GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
    3287             :                       bool allDbs, int excludeVacuum,
    3288             :                       int *nvxids)
    3289             : {
    3290             :     VirtualTransactionId *vxids;
    3291         752 :     ProcArrayStruct *arrayP = procArray;
    3292         752 :     int         count = 0;
    3293             :     int         index;
    3294             : 
    3295             :     /* allocate what's certainly enough result space */
    3296             :     vxids = (VirtualTransactionId *)
    3297         752 :         palloc(sizeof(VirtualTransactionId) * arrayP->maxProcs);
    3298             : 
    3299         752 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3300             : 
    3301        4428 :     for (index = 0; index < arrayP->numProcs; index++)
    3302             :     {
    3303        3676 :         int         pgprocno = arrayP->pgprocnos[index];
    3304        3676 :         PGPROC     *proc = &allProcs[pgprocno];
    3305        3676 :         uint8       statusFlags = ProcGlobal->statusFlags[index];
    3306             : 
    3307        3676 :         if (proc == MyProc)
    3308         752 :             continue;
    3309             : 
    3310        2924 :         if (excludeVacuum & statusFlags)
    3311          12 :             continue;
    3312             : 
    3313        2912 :         if (allDbs || proc->databaseId == MyDatabaseId)
    3314             :         {
    3315             :             /* Fetch xmin just once - might change on us */
    3316        1256 :             TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin);
    3317             : 
    3318        1256 :             if (excludeXmin0 && !TransactionIdIsValid(pxmin))
    3319         772 :                 continue;
    3320             : 
    3321             :             /*
    3322             :              * InvalidTransactionId precedes all other XIDs, so a proc that
    3323             :              * hasn't set xmin yet will not be rejected by this test.
    3324             :              */
    3325         968 :             if (!TransactionIdIsValid(limitXmin) ||
    3326         484 :                 TransactionIdPrecedesOrEquals(pxmin, limitXmin))
    3327             :             {
    3328             :                 VirtualTransactionId vxid;
    3329             : 
    3330         442 :                 GET_VXID_FROM_PGPROC(vxid, *proc);
    3331         442 :                 if (VirtualTransactionIdIsValid(vxid))
    3332         442 :                     vxids[count++] = vxid;
    3333             :             }
    3334             :         }
    3335             :     }
    3336             : 
    3337         752 :     LWLockRelease(ProcArrayLock);
    3338             : 
    3339         752 :     *nvxids = count;
    3340         752 :     return vxids;
    3341             : }
    3342             : 
    3343             : /*
    3344             :  * GetConflictingVirtualXIDs -- returns an array of currently active VXIDs.
    3345             :  *
    3346             :  * Usage is limited to conflict resolution during recovery on standby servers.
    3347             :  * limitXmin is supplied as either a cutoff with snapshotConflictHorizon
    3348             :  * semantics, or InvalidTransactionId in cases where caller cannot accurately
    3349             :  * determine a safe snapshotConflictHorizon value.
    3350             :  *
    3351             :  * If limitXmin is InvalidTransactionId then we want to kill everybody,
    3352             :  * so we're not worried if they have a snapshot or not, nor does it really
    3353             :  * matter what type of lock we hold.  Caller must avoid calling here with
    3354             :  * snapshotConflictHorizon style cutoffs that were set to InvalidTransactionId
    3355             :  * during original execution, since that actually indicates that there is
    3356             :  * definitely no need for a recovery conflict (the snapshotConflictHorizon
    3357             :  * convention for InvalidTransactionId values is the opposite of our own!).
    3358             :  *
    3359             :  * All callers that are checking xmins always now supply a valid and useful
    3360             :  * value for limitXmin. The limitXmin is always lower than the lowest
    3361             :  * numbered KnownAssignedXid that is not already a FATAL error. This is
    3362             :  * because we only care about cleanup records that are cleaning up tuple
    3363             :  * versions from committed transactions. In that case they will only occur
    3364             :  * at the point where the record is less than the lowest running xid. That
    3365             :  * allows us to say that if any backend takes a snapshot concurrently with
    3366             :  * us then the conflict assessment made here would never include the snapshot
    3367             :  * that is being derived. So we take LW_SHARED on the ProcArray and allow
    3368             :  * concurrent snapshots when limitXmin is valid. We might think about adding
    3369             :  *   Assert(limitXmin < lowest(KnownAssignedXids))
    3370             :  * but that would not be true in the case of FATAL errors lagging in array,
    3371             :  * but we already know those are bogus anyway, so we skip that test.
    3372             :  *
    3373             :  * If dbOid is valid we skip backends attached to other databases.
    3374             :  *
    3375             :  * Be careful to *not* pfree the result from this function. We reuse
    3376             :  * this array sufficiently often that we use malloc for the result.
    3377             :  */
    3378             : VirtualTransactionId *
    3379       28022 : GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
    3380             : {
    3381             :     static VirtualTransactionId *vxids;
    3382       28022 :     ProcArrayStruct *arrayP = procArray;
    3383       28022 :     int         count = 0;
    3384             :     int         index;
    3385             : 
    3386             :     /*
    3387             :      * If first time through, get workspace to remember main XIDs in. We
    3388             :      * malloc it permanently to avoid repeated palloc/pfree overhead. Allow
    3389             :      * result space, remembering room for a terminator.
    3390             :      */
    3391       28022 :     if (vxids == NULL)
    3392             :     {
    3393          54 :         vxids = (VirtualTransactionId *)
    3394          54 :             malloc(sizeof(VirtualTransactionId) * (arrayP->maxProcs + 1));
    3395          54 :         if (vxids == NULL)
    3396           0 :             ereport(ERROR,
    3397             :                     (errcode(ERRCODE_OUT_OF_MEMORY),
    3398             :                      errmsg("out of memory")));
    3399             :     }
    3400             : 
    3401       28022 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3402             : 
    3403       28316 :     for (index = 0; index < arrayP->numProcs; index++)
    3404             :     {
    3405         294 :         int         pgprocno = arrayP->pgprocnos[index];
    3406         294 :         PGPROC     *proc = &allProcs[pgprocno];
    3407             : 
    3408             :         /* Exclude prepared transactions */
    3409         294 :         if (proc->pid == 0)
    3410           0 :             continue;
    3411             : 
    3412         294 :         if (!OidIsValid(dbOid) ||
    3413         280 :             proc->databaseId == dbOid)
    3414             :         {
    3415             :             /* Fetch xmin just once - can't change on us, but good coding */
    3416          36 :             TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin);
    3417             : 
    3418             :             /*
    3419             :              * We ignore an invalid pxmin because this means that backend has
    3420             :              * no snapshot currently. We hold a Share lock to avoid contention
    3421             :              * with users taking snapshots.  That is not a problem because the
    3422             :              * current xmin is always at least one higher than the latest
    3423             :              * removed xid, so any new snapshot would never conflict with the
    3424             :              * test here.
    3425             :              */
    3426          36 :             if (!TransactionIdIsValid(limitXmin) ||
    3427           8 :                 (TransactionIdIsValid(pxmin) && !TransactionIdFollows(pxmin, limitXmin)))
    3428             :             {
    3429             :                 VirtualTransactionId vxid;
    3430             : 
    3431           4 :                 GET_VXID_FROM_PGPROC(vxid, *proc);
    3432           4 :                 if (VirtualTransactionIdIsValid(vxid))
    3433           4 :                     vxids[count++] = vxid;
    3434             :             }
    3435             :         }
    3436             :     }
    3437             : 
    3438       28022 :     LWLockRelease(ProcArrayLock);
    3439             : 
    3440             :     /* add the terminator */
    3441       28022 :     vxids[count].procNumber = INVALID_PROC_NUMBER;
    3442       28022 :     vxids[count].localTransactionId = InvalidLocalTransactionId;
    3443             : 
    3444       28022 :     return vxids;
    3445             : }
    3446             : 
    3447             : /*
    3448             :  * CancelVirtualTransaction - used in recovery conflict processing
    3449             :  *
    3450             :  * Returns pid of the process signaled, or 0 if not found.
    3451             :  */
    3452             : pid_t
    3453           6 : CancelVirtualTransaction(VirtualTransactionId vxid, ProcSignalReason sigmode)
    3454             : {
    3455           6 :     return SignalVirtualTransaction(vxid, sigmode, true);
    3456             : }
    3457             : 
    3458             : pid_t
    3459          10 : SignalVirtualTransaction(VirtualTransactionId vxid, ProcSignalReason sigmode,
    3460             :                          bool conflictPending)
    3461             : {
    3462          10 :     ProcArrayStruct *arrayP = procArray;
    3463             :     int         index;
    3464          10 :     pid_t       pid = 0;
    3465             : 
    3466          10 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3467             : 
    3468          10 :     for (index = 0; index < arrayP->numProcs; index++)
    3469             :     {
    3470          10 :         int         pgprocno = arrayP->pgprocnos[index];
    3471          10 :         PGPROC     *proc = &allProcs[pgprocno];
    3472             :         VirtualTransactionId procvxid;
    3473             : 
    3474          10 :         GET_VXID_FROM_PGPROC(procvxid, *proc);
    3475             : 
    3476          10 :         if (procvxid.procNumber == vxid.procNumber &&
    3477          10 :             procvxid.localTransactionId == vxid.localTransactionId)
    3478             :         {
    3479          10 :             proc->recoveryConflictPending = conflictPending;
    3480          10 :             pid = proc->pid;
    3481          10 :             if (pid != 0)
    3482             :             {
    3483             :                 /*
    3484             :                  * Kill the pid if it's still here. If not, that's what we
    3485             :                  * wanted so ignore any errors.
    3486             :                  */
    3487          10 :                 (void) SendProcSignal(pid, sigmode, vxid.procNumber);
    3488             :             }
    3489          10 :             break;
    3490             :         }
    3491             :     }
    3492             : 
    3493          10 :     LWLockRelease(ProcArrayLock);
    3494             : 
    3495          10 :     return pid;
    3496             : }
    3497             : 
    3498             : /*
    3499             :  * MinimumActiveBackends --- count backends (other than myself) that are
    3500             :  *      in active transactions.  Return true if the count exceeds the
    3501             :  *      minimum threshold passed.  This is used as a heuristic to decide if
    3502             :  *      a pre-XLOG-flush delay is worthwhile during commit.
    3503             :  *
    3504             :  * Do not count backends that are blocked waiting for locks, since they are
    3505             :  * not going to get to run until someone else commits.
    3506             :  */
    3507             : bool
    3508           0 : MinimumActiveBackends(int min)
    3509             : {
    3510           0 :     ProcArrayStruct *arrayP = procArray;
    3511           0 :     int         count = 0;
    3512             :     int         index;
    3513             : 
    3514             :     /* Quick short-circuit if no minimum is specified */
    3515           0 :     if (min == 0)
    3516           0 :         return true;
    3517             : 
    3518             :     /*
    3519             :      * Note: for speed, we don't acquire ProcArrayLock.  This is a little bit
    3520             :      * bogus, but since we are only testing fields for zero or nonzero, it
    3521             :      * should be OK.  The result is only used for heuristic purposes anyway...
