LCOV - code coverage report
Current view: top level - src/backend/access/transam - clog.c (source / functions) Hit Total Coverage
Test: PostgreSQL 18devel Lines: 196 252 77.8 %
Date: 2024-12-12 21:14:38 Functions: 21 24 87.5 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : /*-------------------------------------------------------------------------
       2             :  *
       3             :  * clog.c
       4             :  *      PostgreSQL transaction-commit-log manager
       5             :  *
       6             :  * This module stores two bits per transaction regarding its commit/abort
       7             :  * status; the status for four transactions fit in a byte.
       8             :  *
       9             :  * This would be a pretty simple abstraction on top of slru.c, except that
      10             :  * for performance reasons we allow multiple transactions that are
      11             :  * committing concurrently to form a queue, so that a single process can
      12             :  * update the status for all of them within a single lock acquisition run.
      13             :  *
      14             :  * XLOG interactions: this module generates an XLOG record whenever a new
      15             :  * CLOG page is initialized to zeroes.  Other writes of CLOG come from
      16             :  * recording of transaction commit or abort in xact.c, which generates its
      17             :  * own XLOG records for these events and will re-perform the status update
      18             :  * on redo; so we need make no additional XLOG entry here.  For synchronous
      19             :  * transaction commits, the XLOG is guaranteed flushed through the XLOG commit
      20             :  * record before we are called to log a commit, so the WAL rule "write xlog
      21             :  * before data" is satisfied automatically.  However, for async commits we
      22             :  * must track the latest LSN affecting each CLOG page, so that we can flush
      23             :  * XLOG that far and satisfy the WAL rule.  We don't have to worry about this
      24             :  * for aborts (whether sync or async), since the post-crash assumption would
      25             :  * be that such transactions failed anyway.
      26             :  *
      27             :  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
      28             :  * Portions Copyright (c) 1994, Regents of the University of California
      29             :  *
      30             :  * src/backend/access/transam/clog.c
      31             :  *
      32             :  *-------------------------------------------------------------------------
      33             :  */
      34             : #include "postgres.h"
      35             : 
      36             : #include "access/clog.h"
      37             : #include "access/slru.h"
      38             : #include "access/transam.h"
      39             : #include "access/xlog.h"
      40             : #include "access/xloginsert.h"
      41             : #include "access/xlogutils.h"
      42             : #include "miscadmin.h"
      43             : #include "pg_trace.h"
      44             : #include "pgstat.h"
      45             : #include "storage/proc.h"
      46             : #include "storage/sync.h"
      47             : #include "utils/guc_hooks.h"
      48             : 
      49             : /*
      50             :  * Defines for CLOG page sizes.  A page is the same BLCKSZ as is used
      51             :  * everywhere else in Postgres.
      52             :  *
      53             :  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
      54             :  * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
      55             :  * and CLOG segment numbering at
      56             :  * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need take no
      57             :  * explicit notice of that fact in this module, except when comparing segment
      58             :  * and page numbers in TruncateCLOG (see CLOGPagePrecedes).
      59             :  */
      60             : 
      61             : /* We need two bits per xact, so four xacts fit in a byte */
      62             : #define CLOG_BITS_PER_XACT  2
      63             : #define CLOG_XACTS_PER_BYTE 4
      64             : #define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
      65             : #define CLOG_XACT_BITMASK   ((1 << CLOG_BITS_PER_XACT) - 1)
      66             : 
      67             : /*
      68             :  * Because space used in CLOG by each transaction is so small, we place a
      69             :  * smaller limit on the number of CLOG buffers than SLRU allows.  No other
      70             :  * SLRU needs this.
      71             :  */
      72             : #define CLOG_MAX_ALLOWED_BUFFERS \
      73             :     Min(SLRU_MAX_ALLOWED_BUFFERS, \
      74             :         (((MaxTransactionId / 2) + (CLOG_XACTS_PER_PAGE - 1)) / CLOG_XACTS_PER_PAGE))
      75             : 
      76             : 
      77             : /*
      78             :  * Although we return an int64 the actual value can't currently exceed
      79             :  * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE.
      80             :  */
      81             : static inline int64
      82     2535436 : TransactionIdToPage(TransactionId xid)
      83             : {
      84     2535436 :     return xid / (int64) CLOG_XACTS_PER_PAGE;
      85             : }
      86             : 
      87             : #define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
      88             : #define TransactionIdToByte(xid)    (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
      89             : #define TransactionIdToBIndex(xid)  ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
      90             : 
      91             : /* We store the latest async LSN for each group of transactions */
      92             : #define CLOG_XACTS_PER_LSN_GROUP    32  /* keep this a power of 2 */
      93             : #define CLOG_LSNS_PER_PAGE  (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)
      94             : 
      95             : #define GetLSNIndex(slotno, xid)    ((slotno) * CLOG_LSNS_PER_PAGE + \
      96             :     ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)
      97             : 
      98             : /*
      99             :  * The number of subtransactions below which we consider to apply clog group
     100             :  * update optimization.  Testing reveals that the number higher than this can
     101             :  * hurt performance.
     102             :  */
     103             : #define THRESHOLD_SUBTRANS_CLOG_OPT 5
     104             : 
     105             : /*
     106             :  * Link to shared-memory data structures for CLOG control
     107             :  */
     108             : static SlruCtlData XactCtlData;
     109             : 
     110             : #define XactCtl (&XactCtlData)
     111             : 
     112             : 
     113             : static int  ZeroCLOGPage(int64 pageno, bool writeXlog);
     114             : static bool CLOGPagePrecedes(int64 page1, int64 page2);
     115             : static void WriteZeroPageXlogRec(int64 pageno);
     116             : static void WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact,
     117             :                                  Oid oldestXactDb);
     118             : static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
     119             :                                        TransactionId *subxids, XidStatus status,
     120             :                                        XLogRecPtr lsn, int64 pageno,
     121             :                                        bool all_xact_same_page);
     122             : static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status,
     123             :                                       XLogRecPtr lsn, int slotno);
     124             : static void set_status_by_pages(int nsubxids, TransactionId *subxids,
     125             :                                 XidStatus status, XLogRecPtr lsn);
     126             : static bool TransactionGroupUpdateXidStatus(TransactionId xid,
     127             :                                             XidStatus status, XLogRecPtr lsn, int64 pageno);
     128             : static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
     129             :                                                TransactionId *subxids, XidStatus status,
     130             :                                                XLogRecPtr lsn, int64 pageno);
     131             : 
     132             : 
     133             : /*
     134             :  * TransactionIdSetTreeStatus
     135             :  *
     136             :  * Record the final state of transaction entries in the commit log for
     137             :  * a transaction and its subtransaction tree. Take care to ensure this is
     138             :  * efficient, and as atomic as possible.
     139             :  *
     140             :  * xid is a single xid to set status for. This will typically be
     141             :  * the top level transactionid for a top level commit or abort. It can
     142             :  * also be a subtransaction when we record transaction aborts.
     143             :  *
     144             :  * subxids is an array of xids of length nsubxids, representing subtransactions
     145             :  * in the tree of xid. In various cases nsubxids may be zero.
     146             :  *
     147             :  * lsn must be the WAL location of the commit record when recording an async
     148             :  * commit.  For a synchronous commit it can be InvalidXLogRecPtr, since the
     149             :  * caller guarantees the commit record is already flushed in that case.  It
     150             :  * should be InvalidXLogRecPtr for abort cases, too.
     151             :  *
     152             :  * In the commit case, atomicity is limited by whether all the subxids are in
     153             :  * the same CLOG page as xid.  If they all are, then the lock will be grabbed
     154             :  * only once, and the status will be set to committed directly.  Otherwise
     155             :  * we must
     156             :  *   1. set sub-committed all subxids that are not on the same page as the
     157             :  *      main xid
     158             :  *   2. atomically set committed the main xid and the subxids on the same page
     159             :  *   3. go over the first bunch again and set them committed
     160             :  * Note that as far as concurrent checkers are concerned, main transaction
     161             :  * commit as a whole is still atomic.
     162             :  *
     163             :  * Example:
     164             :  *      TransactionId t commits and has subxids t1, t2, t3, t4
     165             :  *      t is on page p1, t1 is also on p1, t2 and t3 are on p2, t4 is on p3
     166             :  *      1. update pages2-3:
     167             :  *                  page2: set t2,t3 as sub-committed
     168             :  *                  page3: set t4 as sub-committed
     169             :  *      2. update page1:
     170             :  *                  page1: set t,t1 as committed
     171             :  *      3. update pages2-3:
     172             :  *                  page2: set t2,t3 as committed
     173             :  *                  page3: set t4 as committed
     174             :  *
     175             :  * NB: this is a low-level routine and is NOT the preferred entry point
     176             :  * for most uses; functions in transam.c are the intended callers.
     177             :  *
     178             :  * XXX Think about issuing POSIX_FADV_WILLNEED on pages that we will need,
     179             :  * but aren't yet in cache, as well as hinting pages not to fall out of
     180             :  * cache yet.
     181             :  */
     182             : void
     183      288958 : TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
     184             :                            TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
     185             : {
     186      288958 :     int64       pageno = TransactionIdToPage(xid);  /* get page of parent */
     187             :     int         i;
     188             : 
     189             :     Assert(status == TRANSACTION_STATUS_COMMITTED ||
     190             :            status == TRANSACTION_STATUS_ABORTED);
     191             : 
     192             :     /*
     193             :      * See how many subxids, if any, are on the same page as the parent, if
     194             :      * any.
     195             :      */
     196      299076 :     for (i = 0; i < nsubxids; i++)
     197             :     {
     198       10118 :         if (TransactionIdToPage(subxids[i]) != pageno)
     199           0 :             break;
     200             :     }
     201             : 
     202             :     /*
     203             :      * Do all items fit on a single page?
     204             :      */
     205      288958 :     if (i == nsubxids)
     206             :     {
     207             :         /*
     208             :          * Set the parent and all subtransactions in a single call
     209             :          */
     210      288958 :         TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
     211             :                                    pageno, true);
     212             :     }
     213             :     else
     214             :     {
     215           0 :         int         nsubxids_on_first_page = i;
     216             : 
     217             :         /*
     218             :          * If this is a commit then we care about doing this correctly (i.e.
     219             :          * using the subcommitted intermediate status).  By here, we know
     220             :          * we're updating more than one page of clog, so we must mark entries
     221             :          * that are *not* on the first page so that they show as subcommitted
     222             :          * before we then return to update the status to fully committed.
     223             :          *
     224             :          * To avoid touching the first page twice, skip marking subcommitted
     225             :          * for the subxids on that first page.