    3522             :      */
    3523           0 :     for (index = 0; index < arrayP->numProcs; index++)
    3524             :     {
    3525           0 :         int         pgprocno = arrayP->pgprocnos[index];
    3526           0 :         PGPROC     *proc = &allProcs[pgprocno];
    3527             : 
    3528             :         /*
    3529             :          * Since we're not holding a lock, need to be prepared to deal with
    3530             :          * garbage, as someone could have incremented numProcs but not yet
    3531             :          * filled the structure.
    3532             :          *
    3533             :          * If someone just decremented numProcs, 'proc' could also point to a
    3534             :          * PGPROC entry that's no longer in the array. It still points to a
    3535             :          * PGPROC struct, though, because freed PGPROC entries just go to the
    3536             :          * free list and are recycled. Its contents are nonsense in that case,
    3537             :          * but that's acceptable for this function.
    3538             :          */
    3539           0 :         if (pgprocno == -1)
    3540           0 :             continue;           /* do not count deleted entries */
    3541           0 :         if (proc == MyProc)
    3542           0 :             continue;           /* do not count myself */
    3543           0 :         if (proc->xid == InvalidTransactionId)
    3544           0 :             continue;           /* do not count if no XID assigned */
    3545           0 :         if (proc->pid == 0)
    3546           0 :             continue;           /* do not count prepared xacts */
    3547           0 :         if (proc->waitLock != NULL)
    3548           0 :             continue;           /* do not count if blocked on a lock */
    3549           0 :         count++;
    3550           0 :         if (count >= min)
    3551           0 :             break;
    3552             :     }
    3553             : 
    3554           0 :     return count >= min;
    3555             : }
    3556             : 
    3557             : /*
    3558             :  * CountDBBackends --- count backends that are using specified database
    3559             :  */
    3560             : int
    3561          30 : CountDBBackends(Oid databaseid)
    3562             : {
    3563          30 :     ProcArrayStruct *arrayP = procArray;
    3564          30 :     int         count = 0;
    3565             :     int         index;
    3566             : 
    3567          30 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3568             : 
    3569          44 :     for (index = 0; index < arrayP->numProcs; index++)
    3570             :     {
    3571          14 :         int         pgprocno = arrayP->pgprocnos[index];
    3572          14 :         PGPROC     *proc = &allProcs[pgprocno];
    3573             : 
    3574          14 :         if (proc->pid == 0)
    3575           0 :             continue;           /* do not count prepared xacts */
    3576          14 :         if (!OidIsValid(databaseid) ||
    3577          14 :             proc->databaseId == databaseid)
    3578           4 :             count++;
    3579             :     }
    3580             : 
    3581          30 :     LWLockRelease(ProcArrayLock);
    3582             : 
    3583          30 :     return count;
    3584             : }
    3585             : 
    3586             : /*
    3587             :  * CountDBConnections --- counts database backends (only regular backends)
    3588             :  */
    3589             : int
    3590           0 : CountDBConnections(Oid databaseid)
    3591             : {
    3592           0 :     ProcArrayStruct *arrayP = procArray;
    3593           0 :     int         count = 0;
    3594             :     int         index;
    3595             : 
    3596           0 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3597             : 
    3598           0 :     for (index = 0; index < arrayP->numProcs; index++)
    3599             :     {
    3600           0 :         int         pgprocno = arrayP->pgprocnos[index];
    3601           0 :         PGPROC     *proc = &allProcs[pgprocno];
    3602             : 
    3603           0 :         if (proc->pid == 0)
    3604           0 :             continue;           /* do not count prepared xacts */
    3605           0 :         if (!proc->isRegularBackend)
    3606           0 :             continue;           /* count only regular backend processes */
    3607           0 :         if (!OidIsValid(databaseid) ||
    3608           0 :             proc->databaseId == databaseid)
    3609           0 :             count++;
    3610             :     }
    3611             : 
    3612           0 :     LWLockRelease(ProcArrayLock);
    3613             : 
    3614           0 :     return count;
    3615             : }
    3616             : 
    3617             : /*
    3618             :  * CancelDBBackends --- cancel backends that are using specified database
    3619             :  */
    3620             : void
    3621          18 : CancelDBBackends(Oid databaseid, ProcSignalReason sigmode, bool conflictPending)
    3622             : {
    3623          18 :     ProcArrayStruct *arrayP = procArray;
    3624             :     int         index;
    3625             : 
    3626             :     /* tell all backends to die */
    3627          18 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    3628             : 
    3629          36 :     for (index = 0; index < arrayP->numProcs; index++)
    3630             :     {
    3631          18 :         int         pgprocno = arrayP->pgprocnos[index];
    3632          18 :         PGPROC     *proc = &allProcs[pgprocno];
    3633             : 
    3634          18 :         if (databaseid == InvalidOid || proc->databaseId == databaseid)
    3635             :         {
    3636             :             VirtualTransactionId procvxid;
    3637             :             pid_t       pid;
    3638             : 
    3639          18 :             GET_VXID_FROM_PGPROC(procvxid, *proc);
    3640             : 
    3641          18 :             proc->recoveryConflictPending = conflictPending;
    3642          18 :             pid = proc->pid;
    3643          18 :             if (pid != 0)
    3644             :             {
    3645             :                 /*
    3646             :                  * Kill the pid if it's still here. If not, that's what we
    3647             :                  * wanted so ignore any errors.
    3648             :                  */
    3649          18 :                 (void) SendProcSignal(pid, sigmode, procvxid.procNumber);
    3650             :             }
    3651             :         }
    3652             :     }
    3653             : 
    3654          18 :     LWLockRelease(ProcArrayLock);
    3655          18 : }
    3656             : 
    3657             : /*
    3658             :  * CountUserBackends --- count backends that are used by specified user
    3659             :  * (only regular backends, not any type of background worker)
    3660             :  */
    3661             : int
    3662           0 : CountUserBackends(Oid roleid)
    3663             : {
    3664           0 :     ProcArrayStruct *arrayP = procArray;
    3665           0 :     int         count = 0;
    3666             :     int         index;
    3667             : 
    3668           0 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3669             : 
    3670           0 :     for (index = 0; index < arrayP->numProcs; index++)
    3671             :     {
    3672           0 :         int         pgprocno = arrayP->pgprocnos[index];
    3673           0 :         PGPROC     *proc = &allProcs[pgprocno];
    3674             : 
    3675           0 :         if (proc->pid == 0)
    3676           0 :             continue;           /* do not count prepared xacts */
    3677           0 :         if (!proc->isRegularBackend)
    3678           0 :             continue;           /* count only regular backend processes */
    3679           0 :         if (proc->roleId == roleid)
    3680           0 :             count++;
    3681             :     }
    3682             : 
    3683           0 :     LWLockRelease(ProcArrayLock);
    3684             : 
    3685           0 :     return count;
    3686             : }
    3687             : 
    3688             : /*
    3689             :  * CountOtherDBBackends -- check for other backends running in the given DB
    3690             :  *
    3691             :  * If there are other backends in the DB, we will wait a maximum of 5 seconds
    3692             :  * for them to exit.  Autovacuum backends are encouraged to exit early by
    3693             :  * sending them SIGTERM, but normal user backends are just waited for.
    3694             :  *
    3695             :  * The current backend is always ignored; it is caller's responsibility to
    3696             :  * check whether the current backend uses the given DB, if it's important.
    3697             :  *
    3698             :  * Returns true if there are (still) other backends in the DB, false if not.
    3699             :  * Also, *nbackends and *nprepared are set to the number of other backends
    3700             :  * and prepared transactions in the DB, respectively.
    3701             :  *
    3702             :  * This function is used to interlock DROP DATABASE and related commands
    3703             :  * against there being any active backends in the target DB --- dropping the
    3704             :  * DB while active backends remain would be a Bad Thing.  Note that we cannot
    3705             :  * detect here the possibility of a newly-started backend that is trying to
    3706             :  * connect to the doomed database, so additional interlocking is needed during
    3707             :  * backend startup.  The caller should normally hold an exclusive lock on the
    3708             :  * target DB before calling this, which is one reason we mustn't wait
    3709             :  * indefinitely.
    3710             :  */
    3711             : bool
    3712         866 : CountOtherDBBackends(Oid databaseId, int *nbackends, int *nprepared)
    3713             : {
    3714         866 :     ProcArrayStruct *arrayP = procArray;
    3715             : 
    3716             : #define MAXAUTOVACPIDS  10      /* max autovacs to SIGTERM per iteration */
    3717             :     int         autovac_pids[MAXAUTOVACPIDS];
    3718             :     int         tries;
    3719             : 
    3720             :     /* 50 tries with 100ms sleep between tries makes 5 sec total wait */
    3721         866 :     for (tries = 0; tries < 50; tries++)
    3722             :     {
    3723         866 :         int         nautovacs = 0;
    3724         866 :         bool        found = false;
    3725             :         int         index;
    3726             : 
    3727         866 :         CHECK_FOR_INTERRUPTS();
    3728             : 
    3729         866 :         *nbackends = *nprepared = 0;
    3730             : 
    3731         866 :         LWLockAcquire(ProcArrayLock, LW_SHARED);
    3732             : 
    3733        3166 :         for (index = 0; index < arrayP->numProcs; index++)
    3734             :         {
    3735        2300 :             int         pgprocno = arrayP->pgprocnos[index];
    3736        2300 :             PGPROC     *proc = &allProcs[pgprocno];
    3737        2300 :             uint8       statusFlags = ProcGlobal->statusFlags[index];
    3738             : 
    3739        2300 :             if (proc->databaseId != databaseId)
    3740        2104 :                 continue;
    3741         196 :             if (proc == MyProc)
    3742         196 :                 continue;
    3743             : 
    3744           0 :             found = true;
    3745             : 
    3746           0 :             if (proc->pid == 0)
    3747           0 :                 (*nprepared)++;
    3748             :             else
    3749             :             {
    3750           0 :                 (*nbackends)++;
    3751           0 :                 if ((statusFlags & PROC_IS_AUTOVACUUM) &&
    3752             :                     nautovacs < MAXAUTOVACPIDS)
    3753           0 :                     autovac_pids[nautovacs++] = proc->pid;
    3754             :             }
    3755             :         }
    3756             : 
    3757         866 :         LWLockRelease(ProcArrayLock);
    3758             : 
    3759         866 :         if (!found)
    3760         866 :             return false;       /* no conflicting backends, so done */
    3761             : 
    3762             :         /*
    3763             :          * Send SIGTERM to any conflicting autovacuums before sleeping. We
    3764             :          * postpone this step until after the loop because we don't want to
    3765             :          * hold ProcArrayLock while issuing kill(). We have no idea what might
    3766             :          * block kill() inside the kernel...
    3767             :          */
    3768           0 :         for (index = 0; index < nautovacs; index++)
    3769           0 :             (void) kill(autovac_pids[index], SIGTERM);  /* ignore any error */
    3770             : 
    3771             :         /* sleep, then try again */
    3772           0 :         pg_usleep(100 * 1000L); /* 100ms */
    3773             :     }
    3774             : 
    3775           0 :     return true;                /* timed out, still conflicts */
    3776             : }
    3777             : 
    3778             : /*
    3779             :  * Terminate existing connections to the specified database. This routine
    3780             :  * is used by the DROP DATABASE command when user has asked to forcefully
    3781             :  * drop the database.
    3782             :  *
    3783             :  * The current backend is always ignored; it is caller's responsibility to
    3784             :  * check whether the current backend uses the given DB, if it's important.