     226             :          */
     227           0 :         if (status == TRANSACTION_STATUS_COMMITTED)
     228           0 :             set_status_by_pages(nsubxids - nsubxids_on_first_page,
     229           0 :                                 subxids + nsubxids_on_first_page,
     230             :                                 TRANSACTION_STATUS_SUB_COMMITTED, lsn);
     231             : 
     232             :         /*
     233             :          * Now set the parent and subtransactions on same page as the parent,
     234             :          * if any
     235             :          */
     236           0 :         pageno = TransactionIdToPage(xid);
     237           0 :         TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
     238             :                                    lsn, pageno, false);
     239             : 
     240             :         /*
     241             :          * Now work through the rest of the subxids one clog page at a time,
     242             :          * starting from the second page onwards, like we did above.
     243             :          */
     244           0 :         set_status_by_pages(nsubxids - nsubxids_on_first_page,
     245           0 :                             subxids + nsubxids_on_first_page,
     246             :                             status, lsn);
     247             :     }
     248      288958 : }
     249             : 
     250             : /*
     251             :  * Helper for TransactionIdSetTreeStatus: set the status for a bunch of
     252             :  * transactions, chunking in the separate CLOG pages involved. We never
     253             :  * pass the whole transaction tree to this function, only subtransactions
     254             :  * that are on different pages to the top level transaction id.
     255             :  */
     256             : static void
     257           0 : set_status_by_pages(int nsubxids, TransactionId *subxids,
     258             :                     XidStatus status, XLogRecPtr lsn)
     259             : {
     260           0 :     int64       pageno = TransactionIdToPage(subxids[0]);
     261           0 :     int         offset = 0;
     262           0 :     int         i = 0;
     263             : 
     264             :     Assert(nsubxids > 0);        /* else the pageno fetch above is unsafe */
     265             : 
     266           0 :     while (i < nsubxids)
     267             :     {
     268           0 :         int         num_on_page = 0;
     269             :         int64       nextpageno;
     270             : 
     271             :         do
     272             :         {
     273           0 :             nextpageno = TransactionIdToPage(subxids[i]);
     274           0 :             if (nextpageno != pageno)
     275           0 :                 break;
     276           0 :             num_on_page++;
     277           0 :             i++;
     278           0 :         } while (i < nsubxids);
     279             : 
     280           0 :         TransactionIdSetPageStatus(InvalidTransactionId,
     281           0 :                                    num_on_page, subxids + offset,
     282             :                                    status, lsn, pageno, false);
     283           0 :         offset = i;
     284           0 :         pageno = nextpageno;
     285             :     }
     286           0 : }
     287             : 
     288             : /*
     289             :  * Record the final state of transaction entries in the commit log for all
     290             :  * entries on a single page.  Atomic only on this page.
     291             :  */
     292             : static void
     293      288958 : TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
     294             :                            TransactionId *subxids, XidStatus status,
     295             :                            XLogRecPtr lsn, int64 pageno,
     296             :                            bool all_xact_same_page)
     297             : {
     298             :     LWLock     *lock;
     299             : 
     300             :     /* Can't use group update when PGPROC overflows. */
     301             :     StaticAssertDecl(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS,
     302             :                      "group clog threshold less than PGPROC cached subxids");
     303             : 
     304             :     /* Get the SLRU bank lock for the page we are going to access. */
     305      288958 :     lock = SimpleLruGetBankLock(XactCtl, pageno);
     306             : 
     307             :     /*
     308             :      * When there is contention on the SLRU bank lock we need, we try to group
     309             :      * multiple updates; a single leader process will perform transaction
     310             :      * status updates for multiple backends so that the number of times the
     311             :      * bank lock needs to be acquired is reduced.
     312             :      *
     313             :      * For this optimization to be safe, the XID and subxids in MyProc must be
     314             :      * the same as the ones for which we're setting the status.  Check that
     315             :      * this is the case.
     316             :      *
     317             :      * For this optimization to be efficient, we shouldn't have too many
     318             :      * sub-XIDs and all of the XIDs for which we're adjusting clog should be
     319             :      * on the same page.  Check those conditions, too.
     320             :      */
     321      288958 :     if (all_xact_same_page && xid == MyProc->xid &&
     322      242098 :         nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
     323      242098 :         nsubxids == MyProc->subxidStatus.count &&
     324         888 :         (nsubxids == 0 ||
     325         888 :          memcmp(subxids, MyProc->subxids.xids,
     326             :                 nsubxids * sizeof(TransactionId)) == 0))
     327             :     {
     328             :         /*
     329             :          * If we can immediately acquire the lock, we update the status of our
     330             :          * own XID and release the lock.  If not, try use group XID update. If
     331             :          * that doesn't work out, fall back to waiting for the lock to perform
     332             :          * an update for this transaction only.
     333             :          */
     334      241868 :         if (LWLockConditionalAcquire(lock, LW_EXCLUSIVE))
     335             :         {
     336             :             /* Got the lock without waiting!  Do the update. */
     337      241674 :             TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
     338             :                                                lsn, pageno);
     339      241674 :             LWLockRelease(lock);
     340      241674 :             return;
     341             :         }
     342         194 :         else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
     343             :         {
     344             :             /* Group update mechanism has done the work. */
     345         194 :             return;
     346             :         }
     347             : 
     348             :         /* Fall through only if update isn't done yet. */
     349             :     }
     350             : 
     351             :     /* Group update not applicable, or couldn't accept this page number. */
     352       47090 :     LWLockAcquire(lock, LW_EXCLUSIVE);
     353       47090 :     TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
     354             :                                        lsn, pageno);
     355       47090 :     LWLockRelease(lock);
     356             : }
     357             : 
     358             : /*
     359             :  * Record the final state of transaction entry in the commit log
     360             :  *
     361             :  * We don't do any locking here; caller must handle that.
     362             :  */
     363             : static void
     364      288958 : TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
     365             :                                    TransactionId *subxids, XidStatus status,
     366             :                                    XLogRecPtr lsn, int64 pageno)
     367             : {
     368             :     int         slotno;
     369             :     int         i;
     370             : 
     371             :     Assert(status == TRANSACTION_STATUS_COMMITTED ||
     372             :            status == TRANSACTION_STATUS_ABORTED ||
     373             :            (status == TRANSACTION_STATUS_SUB_COMMITTED && !TransactionIdIsValid(xid)));
     374             :     Assert(LWLockHeldByMeInMode(SimpleLruGetBankLock(XactCtl, pageno),
     375             :                                 LW_EXCLUSIVE));
     376             : 
     377             :     /*
     378             :      * If we're doing an async commit (ie, lsn is valid), then we must wait
     379             :      * for any active write on the page slot to complete.  Otherwise our
     380             :      * update could reach disk in that write, which will not do since we
     381             :      * mustn't let it reach disk until we've done the appropriate WAL flush.
     382             :      * But when lsn is invalid, it's OK to scribble on a page while it is
     383             :      * write-busy, since we don't care if the update reaches disk sooner than
     384             :      * we think.
     385             :      */
     386      288958 :     slotno = SimpleLruReadPage(XactCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
     387             : 
     388             :     /*
     389             :      * Set the main transaction id, if any.
     390             :      *
     391             :      * If we update more than one xid on this page while it is being written
     392             :      * out, we might find that some of the bits go to disk and others don't.
     393             :      * If we are updating commits on the page with the top-level xid that
     394             :      * could break atomicity, so we subcommit the subxids first before we mark
     395             :      * the top-level commit.
     396             :      */
     397      288958 :     if (TransactionIdIsValid(xid))
     398             :     {
     399             :         /* Subtransactions first, if needed ... */
     400      288958 :         if (status == TRANSACTION_STATUS_COMMITTED)
     401             :         {
     402      283926 :             for (i = 0; i < nsubxids; i++)
     403             :             {
     404             :                 Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
     405        9476 :                 TransactionIdSetStatusBit(subxids[i],
     406             :                                           TRANSACTION_STATUS_SUB_COMMITTED,
     407             :                                           lsn, slotno);
     408             :             }
     409             :         }
     410             : 
     411             :         /* ... then the main transaction */
     412      288958 :         TransactionIdSetStatusBit(xid, status, lsn, slotno);
     413             :     }
     414             : 
     415             :     /* Set the subtransactions */
     416      299076 :     for (i = 0; i < nsubxids; i++)
     417             :     {
     418             :         Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
     419       10118 :         TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
     420             :     }
     421             : 
     422      288958 :     XactCtl->shared->page_dirty[slotno] = true;
     423      288958 : }
     424             : 
     425             : /*
     426             :  * Subroutine for TransactionIdSetPageStatus, q.v.
     427             :  *
     428             :  * When we cannot immediately acquire the SLRU bank lock in exclusive mode at
     429             :  * commit time, add ourselves to a list of processes that need their XIDs
     430             :  * status update.  The first process to add itself to the list will acquire
     431             :  * the lock in exclusive mode and set transaction status as required on behalf
     432             :  * of all group members.  This avoids a great deal of contention when many
     433             :  * processes are trying to commit at once, since the lock need not be
     434             :  * repeatedly handed off from one committing process to the next.
     435             :  *
     436             :  * Returns true when transaction status has been updated in clog; returns
     437             :  * false if we decided against applying the optimization because the page
     438             :  * number we need to update differs from those processes already waiting.
     439             :  */
     440             : static bool
     441         194 : TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
     442             :                                 XLogRecPtr lsn, int64 pageno)
     443             : {
     444         194 :     volatile PROC_HDR *procglobal = ProcGlobal;
     445         194 :     PGPROC     *proc = MyProc;
     446             :     uint32      nextidx;
     447             :     uint32      wakeidx;
     448             :     int64       prevpageno;
     449         194 :     LWLock     *prevlock = NULL;
     450             : 
     451             :     /* We should definitely have an XID whose status needs to be updated. */
     452             :     Assert(TransactionIdIsValid(xid));
     453             : 
     454             :     /*
     455             :      * Prepare to add ourselves to the list of processes needing a group XID
     456             :      * status update.
     457             :      */
     458         194 :     proc->clogGroupMember = true;
     459         194 :     proc->clogGroupMemberXid = xid;
     460         194 :     proc->clogGroupMemberXidStatus = status;
     461         194 :     proc->clogGroupMemberPage = pageno;
     462         194 :     proc->clogGroupMemberLsn = lsn;
     463             : 
     464             :     /*
     465             :      * We put ourselves in the queue by writing MyProcNumber to
     466             :      * ProcGlobal->clogGroupFirst.  However, if there's already a process
     467             :      * listed there, we compare our pageno with that of that process; if it
     468             :      * differs, we cannot participate in the group, so we return for caller to
     469             :      * update pg_xact in the normal way.
     470             :      *
     471             :      * If we're not the first process in the list, we must follow the leader.
     472             :      * We do this by storing the data we want updated in our PGPROC entry
     473             :      * where the leader can find it, then going to sleep.
     474             :      *
     475             :      * If no process is already in the list, we're the leader; our first step
     476             :      * is to lock the SLRU bank to which our page belongs, then we close out
     477             :      * the group by resetting the list pointer from ProcGlobal->clogGroupFirst
     478             :      * (this lets other processes set up other groups later); finally we do
     479             :      * the SLRU updates, release the SLRU bank lock, and wake up the sleeping
     480             :      * processes.