    3785             :  *
    3786             :  * If the target database has a prepared transaction or permissions checks
    3787             :  * fail for a connection, this fails without terminating anything.
    3788             :  */
    3789             : void
    3790           2 : TerminateOtherDBBackends(Oid databaseId)
    3791             : {
    3792           2 :     ProcArrayStruct *arrayP = procArray;
    3793           2 :     List       *pids = NIL;
    3794           2 :     int         nprepared = 0;
    3795             :     int         i;
    3796             : 
    3797           2 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3798             : 
    3799           8 :     for (i = 0; i < procArray->numProcs; i++)
    3800             :     {
    3801           6 :         int         pgprocno = arrayP->pgprocnos[i];
    3802           6 :         PGPROC     *proc = &allProcs[pgprocno];
    3803             : 
    3804           6 :         if (proc->databaseId != databaseId)
    3805           6 :             continue;
    3806           0 :         if (proc == MyProc)
    3807           0 :             continue;
    3808             : 
    3809           0 :         if (proc->pid != 0)
    3810           0 :             pids = lappend_int(pids, proc->pid);
    3811             :         else
    3812           0 :             nprepared++;
    3813             :     }
    3814             : 
    3815           2 :     LWLockRelease(ProcArrayLock);
    3816             : 
    3817           2 :     if (nprepared > 0)
    3818           0 :         ereport(ERROR,
    3819             :                 (errcode(ERRCODE_OBJECT_IN_USE),
    3820             :                  errmsg("database \"%s\" is being used by prepared transactions",
    3821             :                         get_database_name(databaseId)),
    3822             :                  errdetail_plural("There is %d prepared transaction using the database.",
    3823             :                                   "There are %d prepared transactions using the database.",
    3824             :                                   nprepared,
    3825             :                                   nprepared)));
    3826             : 
    3827           2 :     if (pids)
    3828             :     {
    3829             :         ListCell   *lc;
    3830             : 
    3831             :         /*
    3832             :          * Permissions checks relax the pg_terminate_backend checks in two
    3833             :          * ways, both by omitting the !OidIsValid(proc->roleId) check:
    3834             :          *
    3835             :          * - Accept terminating autovacuum workers, since DROP DATABASE
    3836             :          * without FORCE terminates them.
    3837             :          *
    3838             :          * - Accept terminating bgworkers.  For bgworker authors, it's
    3839             :          * convenient to be able to recommend FORCE if a worker is blocking
    3840             :          * DROP DATABASE unexpectedly.
    3841             :          *
    3842             :          * Unlike pg_terminate_backend, we don't raise some warnings - like
    3843             :          * "PID %d is not a PostgreSQL server process", because for us already
    3844             :          * finished session is not a problem.
    3845             :          */
    3846           0 :         foreach(lc, pids)
    3847             :         {
    3848           0 :             int         pid = lfirst_int(lc);
    3849           0 :             PGPROC     *proc = BackendPidGetProc(pid);
    3850             : 
    3851           0 :             if (proc != NULL)
    3852             :             {
    3853           0 :                 if (superuser_arg(proc->roleId) && !superuser())
    3854           0 :                     ereport(ERROR,
    3855             :                             (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
    3856             :                              errmsg("permission denied to terminate process"),
    3857             :                              errdetail("Only roles with the %s attribute may terminate processes of roles with the %s attribute.",
    3858             :                                        "SUPERUSER", "SUPERUSER")));
    3859             : 
    3860           0 :                 if (!has_privs_of_role(GetUserId(), proc->roleId) &&
    3861           0 :                     !has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_BACKEND))
    3862           0 :                     ereport(ERROR,
    3863             :                             (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
    3864             :                              errmsg("permission denied to terminate process"),
    3865             :                              errdetail("Only roles with privileges of the role whose process is being terminated or with privileges of the \"%s\" role may terminate this process.",
    3866             :                                        "pg_signal_backend")));
    3867             :             }
    3868             :         }
    3869             : 
    3870             :         /*
    3871             :          * There's a race condition here: once we release the ProcArrayLock,
    3872             :          * it's possible for the session to exit before we issue kill.  That
    3873             :          * race condition possibility seems too unlikely to worry about.  See
    3874             :          * pg_signal_backend.
    3875             :          */
    3876           0 :         foreach(lc, pids)
    3877             :         {
    3878           0 :             int         pid = lfirst_int(lc);
    3879           0 :             PGPROC     *proc = BackendPidGetProc(pid);
    3880             : 
    3881           0 :             if (proc != NULL)
    3882             :             {
    3883             :                 /*
    3884             :                  * If we have setsid(), signal the backend's whole process
    3885             :                  * group
    3886             :                  */
    3887             : #ifdef HAVE_SETSID
    3888           0 :                 (void) kill(-pid, SIGTERM);
    3889             : #else
    3890             :                 (void) kill(pid, SIGTERM);
    3891             : #endif
    3892             :             }
    3893             :         }
    3894             :     }
    3895           2 : }
    3896             : 
    3897             : /*
    3898             :  * ProcArraySetReplicationSlotXmin
    3899             :  *
    3900             :  * Install limits to future computations of the xmin horizon to prevent vacuum
    3901             :  * and HOT pruning from removing affected rows still needed by clients with
    3902             :  * replication slots.
    3903             :  */
    3904             : void
    3905        4588 : ProcArraySetReplicationSlotXmin(TransactionId xmin, TransactionId catalog_xmin,
    3906             :                                 bool already_locked)
    3907             : {
    3908             :     Assert(!already_locked || LWLockHeldByMe(ProcArrayLock));
    3909             : 
    3910        4588 :     if (!already_locked)
    3911        3664 :         LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    3912             : 
    3913        4588 :     procArray->replication_slot_xmin = xmin;
    3914        4588 :     procArray->replication_slot_catalog_xmin = catalog_xmin;
    3915             : 
    3916        4588 :     if (!already_locked)
    3917        3664 :         LWLockRelease(ProcArrayLock);
    3918             : 
    3919        4588 :     elog(DEBUG1, "xmin required by slots: data %u, catalog %u",
    3920             :          xmin, catalog_xmin);
    3921        4588 : }
    3922             : 
    3923             : /*
    3924             :  * ProcArrayGetReplicationSlotXmin
    3925             :  *
    3926             :  * Return the current slot xmin limits. That's useful to be able to remove
    3927             :  * data that's older than those limits.
    3928             :  */
    3929             : void
    3930          44 : ProcArrayGetReplicationSlotXmin(TransactionId *xmin,
    3931             :                                 TransactionId *catalog_xmin)
    3932             : {
    3933          44 :     LWLockAcquire(ProcArrayLock, LW_SHARED);
    3934             : 
    3935          44 :     if (xmin != NULL)
    3936           0 :         *xmin = procArray->replication_slot_xmin;
    3937             : 
    3938          44 :     if (catalog_xmin != NULL)
    3939          44 :         *catalog_xmin = procArray->replication_slot_catalog_xmin;
    3940             : 
    3941          44 :     LWLockRelease(ProcArrayLock);
    3942          44 : }
    3943             : 
    3944             : /*
    3945             :  * XidCacheRemoveRunningXids
    3946             :  *
    3947             :  * Remove a bunch of TransactionIds from the list of known-running
    3948             :  * subtransactions for my backend.  Both the specified xid and those in
    3949             :  * the xids[] array (of length nxids) are removed from the subxids cache.
    3950             :  * latestXid must be the latest XID among the group.
    3951             :  */
    3952             : void
    3953        1298 : XidCacheRemoveRunningXids(TransactionId xid,
    3954             :                           int nxids, const TransactionId *xids,
    3955             :                           TransactionId latestXid)
    3956             : {
    3957             :     int         i,
    3958             :                 j;
    3959             :     XidCacheStatus *mysubxidstat;
    3960             : 
    3961             :     Assert(TransactionIdIsValid(xid));
    3962             : 
    3963             :     /*
    3964             :      * We must hold ProcArrayLock exclusively in order to remove transactions
    3965             :      * from the PGPROC array.  (See src/backend/access/transam/README.)  It's
    3966             :      * possible this could be relaxed since we know this routine is only used
    3967             :      * to abort subtransactions, but pending closer analysis we'd best be
    3968             :      * conservative.
    3969             :      *
    3970             :      * Note that we do not have to be careful about memory ordering of our own
    3971             :      * reads wrt. GetNewTransactionId() here - only this process can modify
    3972             :      * relevant fields of MyProc/ProcGlobal->xids[].  But we do have to be
    3973             :      * careful about our own writes being well ordered.
    3974             :      */
    3975        1298 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    3976             : 
    3977        1298 :     mysubxidstat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
    3978             : 
    3979             :     /*
    3980             :      * Under normal circumstances xid and xids[] will be in increasing order,
    3981             :      * as will be the entries in subxids.  Scan backwards to avoid O(N^2)
    3982             :      * behavior when removing a lot of xids.
    3983             :      */
    3984        1358 :     for (i = nxids - 1; i >= 0; i--)
    3985             :     {
    3986          60 :         TransactionId anxid = xids[i];
    3987             : 
    3988          60 :         for (j = MyProc->subxidStatus.count - 1; j >= 0; j--)
    3989             :         {
    3990          60 :             if (TransactionIdEquals(MyProc->subxids.xids[j], anxid))
    3991             :             {
    3992          60 :                 MyProc->subxids.xids[j] = MyProc->subxids.xids[MyProc->subxidStatus.count - 1];
    3993          60 :                 pg_write_barrier();
    3994          60 :                 mysubxidstat->count--;
    3995          60 :                 MyProc->subxidStatus.count--;
    3996          60 :                 break;
    3997             :             }
    3998             :         }
    3999             : 
    4000             :         /*
    4001             :          * Ordinarily we should have found it, unless the cache has
    4002             :          * overflowed. However it's also possible for this routine to be
    4003             :          * invoked multiple times for the same subtransaction, in case of an
    4004             :          * error during AbortSubTransaction.  So instead of Assert, emit a
    4005             :          * debug warning.
    4006             :          */
    4007          60 :         if (j < 0 && !MyProc->subxidStatus.overflowed)
    4008           0 :             elog(WARNING, "did not find subXID %u in MyProc", anxid);
    4009             :     }
    4010             : 
    4011        1426 :     for (j = MyProc->subxidStatus.count - 1; j >= 0; j--)
    4012             :     {
    4013        1424 :         if (TransactionIdEquals(MyProc->subxids.xids[j], xid))
    4014             :         {
    4015        1296 :             MyProc->subxids.xids[j] = MyProc->subxids.xids[MyProc->subxidStatus.count - 1];
    4016        1296 :             pg_write_barrier();
    4017        1296 :             mysubxidstat->count--;
    4018        1296 :             MyProc->subxidStatus.count--;
    4019        1296 :             break;
    4020             :         }
    4021             :     }
    4022             :     /* Ordinarily we should have found it, unless the cache has overflowed */
    4023        1298 :     if (j < 0 && !MyProc->subxidStatus.overflowed)
    4024           0 :         elog(WARNING, "did not find subXID %u in MyProc", xid);
    4025             : 
    4026             :     /* Also advance global latestCompletedXid while holding the lock */
    4027        1298 :     MaintainLatestCompletedXid(latestXid);
    4028             : 
    4029             :     /* ... and xactCompletionCount */
    4030        1298 :     TransamVariables->xactCompletionCount++;
    4031             : 
    4032        1298 :     LWLockRelease(ProcArrayLock);
    4033        1298 : }
    4034             : 
    4035             : #ifdef XIDCACHE_DEBUG
    4036             : 
    4037             : /*
    4038             :  * Print stats about effectiveness of XID cache
    4039             :  */
    4040             : static void
    4041             : DisplayXidCache(void)
    4042             : {
    4043             :     fprintf(stderr,
    4044             :             "XidCache: xmin: %ld, known: %ld, myxact: %ld, latest: %ld, mainxid: %ld, childxid: %ld, knownassigned: %ld, nooflo: %ld, slow: %ld\n",
    4045             :             xc_by_recent_xmin,
    4046             :             xc_by_known_xact,
    4047             :             xc_by_my_xact,
    4048             :             xc_by_latest_xid,
    4049             :             xc_by_main_xid,
    4050             :             xc_by_child_xid,
    4051             :             xc_by_known_assigned,
    4052             :             xc_no_overflow,
    4053             :             xc_slow_answer);
    4054             : }
    4055             : #endif                          /* XIDCACHE_DEBUG */
    4056             : 
    4057             : /*
    4058             :  * If rel != NULL, return test state appropriate for relation, otherwise
    4059             :  * return state usable for all relations.  The latter may consider XIDs as
    4060             :  * not-yet-visible-to-everyone that a state for a specific relation would
    4061             :  * already consider visible-to-everyone.
    4062             :  *
    4063             :  * This needs to be called while a snapshot is active or registered, otherwise
    4064             :  * there are wraparound and other dangers.
    4065             :  *
    4066             :  * See comment for GlobalVisState for details.
    4067             :  */
    4068             : GlobalVisState *
    4069    28569152 : GlobalVisTestFor(Relation rel)
    4070             : {
    4071    28569152 :     GlobalVisState *state = NULL;
    4072             : 
    4073             :     /* XXX: we should assert that a snapshot is pushed or registered */
    4074             :     Assert(RecentXmin);
    4075             : 
    4076    28569152 :     switch (GlobalVisHorizonKindForRel(rel))
    4077             :     {
    4078      176236 :         case VISHORIZON_SHARED:
    4079      176236 :             state = &GlobalVisSharedRels;
    4080      176236 :             break;
    4081     5708342 :         case VISHORIZON_CATALOG:
    4082     5708342 :             state = &GlobalVisCatalogRels;
    4083     5708342 :             break;
    4084    22600686 :         case VISHORIZON_DATA:
    4085    22600686 :             state = &GlobalVisDataRels;
    4086    22600686 :             break;
    4087       83888 :         case VISHORIZON_TEMP:
    4088       83888 :             state = &GlobalVisTempRels;
    4089       83888 :             break;
    4090             :     }
    4091             : 
    4092             :     Assert(FullTransactionIdIsValid(state->definitely_needed) &&
    4093             :            FullTransactionIdIsValid(state->maybe_needed));
    4094             : 
    4095    28569152 :     return state;
    4096             : }
    4097             : 
    4098             : /*
    4099             :  * Return true if it's worth updating the accurate maybe_needed boundary.
    4100             :  *
    4101             :  * As it is somewhat expensive to determine xmin horizons, we don't want to
    4102             :  * repeatedly do so when there is a low likelihood of it being beneficial.
    4103             :  *
    4104             :  * The current heuristic is that we update only if RecentXmin has changed
    4105             :  * since the last update. If the oldest currently running transaction has not
    4106             :  * finished, it is unlikely that recomputing the horizon would be useful.
    4107             :  */
    4108             : static bool
    4109      821246 : GlobalVisTestShouldUpdate(GlobalVisState *state)
    4110             : {
    4111             :     /* hasn't been updated yet */
    4112      821246 :     if (!TransactionIdIsValid(ComputeXidHorizonsResultLastXmin))
    4113       16958 :         return true;
    4114             : 
    4115             :     /*
    4116             :      * If the maybe_needed/definitely_needed boundaries are the same, it's
    4117             :      * unlikely to be beneficial to refresh boundaries.
    4118             :      */
    4119      804288 :     if (FullTransactionIdFollowsOrEquals(state->maybe_needed,
    4120             :                                          state->definitely_needed))
    4121           0 :         return false;
    4122             : 
    4123             :     /* does the last snapshot built have a different xmin? */
    4124      804288 :     return RecentXmin != ComputeXidHorizonsResultLastXmin;
    4125             : }
    4126             : 
    4127             : static void
    4128      420090 : GlobalVisUpdateApply(ComputeXidHorizonsResult *horizons)
    4129             : {
    4130             :     GlobalVisSharedRels.maybe_needed =
    4131      420090 :         FullXidRelativeTo(horizons->latest_completed,
    4132             :                           horizons->shared_oldest_nonremovable);
    4133             :     GlobalVisCatalogRels.maybe_needed =
    4134      420090 :         FullXidRelativeTo(horizons->latest_completed,
    4135             :                           horizons->catalog_oldest_nonremovable);
    4136             :     GlobalVisDataRels.maybe_needed =
    4137      420090 :         FullXidRelativeTo(horizons->latest_completed,
    4138             :                           horizons->data_oldest_nonremovable);
    4139             :     GlobalVisTempRels.maybe_needed =
    4140      420090 :         FullXidRelativeTo(horizons->latest_completed,
    4141             :                           horizons->temp_oldest_nonremovable);
    4142             : 
    4143             :     /*
    4144             :      * In longer running transactions it's possible that transactions we
    4145             :      * previously needed to treat as running aren't around anymore. So update
    4146             :      * definitely_needed to not be earlier than maybe_needed.
    4147             :      */
    4148             :     GlobalVisSharedRels.definitely_needed =
    4149      420090 :         FullTransactionIdNewer(GlobalVisSharedRels.maybe_needed,
    4150             :                                GlobalVisSharedRels.definitely_needed);
    4151             :     GlobalVisCatalogRels.definitely_needed =
    4152      420090 :         FullTransactionIdNewer(GlobalVisCatalogRels.maybe_needed,
    4153             :                                GlobalVisCatalogRels.definitely_needed);
    4154             :     GlobalVisDataRels.definitely_needed =
    4155      420090 :         FullTransactionIdNewer(GlobalVisDataRels.maybe_needed,
    4156             :                                GlobalVisDataRels.definitely_needed);
    4157      420090 :     GlobalVisTempRels.definitely_needed = GlobalVisTempRels.maybe_needed;
    4158             : 
    4159      420090 :     ComputeXidHorizonsResultLastXmin = RecentXmin;
    4160      420090 : }
    4161             : 
    4162             : /*
    4163             :  * Update boundaries in GlobalVis{Shared,Catalog, Data}Rels
    4164             :  * using ComputeXidHorizons().
    4165             :  */
    4166             : static void
    4167      110496 : GlobalVisUpdate(void)
    4168             : {
    4169             :     ComputeXidHorizonsResult horizons;
    4170             : 
    4171             :     /* updates the horizons as a side-effect */
    4172      110496 :     ComputeXidHorizons(&horizons);
    4173      110496 : }
    4174             : 
    4175             : /*
    4176             :  * Return true if no snapshot still considers fxid to be running.
    4177             :  *
    4178             :  * The state passed needs to have been initialized for the relation fxid is
    4179             :  * from (NULL is also OK), otherwise the result may not be correct.
    4180             :  *
    4181             :  * See comment for GlobalVisState for details.
    4182             :  */
    4183             : bool
    4184    19080788 : GlobalVisTestIsRemovableFullXid(GlobalVisState *state,
    4185             :                                 FullTransactionId fxid)
    4186             : {
    4187             :     /*
    4188             :      * If fxid is older than maybe_needed bound, it definitely is visible to
    4189             :      * everyone.
    4190             :      */
    4191    19080788 :     if (FullTransactionIdPrecedes(fxid, state->maybe_needed))
    4192     4840712 :         return true;
    4193             : 
    4194             :     /*
    4195             :      * If fxid is >= definitely_needed bound, it is very likely to still be
    4196             :      * considered running.
    4197             :      */
    4198    14240076 :     if (FullTransactionIdFollowsOrEquals(fxid, state->definitely_needed))
    4199    13418830 :         return false;
    4200             : 
    4201             :     /*
    4202             :      * fxid is between maybe_needed and definitely_needed, i.e. there might or
    4203             :      * might not exist a snapshot considering fxid running. If it makes sense,
    4204             :      * update boundaries and recheck.
    4205             :      */
    4206      821246 :     if (GlobalVisTestShouldUpdate(state))
    4207             :     {
    4208      110496 :         GlobalVisUpdate();
    4209             : 
    4210             :         Assert(FullTransactionIdPrecedes(fxid, state->definitely_needed));
    4211             : 
    4212      110496 :         return FullTransactionIdPrecedes(fxid, state->maybe_needed);
    4213             :     }
    4214             :     else
    4215      710750 :         return false;
    4216             : }
    4217             : 
    4218             : /*
    4219             :  * Wrapper around GlobalVisTestIsRemovableFullXid() for 32bit xids.
    4220             :  *
    4221             :  * It is crucial that this only gets called for xids from a source that
    4222             :  * protects against xid wraparounds (e.g. from a table and thus protected by
    4223             :  * relfrozenxid).
    4224             :  */
    4225             : bool
    4226    19080026 : GlobalVisTestIsRemovableXid(GlobalVisState *state, TransactionId xid)
    4227             : {
    4228             :     FullTransactionId fxid;
    4229             : 
    4230             :     /*
    4231             :      * Convert 32 bit argument to FullTransactionId. We can do so safely
    4232             :      * because we know the xid has to, at the very least, be between
    4233             :      * [oldestXid, nextXid), i.e. within 2 billion of xid. To avoid taking a
    4234             :      * lock to determine either, we can just compare with
    4235             :      * state->definitely_needed, which was based on those value at the time
    4236             :      * the current snapshot was built.
    4237             :      */
    4238    19080026 :     fxid = FullXidRelativeTo(state->definitely_needed, xid);
    4239             : 
    4240    19080026 :     return GlobalVisTestIsRemovableFullXid(state, fxid);
    4241             : }
    4242             : 
    4243             : /*
    4244             :  * Convenience wrapper around GlobalVisTestFor() and
    4245             :  * GlobalVisTestIsRemovableFullXid(), see their comments.
    4246             :  */
    4247             : bool
    4248         762 : GlobalVisCheckRemovableFullXid(Relation rel, FullTransactionId fxid)
    4249             : {
    4250             :     GlobalVisState *state;
    4251             : 
    4252         762 :     state = GlobalVisTestFor(rel);
    4253             : 
    4254         762 :     return GlobalVisTestIsRemovableFullXid(state, fxid);
    4255             : }
    4256             : 
    4257             : /*
    4258             :  * Convenience wrapper around GlobalVisTestFor() and
    4259             :  * GlobalVisTestIsRemovableXid(), see their comments.
    4260             :  */
    4261             : bool
    4262          12 : GlobalVisCheckRemovableXid(Relation rel, TransactionId xid)
    4263             : {
    4264             :     GlobalVisState *state;
    4265             : 
    4266          12 :     state = GlobalVisTestFor(rel);
    4267             : 
    4268          12 :     return GlobalVisTestIsRemovableXid(state, xid);
    4269             : }
    4270             : 
    4271             : /*
    4272             :  * Convert a 32 bit transaction id into 64 bit transaction id, by assuming it
    4273             :  * is within MaxTransactionId / 2 of XidFromFullTransactionId(rel).