     481             :      *
     482             :      * If another group starts to update a page in a different SLRU bank, they
     483             :      * can proceed concurrently, since the bank lock they're going to use is
     484             :      * different from ours.  If another group starts to update a page in the
     485             :      * same bank as ours, they wait until we release the lock.
     486             :      */
     487         194 :     nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
     488             : 
     489             :     while (true)
     490             :     {
     491             :         /*
     492             :          * Add the proc to list, if the clog page where we need to update the
     493             :          * current transaction status is same as group leader's clog page.
     494             :          *
     495             :          * There is a race condition here, which is that after doing the below
     496             :          * check and before adding this proc's clog update to a group, the
     497             :          * group leader might have already finished the group update for this
     498             :          * page and becomes group leader of another group, updating a
     499             :          * different page.  This will lead to a situation where a single group
     500             :          * can have different clog page updates.  This isn't likely and will
     501             :          * still work, just less efficiently -- we handle this case by
     502             :          * switching to a different bank lock in the loop below.
     503             :          */
     504         194 :         if (nextidx != INVALID_PROC_NUMBER &&
     505           6 :             GetPGProcByNumber(nextidx)->clogGroupMemberPage != proc->clogGroupMemberPage)
     506             :         {
     507             :             /*
     508             :              * Ensure that this proc is not a member of any clog group that
     509             :              * needs an XID status update.
     510             :              */
     511           0 :             proc->clogGroupMember = false;
     512           0 :             pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PROC_NUMBER);
     513           0 :             return false;
     514             :         }
     515             : 
     516         194 :         pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
     517             : 
     518         194 :         if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst,
     519             :                                            &nextidx,
     520             :                                            (uint32) MyProcNumber))
     521         194 :             break;
     522             :     }
     523             : 
     524             :     /*
     525             :      * If the list was not empty, the leader will update the status of our
     526             :      * XID. It is impossible to have followers without a leader because the
     527             :      * first process that has added itself to the list will always have
     528             :      * nextidx as INVALID_PROC_NUMBER.
     529             :      */
     530         194 :     if (nextidx != INVALID_PROC_NUMBER)
     531             :     {
     532           6 :         int         extraWaits = 0;
     533             : 
     534             :         /* Sleep until the leader updates our XID status. */
     535           6 :         pgstat_report_wait_start(WAIT_EVENT_XACT_GROUP_UPDATE);
     536             :         for (;;)
     537             :         {
     538             :             /* acts as a read barrier */
     539           6 :             PGSemaphoreLock(proc->sem);
     540           6 :             if (!proc->clogGroupMember)
     541           6 :                 break;
     542           0 :             extraWaits++;
     543             :         }
     544           6 :         pgstat_report_wait_end();
     545             : 
     546             :         Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PROC_NUMBER);
     547             : 
     548             :         /* Fix semaphore count for any absorbed wakeups */
     549           6 :         while (extraWaits-- > 0)
     550           0 :             PGSemaphoreUnlock(proc->sem);
     551           6 :         return true;
     552             :     }
     553             : 
     554             :     /*
     555             :      * By here, we know we're the leader process.  Acquire the SLRU bank lock
     556             :      * that corresponds to the page we originally wanted to modify.
     557             :      */
     558         188 :     prevpageno = proc->clogGroupMemberPage;
     559         188 :     prevlock = SimpleLruGetBankLock(XactCtl, prevpageno);
     560         188 :     LWLockAcquire(prevlock, LW_EXCLUSIVE);
     561             : 
     562             :     /*
     563             :      * Now that we've got the lock, clear the list of processes waiting for
     564             :      * group XID status update, saving a pointer to the head of the list.
     565             :      * (Trying to pop elements one at a time could lead to an ABA problem.)
     566             :      *
     567             :      * At this point, any processes trying to do this would create a separate
     568             :      * group.
     569             :      */
     570         188 :     nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
     571             :                                      INVALID_PROC_NUMBER);
     572             : 
     573             :     /* Remember head of list so we can perform wakeups after dropping lock. */
     574         188 :     wakeidx = nextidx;
     575             : 
     576             :     /* Walk the list and update the status of all XIDs. */
     577         382 :     while (nextidx != INVALID_PROC_NUMBER)
     578             :     {
     579         194 :         PGPROC     *nextproc = &ProcGlobal->allProcs[nextidx];
     580         194 :         int64       thispageno = nextproc->clogGroupMemberPage;
     581             : 
     582             :         /*
     583             :          * If the page to update belongs to a different bank than the previous
     584             :          * one, exchange bank lock to the new one.  This should be quite rare,
     585             :          * as described above.
     586             :          *
     587             :          * (We could try to optimize this by waking up the processes for which
     588             :          * we have already updated the status while we exchange the lock, but
     589             :          * the code doesn't do that at present.  I think it'd require
     590             :          * additional bookkeeping, making the common path slower in order to
     591             :          * improve an infrequent case.)
     592             :          */
     593         194 :         if (thispageno != prevpageno)
     594             :         {
     595           0 :             LWLock     *lock = SimpleLruGetBankLock(XactCtl, thispageno);
     596             : 
     597           0 :             if (prevlock != lock)
     598             :             {
     599           0 :                 LWLockRelease(prevlock);
     600           0 :                 LWLockAcquire(lock, LW_EXCLUSIVE);
     601             :             }
     602           0 :             prevlock = lock;
     603           0 :             prevpageno = thispageno;
     604             :         }
     605             : 
     606             :         /*
     607             :          * Transactions with more than THRESHOLD_SUBTRANS_CLOG_OPT sub-XIDs
     608             :          * should not use group XID status update mechanism.
     609             :          */
     610             :         Assert(nextproc->subxidStatus.count <= THRESHOLD_SUBTRANS_CLOG_OPT);
     611             : 
     612         194 :         TransactionIdSetPageStatusInternal(nextproc->clogGroupMemberXid,
     613         194 :                                            nextproc->subxidStatus.count,
     614         194 :                                            nextproc->subxids.xids,
     615             :                                            nextproc->clogGroupMemberXidStatus,
     616             :                                            nextproc->clogGroupMemberLsn,
     617             :                                            nextproc->clogGroupMemberPage);
     618             : 
     619             :         /* Move to next proc in list. */
     620         194 :         nextidx = pg_atomic_read_u32(&nextproc->clogGroupNext);
     621             :     }
     622             : 
     623             :     /* We're done with the lock now. */
     624         188 :     if (prevlock != NULL)
     625         188 :         LWLockRelease(prevlock);
     626             : 
     627             :     /*
     628             :      * Now that we've released the lock, go back and wake everybody up.  We
     629             :      * don't do this under the lock so as to keep lock hold times to a
     630             :      * minimum.
     631             :      *
     632             :      * (Perhaps we could do this in two passes, the first setting
     633             :      * clogGroupNext to invalid while saving the semaphores to an array, then
     634             :      * a single write barrier, then another pass unlocking the semaphores.)
     635             :      */
     636         382 :     while (wakeidx != INVALID_PROC_NUMBER)
     637             :     {
     638         194 :         PGPROC     *wakeproc = &ProcGlobal->allProcs[wakeidx];
     639             : 
     640         194 :         wakeidx = pg_atomic_read_u32(&wakeproc->clogGroupNext);
     641         194 :         pg_atomic_write_u32(&wakeproc->clogGroupNext, INVALID_PROC_NUMBER);
     642             : 
     643             :         /* ensure all previous writes are visible before follower continues. */
     644         194 :         pg_write_barrier();
     645             : 
     646         194 :         wakeproc->clogGroupMember = false;
     647             : 
     648         194 :         if (wakeproc != MyProc)
     649           6 :             PGSemaphoreUnlock(wakeproc->sem);
     650             :     }
     651             : 
     652         188 :     return true;
     653             : }
     654             : 
     655             : /*
     656             :  * Sets the commit status of a single transaction.
     657             :  *
     658             :  * Caller must hold the corresponding SLRU bank lock, will be held at exit.
     659             :  */
     660             : static void
     661      308552 : TransactionIdSetStatusBit(TransactionId xid, XidStatus status, XLogRecPtr lsn, int slotno)
     662             : {
     663      308552 :     int         byteno = TransactionIdToByte(xid);
     664      308552 :     int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
     665             :     char       *byteptr;
     666             :     char        byteval;
     667             :     char        curval;
     668             : 
     669             :     Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(xid));
     670             :     Assert(LWLockHeldByMeInMode(SimpleLruGetBankLock(XactCtl,
     671             :                                                      XactCtl->shared->page_number[slotno]),
     672             :                                 LW_EXCLUSIVE));
     673             : 
     674      308552 :     byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
     675      308552 :     curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
     676             : 
     677             :     /*
     678             :      * When replaying transactions during recovery we still need to perform
     679             :      * the two phases of subcommit and then commit. However, some transactions
     680             :      * are already correctly marked, so we just treat those as a no-op which
     681             :      * allows us to keep the following Assert as restrictive as possible.
     682             :      */
     683      308552 :     if (InRecovery && status == TRANSACTION_STATUS_SUB_COMMITTED &&
     684             :         curval == TRANSACTION_STATUS_COMMITTED)
     685           0 :         return;
     686             : 
     687             :     /*
     688             :      * Current state change should be from 0 or subcommitted to target state
     689             :      * or we should already be there when replaying changes during recovery.
     690             :      */
     691             :     Assert(curval == 0 ||
     692             :            (curval == TRANSACTION_STATUS_SUB_COMMITTED &&
     693             :             status != TRANSACTION_STATUS_IN_PROGRESS) ||
     694             :            curval == status);
     695             : 
     696             :     /* note this assumes exclusive access to the clog page */
     697      308552 :     byteval = *byteptr;
     698      308552 :     byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
     699      308552 :     byteval |= (status << bshift);
     700      308552 :     *byteptr = byteval;
     701             : 
     702             :     /*
     703             :      * Update the group LSN if the transaction completion LSN is higher.
     704             :      *
     705             :      * Note: lsn will be invalid when supplied during InRecovery processing,
     706             :      * so we don't need to do anything special to avoid LSN updates during
     707             :      * recovery. After recovery completes the next clog change will set the
     708             :      * LSN correctly.
     709             :      */
     710      308552 :     if (!XLogRecPtrIsInvalid(lsn))
     711             :     {
     712       53302 :         int         lsnindex = GetLSNIndex(slotno, xid);
     713             : 
     714       53302 :         if (XactCtl->shared->group_lsn[lsnindex] < lsn)
     715       48080 :             XactCtl->shared->group_lsn[lsnindex] = lsn;
     716             :     }
     717             : }
     718             : 
     719             : /*
     720             :  * Interrogate the state of a transaction in the commit log.
     721             :  *
     722             :  * Aside from the actual commit status, this function returns (into *lsn)
     723             :  * an LSN that is late enough to be able to guarantee that if we flush up to
     724             :  * that LSN then we will have flushed the transaction's commit record to disk.