    4274             :  *
    4275             :  * Be very careful about when to use this function. It can only safely be used
    4276             :  * when there is a guarantee that xid is within MaxTransactionId / 2 xids of
    4277             :  * rel. That e.g. can be guaranteed if the caller assures a snapshot is
    4278             :  * held by the backend and xid is from a table (where vacuum/freezing ensures
    4279             :  * the xid has to be within that range), or if xid is from the procarray and
    4280             :  * prevents xid wraparound that way.
    4281             :  */
    4282             : static inline FullTransactionId
    4283    23424148 : FullXidRelativeTo(FullTransactionId rel, TransactionId xid)
    4284             : {
    4285    23424148 :     TransactionId rel_xid = XidFromFullTransactionId(rel);
    4286             : 
    4287             :     Assert(TransactionIdIsValid(xid));
    4288             :     Assert(TransactionIdIsValid(rel_xid));
    4289             : 
    4290             :     /* not guaranteed to find issues, but likely to catch mistakes */
    4291             :     AssertTransactionIdInAllowableRange(xid);
    4292             : 
    4293    46848296 :     return FullTransactionIdFromU64(U64FromFullTransactionId(rel)
    4294    23424148 :                                     + (int32) (xid - rel_xid));
    4295             : }
    4296             : 
    4297             : 
    4298             : /* ----------------------------------------------
    4299             :  *      KnownAssignedTransactionIds sub-module
    4300             :  * ----------------------------------------------
    4301             :  */
    4302             : 
    4303             : /*
    4304             :  * In Hot Standby mode, we maintain a list of transactions that are (or were)
    4305             :  * running on the primary at the current point in WAL.  These XIDs must be
    4306             :  * treated as running by standby transactions, even though they are not in
    4307             :  * the standby server's PGPROC array.
    4308             :  *
    4309             :  * We record all XIDs that we know have been assigned.  That includes all the
    4310             :  * XIDs seen in WAL records, plus all unobserved XIDs that we can deduce have
    4311             :  * been assigned.  We can deduce the existence of unobserved XIDs because we
    4312             :  * know XIDs are assigned in sequence, with no gaps.  The KnownAssignedXids
    4313             :  * list expands as new XIDs are observed or inferred, and contracts when
    4314             :  * transaction completion records arrive.
    4315             :  *
    4316             :  * During hot standby we do not fret too much about the distinction between
    4317             :  * top-level XIDs and subtransaction XIDs. We store both together in the
    4318             :  * KnownAssignedXids list.  In backends, this is copied into snapshots in
    4319             :  * GetSnapshotData(), taking advantage of the fact that XidInMVCCSnapshot()
    4320             :  * doesn't care about the distinction either.  Subtransaction XIDs are
    4321             :  * effectively treated as top-level XIDs and in the typical case pg_subtrans
    4322             :  * links are *not* maintained (which does not affect visibility).
    4323             :  *
    4324             :  * We have room in KnownAssignedXids and in snapshots to hold maxProcs *
    4325             :  * (1 + PGPROC_MAX_CACHED_SUBXIDS) XIDs, so every primary transaction must
    4326             :  * report its subtransaction XIDs in a WAL XLOG_XACT_ASSIGNMENT record at
    4327             :  * least every PGPROC_MAX_CACHED_SUBXIDS.  When we receive one of these
    4328             :  * records, we mark the subXIDs as children of the top XID in pg_subtrans,
    4329             :  * and then remove them from KnownAssignedXids.  This prevents overflow of
    4330             :  * KnownAssignedXids and snapshots, at the cost that status checks for these
    4331             :  * subXIDs will take a slower path through TransactionIdIsInProgress().
    4332             :  * This means that KnownAssignedXids is not necessarily complete for subXIDs,
    4333             :  * though it should be complete for top-level XIDs; this is the same situation
    4334             :  * that holds with respect to the PGPROC entries in normal running.
    4335             :  *
    4336             :  * When we throw away subXIDs from KnownAssignedXids, we need to keep track of
    4337             :  * that, similarly to tracking overflow of a PGPROC's subxids array.  We do
    4338             :  * that by remembering the lastOverflowedXid, ie the last thrown-away subXID.
    4339             :  * As long as that is within the range of interesting XIDs, we have to assume
    4340             :  * that subXIDs are missing from snapshots.  (Note that subXID overflow occurs
    4341             :  * on primary when 65th subXID arrives, whereas on standby it occurs when 64th
    4342             :  * subXID arrives - that is not an error.)
    4343             :  *
    4344             :  * Should a backend on primary somehow disappear before it can write an abort
    4345             :  * record, then we just leave those XIDs in KnownAssignedXids. They actually
    4346             :  * aborted but we think they were running; the distinction is irrelevant
    4347             :  * because either way any changes done by the transaction are not visible to
    4348             :  * backends in the standby.  We prune KnownAssignedXids when
    4349             :  * XLOG_RUNNING_XACTS arrives, to forestall possible overflow of the
    4350             :  * array due to such dead XIDs.
    4351             :  */
    4352             : 
    4353             : /*
    4354             :  * RecordKnownAssignedTransactionIds
    4355             :  *      Record the given XID in KnownAssignedXids, as well as any preceding
    4356             :  *      unobserved XIDs.
    4357             :  *
    4358             :  * RecordKnownAssignedTransactionIds() should be run for *every* WAL record
    4359             :  * associated with a transaction. Must be called for each record after we
    4360             :  * have executed StartupCLOG() et al, since we must ExtendCLOG() etc..
    4361             :  *
    4362             :  * Called during recovery in analogy with and in place of GetNewTransactionId()
    4363             :  */
    4364             : void
    4365     4922904 : RecordKnownAssignedTransactionIds(TransactionId xid)
    4366             : {
    4367             :     Assert(standbyState >= STANDBY_INITIALIZED);
    4368             :     Assert(TransactionIdIsValid(xid));
    4369             :     Assert(TransactionIdIsValid(latestObservedXid));
    4370             : 
    4371     4922904 :     elog(DEBUG4, "record known xact %u latestObservedXid %u",
    4372             :          xid, latestObservedXid);
    4373             : 
    4374             :     /*
    4375             :      * When a newly observed xid arrives, it is frequently the case that it is
    4376             :      * *not* the next xid in sequence. When this occurs, we must treat the
    4377             :      * intervening xids as running also.
    4378             :      */
    4379     4922904 :     if (TransactionIdFollows(xid, latestObservedXid))
    4380             :     {
    4381             :         TransactionId next_expected_xid;
    4382             : 
    4383             :         /*
    4384             :          * Extend subtrans like we do in GetNewTransactionId() during normal
    4385             :          * operation using individual extend steps. Note that we do not need
    4386             :          * to extend clog since its extensions are WAL logged.
    4387             :          *
    4388             :          * This part has to be done regardless of standbyState since we
    4389             :          * immediately start assigning subtransactions to their toplevel
    4390             :          * transactions.
    4391             :          */
    4392       44128 :         next_expected_xid = latestObservedXid;
    4393       89682 :         while (TransactionIdPrecedes(next_expected_xid, xid))
    4394             :         {
    4395       45554 :             TransactionIdAdvance(next_expected_xid);
    4396       45554 :             ExtendSUBTRANS(next_expected_xid);
    4397             :         }
    4398             :         Assert(next_expected_xid == xid);
    4399             : 
    4400             :         /*
    4401             :          * If the KnownAssignedXids machinery isn't up yet, there's nothing
    4402             :          * more to do since we don't track assigned xids yet.
    4403             :          */
    4404       44128 :         if (standbyState <= STANDBY_INITIALIZED)
    4405             :         {
    4406           0 :             latestObservedXid = xid;
    4407           0 :             return;
    4408             :         }
    4409             : 
    4410             :         /*
    4411             :          * Add (latestObservedXid, xid] onto the KnownAssignedXids array.
    4412             :          */
    4413       44128 :         next_expected_xid = latestObservedXid;
    4414       44128 :         TransactionIdAdvance(next_expected_xid);
    4415       44128 :         KnownAssignedXidsAdd(next_expected_xid, xid, false);
    4416             : 
    4417             :         /*
    4418             :          * Now we can advance latestObservedXid
    4419             :          */
    4420       44128 :         latestObservedXid = xid;
    4421             : 
    4422             :         /* TransamVariables->nextXid must be beyond any observed xid */
    4423       44128 :         AdvanceNextFullTransactionIdPastXid(latestObservedXid);
    4424             :     }
    4425             : }
    4426             : 
    4427             : /*
    4428             :  * ExpireTreeKnownAssignedTransactionIds
    4429             :  *      Remove the given XIDs from KnownAssignedXids.
    4430             :  *
    4431             :  * Called during recovery in analogy with and in place of ProcArrayEndTransaction()
    4432             :  */
    4433             : void
    4434       42490 : ExpireTreeKnownAssignedTransactionIds(TransactionId xid, int nsubxids,
    4435             :                                       TransactionId *subxids, TransactionId max_xid)
    4436             : {
    4437             :     Assert(standbyState >= STANDBY_INITIALIZED);
    4438             : 
    4439             :     /*
    4440             :      * Uses same locking as transaction commit
    4441             :      */
    4442       42490 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    4443             : 
    4444       42490 :     KnownAssignedXidsRemoveTree(xid, nsubxids, subxids);
    4445             : 
    4446             :     /* As in ProcArrayEndTransaction, advance latestCompletedXid */
    4447       42490 :     MaintainLatestCompletedXidRecovery(max_xid);
    4448             : 
    4449             :     /* ... and xactCompletionCount */
    4450       42490 :     TransamVariables->xactCompletionCount++;
    4451             : 
    4452       42490 :     LWLockRelease(ProcArrayLock);
    4453       42490 : }
    4454             : 
    4455             : /*
    4456             :  * ExpireAllKnownAssignedTransactionIds
    4457             :  *      Remove all entries in KnownAssignedXids and reset lastOverflowedXid.
    4458             :  */
    4459             : void
    4460         212 : ExpireAllKnownAssignedTransactionIds(void)
    4461             : {
    4462             :     FullTransactionId latestXid;
    4463             : 
    4464         212 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    4465         212 :     KnownAssignedXidsRemovePreceding(InvalidTransactionId);
    4466             : 
    4467             :     /* Reset latestCompletedXid to nextXid - 1 */
    4468             :     Assert(FullTransactionIdIsValid(TransamVariables->nextXid));
    4469         212 :     latestXid = TransamVariables->nextXid;
    4470         212 :     FullTransactionIdRetreat(&latestXid);
    4471         212 :     TransamVariables->latestCompletedXid = latestXid;
    4472             : 
    4473             :     /*
    4474             :      * Any transactions that were in-progress were effectively aborted, so
    4475             :      * advance xactCompletionCount.
    4476             :      */
    4477         212 :     TransamVariables->xactCompletionCount++;
    4478             : 
    4479             :     /*
    4480             :      * Reset lastOverflowedXid.  Currently, lastOverflowedXid has no use after
    4481             :      * the call of this function.  But do this for unification with what
    4482             :      * ExpireOldKnownAssignedTransactionIds() do.
    4483             :      */
    4484         212 :     procArray->lastOverflowedXid = InvalidTransactionId;
    4485         212 :     LWLockRelease(ProcArrayLock);
    4486         212 : }
    4487             : 
    4488             : /*
    4489             :  * ExpireOldKnownAssignedTransactionIds
    4490             :  *      Remove KnownAssignedXids entries preceding the given XID and
    4491             :  *      potentially reset lastOverflowedXid.
    4492             :  */
    4493             : void
    4494        1570 : ExpireOldKnownAssignedTransactionIds(TransactionId xid)
    4495             : {
    4496             :     TransactionId latestXid;
    4497             : 
    4498        1570 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    4499             : 
    4500             :     /* As in ProcArrayEndTransaction, advance latestCompletedXid */
    4501        1570 :     latestXid = xid;
    4502        1570 :     TransactionIdRetreat(latestXid);
    4503        1570 :     MaintainLatestCompletedXidRecovery(latestXid);
    4504             : 
    4505             :     /* ... and xactCompletionCount */
    4506        1570 :     TransamVariables->xactCompletionCount++;
    4507             : 
    4508             :     /*
    4509             :      * Reset lastOverflowedXid if we know all transactions that have been
    4510             :      * possibly running are being gone.  Not doing so could cause an incorrect
    4511             :      * lastOverflowedXid value, which makes extra snapshots be marked as
    4512             :      * suboverflowed.
    4513             :      */
    4514        1570 :     if (TransactionIdPrecedes(procArray->lastOverflowedXid, xid))
    4515        1558 :         procArray->lastOverflowedXid = InvalidTransactionId;
    4516        1570 :     KnownAssignedXidsRemovePreceding(xid);
    4517        1570 :     LWLockRelease(ProcArrayLock);
    4518        1570 : }
    4519             : 
    4520             : /*
    4521             :  * KnownAssignedTransactionIdsIdleMaintenance
    4522             :  *      Opportunistically do maintenance work when the startup process
    4523             :  *      is about to go idle.
    4524             :  */
    4525             : void
    4526       17130 : KnownAssignedTransactionIdsIdleMaintenance(void)
    4527             : {
    4528       17130 :     KnownAssignedXidsCompress(KAX_STARTUP_PROCESS_IDLE, false);
    4529       17130 : }
    4530             : 
    4531             : 
    4532             : /*
    4533             :  * Private module functions to manipulate KnownAssignedXids
    4534             :  *
    4535             :  * There are 5 main uses of the KnownAssignedXids data structure:
    4536             :  *
    4537             :  *  * backends taking snapshots - all valid XIDs need to be copied out
    4538             :  *  * backends seeking to determine presence of a specific XID
    4539             :  *  * startup process adding new known-assigned XIDs
    4540             :  *  * startup process removing specific XIDs as transactions end
    4541             :  *  * startup process pruning array when special WAL records arrive
    4542             :  *
    4543             :  * This data structure is known to be a hot spot during Hot Standby, so we
    4544             :  * go to some lengths to make these operations as efficient and as concurrent
    4545             :  * as possible.
    4546             :  *
    4547             :  * The XIDs are stored in an array in sorted order --- TransactionIdPrecedes
    4548             :  * order, to be exact --- to allow binary search for specific XIDs.  Note:
    4549             :  * in general TransactionIdPrecedes would not provide a total order, but
    4550             :  * we know that the entries present at any instant should not extend across
    4551             :  * a large enough fraction of XID space to wrap around (the primary would
    4552             :  * shut down for fear of XID wrap long before that happens).  So it's OK to
    4553             :  * use TransactionIdPrecedes as a binary-search comparator.
    4554             :  *
    4555             :  * It's cheap to maintain the sortedness during insertions, since new known
    4556             :  * XIDs are always reported in XID order; we just append them at the right.
    4557             :  *
    4558             :  * To keep individual deletions cheap, we need to allow gaps in the array.
    4559             :  * This is implemented by marking array elements as valid or invalid using
    4560             :  * the parallel boolean array KnownAssignedXidsValid[].  A deletion is done
    4561             :  * by setting KnownAssignedXidsValid[i] to false, *without* clearing the
    4562             :  * XID entry itself.  This preserves the property that the XID entries are
    4563             :  * sorted, so we can do binary searches easily.  Periodically we compress
    4564             :  * out the unused entries; that's much cheaper than having to compress the
    4565             :  * array immediately on every deletion.
    4566             :  *
    4567             :  * The actually valid items in KnownAssignedXids[] and KnownAssignedXidsValid[]
    4568             :  * are those with indexes tail <= i < head; items outside this subscript range
    4569             :  * have unspecified contents.  When head reaches the end of the array, we
    4570             :  * force compression of unused entries rather than wrapping around, since
    4571             :  * allowing wraparound would greatly complicate the search logic.  We maintain
    4572             :  * an explicit tail pointer so that pruning of old XIDs can be done without
    4573             :  * immediately moving the array contents.  In most cases only a small fraction
    4574             :  * of the array contains valid entries at any instant.
    4575             :  *
    4576             :  * Although only the startup process can ever change the KnownAssignedXids
    4577             :  * data structure, we still need interlocking so that standby backends will
    4578             :  * not observe invalid intermediate states.  The convention is that backends
    4579             :  * must hold shared ProcArrayLock to examine the array.  To remove XIDs from
    4580             :  * the array, the startup process must hold ProcArrayLock exclusively, for
    4581             :  * the usual transactional reasons (compare commit/abort of a transaction
    4582             :  * during normal running).  Compressing unused entries out of the array
    4583             :  * likewise requires exclusive lock.  To add XIDs to the array, we just insert
    4584             :  * them into slots to the right of the head pointer and then advance the head
    4585             :  * pointer.  This doesn't require any lock at all, but on machines with weak
    4586             :  * memory ordering, we need to be careful that other processors see the array
    4587             :  * element changes before they see the head pointer change.  We handle this by
    4588             :  * using memory barriers when reading or writing the head/tail pointers (unless
    4589             :  * the caller holds ProcArrayLock exclusively).
    4590             :  *
    4591             :  * Algorithmic analysis:
    4592             :  *
    4593             :  * If we have a maximum of M slots, with N XIDs currently spread across
    4594             :  * S elements then we have N <= S <= M always.
    4595             :  *
    4596             :  *  * Adding a new XID is O(1) and needs no lock (unless compression must
    4597             :  *      happen)
    4598             :  *  * Compressing the array is O(S) and requires exclusive lock
    4599             :  *  * Removing an XID is O(logS) and requires exclusive lock
    4600             :  *  * Taking a snapshot is O(S) and requires shared lock
    4601             :  *  * Checking for an XID is O(logS) and requires shared lock
    4602             :  *
    4603             :  * In comparison, using a hash table for KnownAssignedXids would mean that
    4604             :  * taking snapshots would be O(M). If we can maintain S << M then the
    4605             :  * sorted array technique will deliver significantly faster snapshots.
    4606             :  * If we try to keep S too small then we will spend too much time compressing,
    4607             :  * so there is an optimal point for any workload mix. We use a heuristic to
    4608             :  * decide when to compress the array, though trimming also helps reduce
    4609             :  * frequency of compressing. The heuristic requires us to track the number of
    4610             :  * currently valid XIDs in the array (N).  Except in special cases, we'll
    4611             :  * compress when S >= 2N.  Bounding S at 2N in turn bounds the time for
    4612             :  * taking a snapshot to be O(N), which it would have to be anyway.
    4613             :  */
    4614             : 
    4615             : 
    4616             : /*
    4617             :  * Compress KnownAssignedXids by shifting valid data down to the start of the
    4618             :  * array, removing any gaps.
    4619             :  *
    4620             :  * A compression step is forced if "reason" is KAX_NO_SPACE, otherwise
    4621             :  * we do it only if a heuristic indicates it's a good time to do it.
    4622             :  *
    4623             :  * Compression requires holding ProcArrayLock in exclusive mode.
    4624             :  * Caller must pass haveLock = true if it already holds the lock.
    4625             :  */
    4626             : static void
    4627       61232 : KnownAssignedXidsCompress(KAXCompressReason reason, bool haveLock)
    4628             : {
    4629       61232 :     ProcArrayStruct *pArray = procArray;
    4630             :     int         head,
    4631             :                 tail,
    4632             :                 nelements;
    4633             :     int         compress_index;
    4634             :     int         i;
    4635             : 
    4636             :     /* Counters for compression heuristics */
    4637             :     static unsigned int transactionEndsCounter;
    4638             :     static TimestampTz lastCompressTs;
    4639             : 
    4640             :     /* Tuning constants */
    4641             : #define KAX_COMPRESS_FREQUENCY 128  /* in transactions */
    4642             : #define KAX_COMPRESS_IDLE_INTERVAL 1000 /* in ms */
    4643             : 
    4644             :     /*
    4645             :      * Since only the startup process modifies the head/tail pointers, we
    4646             :      * don't need a lock to read them here.
    4647             :      */
    4648       61232 :     head = pArray->headKnownAssignedXids;
    4649       61232 :     tail = pArray->tailKnownAssignedXids;
    4650       61232 :     nelements = head - tail;
    4651             : 
    4652             :     /*
    4653             :      * If we can choose whether to compress, use a heuristic to avoid
    4654             :      * compressing too often or not often enough.  "Compress" here simply
    4655             :      * means moving the values to the beginning of the array, so it is not as
    4656             :      * complex or costly as typical data compression algorithms.
    4657             :      */
    4658       61232 :     if (nelements == pArray->numKnownAssignedXids)
    4659             :     {
    4660             :         /*
    4661             :          * When there are no gaps between head and tail, don't bother to
    4662             :          * compress, except in the KAX_NO_SPACE case where we must compress to
    4663             :          * create some space after the head.
    4664             :          */
    4665       30612 :         if (reason != KAX_NO_SPACE)
    4666       30612 :             return;
    4667             :     }
    4668       30620 :     else if (reason == KAX_TRANSACTION_END)
    4669             :     {
    4670             :         /*
    4671             :          * Consider compressing only once every so many commits.  Frequency
    4672             :          * determined by benchmarks.
    4673             :          */
    4674       26044 :         if ((transactionEndsCounter++) % KAX_COMPRESS_FREQUENCY != 0)
    4675       25822 :             return;
    4676             : 
    4677             :         /*
    4678             :          * Furthermore, compress only if the used part of the array is less
    4679             :          * than 50% full (see comments above).
    4680             :          */
    4681         222 :         if (nelements < 2 * pArray->numKnownAssignedXids)
    4682          14 :             return;
    4683             :     }
    4684        4576 :     else if (reason == KAX_STARTUP_PROCESS_IDLE)
    4685             :     {
    4686             :         /*
    4687             :          * We're about to go idle for lack of new WAL, so we might as well
    4688             :          * compress.  But not too often, to avoid ProcArray lock contention
    4689             :          * with readers.
    4690             :          */
    4691        4340 :         if (lastCompressTs != 0)
    4692             :         {
    4693             :             TimestampTz compress_after;
    4694             : 
    4695        4338 :             compress_after = TimestampTzPlusMilliseconds(lastCompressTs,
    4696             :                                                          KAX_COMPRESS_IDLE_INTERVAL);
    4697        4338 :             if (GetCurrentTimestamp() < compress_after)
    4698        4256 :                 return;
    4699             :         }
    4700             :     }
    4701             : 
    4702             :     /* Need to compress, so get the lock if we don't have it. */
    4703         528 :     if (!haveLock)
    4704          84 :         LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    4705             : 
    4706             :     /*
    4707             :      * We compress the array by reading the valid values from tail to head,
    4708             :      * re-aligning data to 0th element.