     725             :  * The result is not necessarily the exact LSN of the transaction's commit
     726             :  * record!  For example, for long-past transactions (those whose clog pages
     727             :  * already migrated to disk), we'll return InvalidXLogRecPtr.  Also, because
     728             :  * we group transactions on the same clog page to conserve storage, we might
     729             :  * return the LSN of a later transaction that falls into the same group.
     730             :  *
     731             :  * NB: this is a low-level routine and is NOT the preferred entry point
     732             :  * for most uses; TransactionLogFetch() in transam.c is the intended caller.
     733             :  */
     734             : XidStatus
     735     1368796 : TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
     736             : {
     737     1368796 :     int64       pageno = TransactionIdToPage(xid);
     738     1368796 :     int         byteno = TransactionIdToByte(xid);
     739     1368796 :     int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
     740             :     int         slotno;
     741             :     int         lsnindex;
     742             :     char       *byteptr;
     743             :     XidStatus   status;
     744             : 
     745             :     /* lock is acquired by SimpleLruReadPage_ReadOnly */
     746             : 
     747     1368796 :     slotno = SimpleLruReadPage_ReadOnly(XactCtl, pageno, xid);
     748     1368796 :     byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
     749             : 
     750     1368796 :     status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
     751             : 
     752     1368796 :     lsnindex = GetLSNIndex(slotno, xid);
     753     1368796 :     *lsn = XactCtl->shared->group_lsn[lsnindex];
     754             : 
     755     1368796 :     LWLockRelease(SimpleLruGetBankLock(XactCtl, pageno));
     756             : 
     757     1368796 :     return status;
     758             : }
     759             : 
     760             : /*
     761             :  * Number of shared CLOG buffers.
     762             :  *
     763             :  * If asked to autotune, use 2MB for every 1GB of shared buffers, up to 8MB.
     764             :  * Otherwise just cap the configured amount to be between 16 and the maximum
     765             :  * allowed.
     766             :  */
     767             : static int
     768        7352 : CLOGShmemBuffers(void)
     769             : {
     770             :     /* auto-tune based on shared buffers */
     771        7352 :     if (transaction_buffers == 0)
     772        5434 :         return SimpleLruAutotuneBuffers(512, 1024);
     773             : 
     774        1918 :     return Min(Max(16, transaction_buffers), CLOG_MAX_ALLOWED_BUFFERS);
     775             : }
     776             : 
     777             : /*
     778             :  * Initialization of shared memory for CLOG
     779             :  */
     780             : Size
     781        3546 : CLOGShmemSize(void)
     782             : {
     783        3546 :     return SimpleLruShmemSize(CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE);
     784             : }
     785             : 
     786             : void
     787        1908 : CLOGShmemInit(void)
     788             : {
     789             :     /* If auto-tuning is requested, now is the time to do it */
     790        1908 :     if (transaction_buffers == 0)
     791             :     {
     792             :         char        buf[32];
     793             : 
     794        1898 :         snprintf(buf, sizeof(buf), "%d", CLOGShmemBuffers());
     795        1898 :         SetConfigOption("transaction_buffers", buf, PGC_POSTMASTER,
     796             :                         PGC_S_DYNAMIC_DEFAULT);
     797             : 
     798             :         /*
     799             :          * We prefer to report this value's source as PGC_S_DYNAMIC_DEFAULT.
     800             :          * However, if the DBA explicitly set transaction_buffers = 0 in the
     801             :          * config file, then PGC_S_DYNAMIC_DEFAULT will fail to override that
     802             :          * and we must force the matter with PGC_S_OVERRIDE.
     803             :          */
     804        1898 :         if (transaction_buffers == 0)   /* failed to apply it? */
     805           0 :             SetConfigOption("transaction_buffers", buf, PGC_POSTMASTER,
     806             :                             PGC_S_OVERRIDE);
     807             :     }
     808             :     Assert(transaction_buffers != 0);
     809             : 
     810        1908 :     XactCtl->PagePrecedes = CLOGPagePrecedes;
     811        1908 :     SimpleLruInit(XactCtl, "transaction", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
     812             :                   "pg_xact", LWTRANCHE_XACT_BUFFER,
     813             :                   LWTRANCHE_XACT_SLRU, SYNC_HANDLER_CLOG, false);
     814             :     SlruPagePrecedesUnitTests(XactCtl, CLOG_XACTS_PER_PAGE);
     815        1908 : }
     816             : 
     817             : /*
     818             :  * GUC check_hook for transaction_buffers
     819             :  */
     820             : bool
     821        3870 : check_transaction_buffers(int *newval, void **extra, GucSource source)
     822             : {
     823        3870 :     return check_slru_buffers("transaction_buffers", newval);
     824             : }
     825             : 
     826             : /*
     827             :  * This func must be called ONCE on system install.  It creates
     828             :  * the initial CLOG segment.  (The CLOG directory is assumed to
     829             :  * have been created by initdb, and CLOGShmemInit must have been
     830             :  * called already.)
     831             :  */
     832             : void
     833          90 : BootStrapCLOG(void)
     834             : {
     835             :     int         slotno;
     836          90 :     LWLock     *lock = SimpleLruGetBankLock(XactCtl, 0);
     837             : 
     838          90 :     LWLockAcquire(lock, LW_EXCLUSIVE);
     839             : 
     840             :     /* Create and zero the first page of the commit log */
     841          90 :     slotno = ZeroCLOGPage(0, false);
     842             : 
     843             :     /* Make sure it's written out */
     844          90 :     SimpleLruWritePage(XactCtl, slotno);
     845             :     Assert(!XactCtl->shared->page_dirty[slotno]);
     846             : 
     847          90 :     LWLockRelease(lock);
     848          90 : }
     849             : 
     850             : /*
     851             :  * Initialize (or reinitialize) a page of CLOG to zeroes.