    4709             :      */
    4710         528 :     compress_index = 0;
    4711       18724 :     for (i = tail; i < head; i++)
    4712             :     {
    4713       18196 :         if (KnownAssignedXidsValid[i])
    4714             :         {
    4715        2048 :             KnownAssignedXids[compress_index] = KnownAssignedXids[i];
    4716        2048 :             KnownAssignedXidsValid[compress_index] = true;
    4717        2048 :             compress_index++;
    4718             :         }
    4719             :     }
    4720             :     Assert(compress_index == pArray->numKnownAssignedXids);
    4721             : 
    4722         528 :     pArray->tailKnownAssignedXids = 0;
    4723         528 :     pArray->headKnownAssignedXids = compress_index;
    4724             : 
    4725         528 :     if (!haveLock)
    4726          84 :         LWLockRelease(ProcArrayLock);
    4727             : 
    4728             :     /* Update timestamp for maintenance.  No need to hold lock for this. */
    4729         528 :     lastCompressTs = GetCurrentTimestamp();
    4730             : }
    4731             : 
    4732             : /*
    4733             :  * Add xids into KnownAssignedXids at the head of the array.
    4734             :  *
    4735             :  * xids from from_xid to to_xid, inclusive, are added to the array.
    4736             :  *
    4737             :  * If exclusive_lock is true then caller already holds ProcArrayLock in
    4738             :  * exclusive mode, so we need no extra locking here.  Else caller holds no
    4739             :  * lock, so we need to be sure we maintain sufficient interlocks against
    4740             :  * concurrent readers.  (Only the startup process ever calls this, so no need
    4741             :  * to worry about concurrent writers.)
    4742             :  */
    4743             : static void
    4744       44136 : KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
    4745             :                      bool exclusive_lock)
    4746             : {
    4747       44136 :     ProcArrayStruct *pArray = procArray;
    4748             :     TransactionId next_xid;
    4749             :     int         head,
    4750             :                 tail;
    4751             :     int         nxids;
    4752             :     int         i;
    4753             : 
    4754             :     Assert(TransactionIdPrecedesOrEquals(from_xid, to_xid));
    4755             : 
    4756             :     /*
    4757             :      * Calculate how many array slots we'll need.  Normally this is cheap; in
    4758             :      * the unusual case where the XIDs cross the wrap point, we do it the hard
    4759             :      * way.
    4760             :      */
    4761       44136 :     if (to_xid >= from_xid)
    4762       44136 :         nxids = to_xid - from_xid + 1;
    4763             :     else
    4764             :     {
    4765           0 :         nxids = 1;
    4766           0 :         next_xid = from_xid;
    4767           0 :         while (TransactionIdPrecedes(next_xid, to_xid))
    4768             :         {
    4769           0 :             nxids++;
    4770           0 :             TransactionIdAdvance(next_xid);
    4771             :         }
    4772             :     }
    4773             : 
    4774             :     /*
    4775             :      * Since only the startup process modifies the head/tail pointers, we
    4776             :      * don't need a lock to read them here.
    4777             :      */
    4778       44136 :     head = pArray->headKnownAssignedXids;
    4779       44136 :     tail = pArray->tailKnownAssignedXids;
    4780             : 
    4781             :     Assert(head >= 0 && head <= pArray->maxKnownAssignedXids);
    4782             :     Assert(tail >= 0 && tail < pArray->maxKnownAssignedXids);
    4783             : 
    4784             :     /*
    4785             :      * Verify that insertions occur in TransactionId sequence.  Note that even
    4786             :      * if the last existing element is marked invalid, it must still have a
    4787             :      * correctly sequenced XID value.
    4788             :      */
    4789       73044 :     if (head > tail &&
    4790       28908 :         TransactionIdFollowsOrEquals(KnownAssignedXids[head - 1], from_xid))
    4791             :     {
    4792           0 :         KnownAssignedXidsDisplay(LOG);
    4793           0 :         elog(ERROR, "out-of-order XID insertion in KnownAssignedXids");
    4794             :     }
    4795             : 
    4796             :     /*
    4797             :      * If our xids won't fit in the remaining space, compress out free space
    4798             :      */
    4799       44136 :     if (head + nxids > pArray->maxKnownAssignedXids)
    4800             :     {
    4801           0 :         KnownAssignedXidsCompress(KAX_NO_SPACE, exclusive_lock);
    4802             : 
    4803           0 :         head = pArray->headKnownAssignedXids;
    4804             :         /* note: we no longer care about the tail pointer */
    4805             : 
    4806             :         /*
    4807             :          * If it still won't fit then we're out of memory
    4808             :          */
    4809           0 :         if (head + nxids > pArray->maxKnownAssignedXids)
    4810           0 :             elog(ERROR, "too many KnownAssignedXids");
    4811             :     }
    4812             : 
    4813             :     /* Now we can insert the xids into the space starting at head */
    4814       44136 :     next_xid = from_xid;
    4815       89698 :     for (i = 0; i < nxids; i++)
    4816             :     {
    4817       45562 :         KnownAssignedXids[head] = next_xid;
    4818       45562 :         KnownAssignedXidsValid[head] = true;
    4819       45562 :         TransactionIdAdvance(next_xid);
    4820       45562 :         head++;
    4821             :     }
    4822             : 
    4823             :     /* Adjust count of number of valid entries */
    4824       44136 :     pArray->numKnownAssignedXids += nxids;
    4825             : 
    4826             :     /*
    4827             :      * Now update the head pointer.  We use a write barrier to ensure that
    4828             :      * other processors see the above array updates before they see the head
    4829             :      * pointer change.  The barrier isn't required if we're holding
    4830             :      * ProcArrayLock exclusively.
    4831             :      */
    4832       44136 :     if (!exclusive_lock)
    4833       44128 :         pg_write_barrier();
    4834             : 
    4835       44136 :     pArray->headKnownAssignedXids = head;
    4836       44136 : }
    4837             : 
    4838             : /*
    4839             :  * KnownAssignedXidsSearch
    4840             :  *
    4841             :  * Searches KnownAssignedXids for a specific xid and optionally removes it.
    4842             :  * Returns true if it was found, false if not.
    4843             :  *
    4844             :  * Caller must hold ProcArrayLock in shared or exclusive mode.
    4845             :  * Exclusive lock must be held for remove = true.
    4846             :  */
    4847             : static bool
    4848       47830 : KnownAssignedXidsSearch(TransactionId xid, bool remove)
    4849             : {
    4850       47830 :     ProcArrayStruct *pArray = procArray;
    4851             :     int         first,
    4852             :                 last;
    4853             :     int         head;
    4854             :     int         tail;
    4855       47830 :     int         result_index = -1;
    4856             : 
    4857       47830 :     tail = pArray->tailKnownAssignedXids;
    4858       47830 :     head = pArray->headKnownAssignedXids;
    4859             : 
    4860             :     /*
    4861             :      * Only the startup process removes entries, so we don't need the read
    4862             :      * barrier in that case.
    4863             :      */
    4864       47830 :     if (!remove)
    4865           2 :         pg_read_barrier();      /* pairs with KnownAssignedXidsAdd */
    4866             : 
    4867             :     /*
    4868             :      * Standard binary search.  Note we can ignore the KnownAssignedXidsValid
    4869             :      * array here, since even invalid entries will contain sorted XIDs.
    4870             :      */
    4871       47830 :     first = tail;
    4872       47830 :     last = head - 1;
    4873      163840 :     while (first <= last)
    4874             :     {
    4875             :         int         mid_index;
    4876             :         TransactionId mid_xid;
    4877             : 
    4878      161416 :         mid_index = (first + last) / 2;
    4879      161416 :         mid_xid = KnownAssignedXids[mid_index];
    4880             : 
    4881      161416 :         if (xid == mid_xid)
    4882             :         {
    4883       45406 :             result_index = mid_index;
    4884       45406 :             break;
    4885             :         }
    4886      116010 :         else if (TransactionIdPrecedes(xid, mid_xid))
    4887       25074 :             last = mid_index - 1;
    4888             :         else
    4889       90936 :             first = mid_index + 1;
    4890             :     }
    4891             : 
    4892       47830 :     if (result_index < 0)
    4893        2424 :         return false;           /* not in array */
    4894             : 
    4895       45406 :     if (!KnownAssignedXidsValid[result_index])
    4896          16 :         return false;           /* in array, but invalid */
    4897             : 
    4898       45390 :     if (remove)
    4899             :     {
    4900       45390 :         KnownAssignedXidsValid[result_index] = false;
    4901             : 
    4902       45390 :         pArray->numKnownAssignedXids--;
    4903             :         Assert(pArray->numKnownAssignedXids >= 0);
    4904             : 
    4905             :         /*
    4906             :          * If we're removing the tail element then advance tail pointer over
    4907             :          * any invalid elements.  This will speed future searches.
    4908             :          */
    4909       45390 :         if (result_index == tail)
    4910             :         {
    4911       17432 :             tail++;
    4912       29242 :             while (tail < head && !KnownAssignedXidsValid[tail])
    4913       11810 :                 tail++;
    4914       17432 :             if (tail >= head)
    4915             :             {
    4916             :                 /* Array is empty, so we can reset both pointers */
    4917       15208 :                 pArray->headKnownAssignedXids = 0;
    4918       15208 :                 pArray->tailKnownAssignedXids = 0;
    4919             :             }
    4920             :             else
    4921             :             {
    4922        2224 :                 pArray->tailKnownAssignedXids = tail;
    4923             :             }
    4924             :         }
    4925             :     }
    4926             : 
    4927       45390 :     return true;
    4928             : }
    4929             : 
    4930             : /*
    4931             :  * Is the specified XID present in KnownAssignedXids[]?
    4932             :  *
    4933             :  * Caller must hold ProcArrayLock in shared or exclusive mode.
    4934             :  */
    4935             : static bool
    4936           2 : KnownAssignedXidExists(TransactionId xid)
    4937             : {
    4938             :     Assert(TransactionIdIsValid(xid));
    4939             : 
    4940           2 :     return KnownAssignedXidsSearch(xid, false);
    4941             : }
    4942             : 
    4943             : /*
    4944             :  * Remove the specified XID from KnownAssignedXids[].
    4945             :  *
    4946             :  * Caller must hold ProcArrayLock in exclusive mode.
    4947             :  */
    4948             : static void
    4949       47828 : KnownAssignedXidsRemove(TransactionId xid)
    4950             : {
    4951             :     Assert(TransactionIdIsValid(xid));
    4952             : 
    4953       47828 :     elog(DEBUG4, "remove KnownAssignedXid %u", xid);
    4954             : 
    4955             :     /*
    4956             :      * Note: we cannot consider it an error to remove an XID that's not
    4957             :      * present.  We intentionally remove subxact IDs while processing
    4958             :      * XLOG_XACT_ASSIGNMENT, to avoid array overflow.  Then those XIDs will be
    4959             :      * removed again when the top-level xact commits or aborts.
    4960             :      *
    4961             :      * It might be possible to track such XIDs to distinguish this case from
    4962             :      * actual errors, but it would be complicated and probably not worth it.
    4963             :      * So, just ignore the search result.
    4964             :      */
    4965       47828 :     (void) KnownAssignedXidsSearch(xid, true);
    4966       47828 : }
    4967             : 
    4968             : /*
    4969             :  * KnownAssignedXidsRemoveTree
    4970             :  *      Remove xid (if it's not InvalidTransactionId) and all the subxids.
    4971             :  *
    4972             :  * Caller must hold ProcArrayLock in exclusive mode.
    4973             :  */
    4974             : static void
    4975       42532 : KnownAssignedXidsRemoveTree(TransactionId xid, int nsubxids,
    4976             :                             TransactionId *subxids)
    4977             : {
    4978             :     int         i;
    4979             : 
    4980       42532 :     if (TransactionIdIsValid(xid))
    4981       42490 :         KnownAssignedXidsRemove(xid);
    4982             : 
    4983       47870 :     for (i = 0; i < nsubxids; i++)
    4984        5338 :         KnownAssignedXidsRemove(subxids[i]);
    4985             : 
    4986             :     /* Opportunistically compress the array */
    4987       42532 :     KnownAssignedXidsCompress(KAX_TRANSACTION_END, true);
    4988       42532 : }
    4989             : 
    4990             : /*
    4991             :  * Prune KnownAssignedXids up to, but *not* including xid. If xid is invalid
    4992             :  * then clear the whole table.
    4993             :  *
    4994             :  * Caller must hold ProcArrayLock in exclusive mode.
    4995             :  */
    4996             : static void
    4997        1782 : KnownAssignedXidsRemovePreceding(TransactionId removeXid)
    4998             : {
    4999        1782 :     ProcArrayStruct *pArray = procArray;
    5000        1782 :     int         count = 0;
    5001             :     int         head,
    5002             :                 tail,
    5003             :                 i;
    5004             : 
    5005        1782 :     if (!TransactionIdIsValid(removeXid))
    5006             :     {
    5007         212 :         elog(DEBUG4, "removing all KnownAssignedXids");
    5008         212 :         pArray->numKnownAssignedXids = 0;
    5009         212 :         pArray->headKnownAssignedXids = pArray->tailKnownAssignedXids = 0;
    5010         212 :         return;
    5011             :     }
    5012             : 
    5013        1570 :     elog(DEBUG4, "prune KnownAssignedXids to %u", removeXid);
    5014             : 
    5015             :     /*
    5016             :      * Mark entries invalid starting at the tail.  Since array is sorted, we
    5017             :      * can stop as soon as we reach an entry >= removeXid.
    5018             :      */
    5019        1570 :     tail = pArray->tailKnownAssignedXids;
    5020        1570 :     head = pArray->headKnownAssignedXids;
    5021             : 
    5022        1570 :     for (i = tail; i < head; i++)
    5023             :     {
    5024         408 :         if (KnownAssignedXidsValid[i])
    5025             :         {
    5026         408 :             TransactionId knownXid = KnownAssignedXids[i];
    5027             : 
    5028         408 :             if (TransactionIdFollowsOrEquals(knownXid, removeXid))
    5029         408 :                 break;
    5030             : 
    5031           0 :             if (!StandbyTransactionIdIsPrepared(knownXid))
    5032             :             {
    5033           0 :                 KnownAssignedXidsValid[i] = false;
    5034           0 :                 count++;
    5035             :             }
    5036             :         }
    5037             :     }
    5038             : 
    5039        1570 :     pArray->numKnownAssignedXids -= count;
    5040             :     Assert(pArray->numKnownAssignedXids >= 0);
    5041             : 
    5042             :     /*
    5043             :      * Advance the tail pointer if we've marked the tail item invalid.
    5044             :      */
    5045        1570 :     for (i = tail; i < head; i++)
    5046             :     {
    5047         408 :         if (KnownAssignedXidsValid[i])
    5048         408 :             break;
    5049             :     }
    5050        1570 :     if (i >= head)
    5051             :     {
    5052             :         /* Array is empty, so we can reset both pointers */
    5053        1162 :         pArray->headKnownAssignedXids = 0;
    5054        1162 :         pArray->tailKnownAssignedXids = 0;
    5055             :     }
    5056             :     else
    5057             :     {
    5058         408 :         pArray->tailKnownAssignedXids = i;
    5059             :     }
    5060             : 
    5061             :     /* Opportunistically compress the array */
    5062        1570 :     KnownAssignedXidsCompress(KAX_PRUNE, true);
    5063             : }
    5064             : 
    5065             : /*
    5066             :  * KnownAssignedXidsGet - Get an array of xids by scanning KnownAssignedXids.
    5067             :  * We filter out anything >= xmax.
    5068             :  *
    5069             :  * Returns the number of XIDs stored into xarray[].  Caller is responsible
    5070             :  * that array is large enough.
    5071             :  *
    5072             :  * Caller must hold ProcArrayLock in (at least) shared mode.
    5073             :  */
    5074             : static int
    5075           0 : KnownAssignedXidsGet(TransactionId *xarray, TransactionId xmax)
    5076             : {
    5077           0 :     TransactionId xtmp = InvalidTransactionId;
    5078             : 
    5079           0 :     return KnownAssignedXidsGetAndSetXmin(xarray, &xtmp, xmax);
    5080             : }
    5081             : 
    5082             : /*
    5083             :  * KnownAssignedXidsGetAndSetXmin - as KnownAssignedXidsGet, plus
    5084             :  * we reduce *xmin to the lowest xid value seen if not already lower.
    5085             :  *
    5086             :  * Caller must hold ProcArrayLock in (at least) shared mode.
    5087             :  */
    5088             : static int
    5089        2076 : KnownAssignedXidsGetAndSetXmin(TransactionId *xarray, TransactionId *xmin,
    5090             :                                TransactionId xmax)
    5091             : {
    5092        2076 :     int         count = 0;
    5093             :     int         head,
    5094             :                 tail;
    5095             :     int         i;
    5096             : 
    5097             :     /*
    5098             :      * Fetch head just once, since it may change while we loop. We can stop
    5099             :      * once we reach the initially seen head, since we are certain that an xid
    5100             :      * cannot enter and then leave the array while we hold ProcArrayLock.  We
    5101             :      * might miss newly-added xids, but they should be >= xmax so irrelevant
    5102             :      * anyway.
    5103             :      */
    5104        2076 :     tail = procArray->tailKnownAssignedXids;
    5105        2076 :     head = procArray->headKnownAssignedXids;
    5106             : 
    5107        2076 :     pg_read_barrier();          /* pairs with KnownAssignedXidsAdd */
    5108             : 
    5109        2152 :     for (i = tail; i < head; i++)
    5110             :     {
    5111             :         /* Skip any gaps in the array */
    5112         204 :         if (KnownAssignedXidsValid[i])
    5113             :         {
    5114         158 :             TransactionId knownXid = KnownAssignedXids[i];
    5115             : 
    5116             :             /*
    5117             :              * Update xmin if required.  Only the first XID need be checked,
    5118             :              * since the array is sorted.
    5119             :              */
    5120         314 :             if (count == 0 &&
    5121         156 :                 TransactionIdPrecedes(knownXid, *xmin))
    5122          30 :                 *xmin = knownXid;
    5123             : 
    5124             :             /*
    5125             :              * Filter out anything >= xmax, again relying on sorted property
    5126             :              * of array.
    5127             :              */
    5128         316 :             if (TransactionIdIsValid(xmax) &&
    5129         158 :                 TransactionIdFollowsOrEquals(knownXid, xmax))
    5130         128 :                 break;
    5131             : 
    5132             :             /* Add knownXid into output array */
    5133          30 :             xarray[count++] = knownXid;
    5134             :         }
    5135             :     }
    5136             : 
    5137        2076 :     return count;
    5138             : }
    5139             : 
    5140             : /*
    5141             :  * Get oldest XID in the KnownAssignedXids array, or InvalidTransactionId
    5142             :  * if nothing there.
    5143             :  */
    5144             : static TransactionId
    5145         700 : KnownAssignedXidsGetOldestXmin(void)
    5146             : {
    5147             :     int         head,
    5148             :                 tail;
    5149             :     int         i;
    5150             : 
    5151             :     /*
    5152             :      * Fetch head just once, since it may change while we loop.
    5153             :      */
    5154         700 :     tail = procArray->tailKnownAssignedXids;
    5155         700 :     head = procArray->headKnownAssignedXids;
    5156             : 
    5157         700 :     pg_read_barrier();          /* pairs with KnownAssignedXidsAdd */
    5158             : 
    5159         700 :     for (i = tail; i < head; i++)
    5160             :     {
    5161             :         /* Skip any gaps in the array */
    5162         288 :         if (KnownAssignedXidsValid[i])
    5163         288 :             return KnownAssignedXids[i];
    5164             :     }
    5165             : 
    5166         412 :     return InvalidTransactionId;
    5167             : }
    5168             : 
    5169             : /*
    5170             :  * Display KnownAssignedXids to provide debug trail
    5171             :  *
    5172             :  * Currently this is only called within startup process, so we need no
    5173             :  * special locking.
    5174             :  *
    5175             :  * Note this is pretty expensive, and much of the expense will be incurred
    5176             :  * even if the elog message will get discarded.  It's not currently called
    5177             :  * in any performance-critical places, however, so no need to be tenser.
    5178             :  */
    5179             : static void
    5180         220 : KnownAssignedXidsDisplay(int trace_level)
    5181             : {
    5182         220 :     ProcArrayStruct *pArray = procArray;
    5183             :     StringInfoData buf;
    5184             :     int         head,
    5185             :                 tail,
    5186             :                 i;
    5187         220 :     int         nxids = 0;
    5188             : 
    5189         220 :     tail = pArray->tailKnownAssignedXids;
    5190         220 :     head = pArray->headKnownAssignedXids;
    5191             : 
    5192         220 :     initStringInfo(&buf);
    5193             : 
    5194         236 :     for (i = tail; i < head; i++)
    5195             :     {
    5196          16 :         if (KnownAssignedXidsValid[i])
    5197             :         {
    5198          16 :             nxids++;
    5199          16 :             appendStringInfo(&buf, "[%d]=%u ", i, KnownAssignedXids[i]);
    5200             :         }
    5201             :     }
    5202             : 
    5203         220 :     elog(trace_level, "%d KnownAssignedXids (num=%d tail=%d head=%d) %s",
    5204             :          nxids,
    5205             :          pArray->numKnownAssignedXids,
    5206             :          pArray->tailKnownAssignedXids,
    5207             :          pArray->headKnownAssignedXids,
    5208             :          buf.data);
    5209             : 
    5210         220 :     pfree(buf.data);
    5211         220 : }
    5212             : 
    5213             : /*
    5214             :  * KnownAssignedXidsReset
    5215             :  *      Resets KnownAssignedXids to be empty
    5216             :  */
    5217             : static void
    5218           0 : KnownAssignedXidsReset(void)
    5219             : {
    5220           0 :     ProcArrayStruct *pArray = procArray;
    5221             : 
    5222           0 :     LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    5223             : 
    5224           0 :     pArray->numKnownAssignedXids = 0;
    5225           0 :     pArray->tailKnownAssignedXids = 0;
    5226           0 :     pArray->headKnownAssignedXids = 0;
    5227             : 
    5228           0 :     LWLockRelease(ProcArrayLock);
    5229           0 : }

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