     852             :  * If writeXlog is true, also emit an XLOG record saying we did this.
     853             :  *
     854             :  * The page is not actually written, just set up in shared memory.
     855             :  * The slot number of the new page is returned.
     856             :  *
     857             :  * Control lock must be held at entry, and will be held at exit.
     858             :  */
     859             : static int
     860      863484 : ZeroCLOGPage(int64 pageno, bool writeXlog)
     861             : {
     862             :     int         slotno;
     863             : 
     864      863484 :     slotno = SimpleLruZeroPage(XactCtl, pageno);
     865             : 
     866      863484 :     if (writeXlog)
     867      863394 :         WriteZeroPageXlogRec(pageno);
     868             : 
     869      863484 :     return slotno;
     870             : }
     871             : 
     872             : /*
     873             :  * This must be called ONCE during postmaster or standalone-backend startup,
     874             :  * after StartupXLOG has initialized TransamVariables->nextXid.
     875             :  */
     876             : void
     877        1640 : StartupCLOG(void)
     878             : {
     879        1640 :     TransactionId xid = XidFromFullTransactionId(TransamVariables->nextXid);
     880        1640 :     int64       pageno = TransactionIdToPage(xid);
     881             : 
     882             :     /*
     883             :      * Initialize our idea of the latest page number.
     884             :      */
     885        1640 :     pg_atomic_write_u64(&XactCtl->shared->latest_page_number, pageno);
     886        1640 : }
     887             : 
     888             : /*
     889             :  * This must be called ONCE at the end of startup/recovery.
     890             :  */
     891             : void
     892        1534 : TrimCLOG(void)
     893             : {
     894        1534 :     TransactionId xid = XidFromFullTransactionId(TransamVariables->nextXid);
     895        1534 :     int64       pageno = TransactionIdToPage(xid);
     896        1534 :     LWLock     *lock = SimpleLruGetBankLock(XactCtl, pageno);
     897             : 
     898        1534 :     LWLockAcquire(lock, LW_EXCLUSIVE);
     899             : 
     900             :     /*
     901             :      * Zero out the remainder of the current clog page.  Under normal
     902             :      * circumstances it should be zeroes already, but it seems at least
     903             :      * theoretically possible that XLOG replay will have settled on a nextXID
     904             :      * value that is less than the last XID actually used and marked by the
     905             :      * previous database lifecycle (since subtransaction commit writes clog
     906             :      * but makes no WAL entry).  Let's just be safe. (We need not worry about
     907             :      * pages beyond the current one, since those will be zeroed when first
     908             :      * used.  For the same reason, there is no need to do anything when
     909             :      * nextXid is exactly at a page boundary; and it's likely that the
     910             :      * "current" page doesn't exist yet in that case.)
     911             :      */
     912        1534 :     if (TransactionIdToPgIndex(xid) != 0)
     913             :     {
     914        1532 :         int         byteno = TransactionIdToByte(xid);
     915        1532 :         int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
     916             :         int         slotno;
     917             :         char       *byteptr;
     918             : 
     919        1532 :         slotno = SimpleLruReadPage(XactCtl, pageno, false, xid);
     920        1532 :         byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
     921             : 
     922             :         /* Zero so-far-unused positions in the current byte */
     923        1532 :         *byteptr &= (1 << bshift) - 1;
     924             :         /* Zero the rest of the page */
     925        1532 :         MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
     926             : 
     927        1532 :         XactCtl->shared->page_dirty[slotno] = true;
     928             :     }
     929             : 
     930        1534 :     LWLockRelease(lock);
     931        1534 : }
     932             : 
     933             : /*
     934             :  * Perform a checkpoint --- either during shutdown, or on-the-fly
     935             :  */
     936             : void
     937        2476 : CheckPointCLOG(void)
     938             : {
     939             :     /*
     940             :      * Write dirty CLOG pages to disk.  This may result in sync requests
     941             :      * queued for later handling by ProcessSyncRequests(), as part of the
     942             :      * checkpoint.
     943             :      */
     944             :     TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
     945        2476 :     SimpleLruWriteAll(XactCtl, true);
     946             :     TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
     947        2476 : }
     948             : 
     949             : 
     950             : /*
     951             :  * Make sure that CLOG has room for a newly-allocated XID.
     952             :  *
     953             :  * NB: this is called while holding XidGenLock.  We want it to be very fast
     954             :  * most of the time; even when it's not so fast, no actual I/O need happen
     955             :  * unless we're forced to write out a dirty clog or xlog page to make room
     956             :  * in shared memory.
     957             :  */
     958             : void
     959    48976204 : ExtendCLOG(TransactionId newestXact)
     960             : {
     961             :     int64       pageno;
     962             :     LWLock     *lock;
     963             : 
     964             :     /*
     965             :      * No work except at first XID of a page.  But beware: just after
     966             :      * wraparound, the first XID of page zero is FirstNormalTransactionId.
     967             :      */
     968    48976204 :     if (TransactionIdToPgIndex(newestXact) != 0 &&
     969             :         !TransactionIdEquals(newestXact, FirstNormalTransactionId))
     970    48112810 :         return;
     971             : 
     972      863394 :     pageno = TransactionIdToPage(newestXact);
     973      863394 :     lock = SimpleLruGetBankLock(XactCtl, pageno);
     974             : 
     975      863394 :     LWLockAcquire(lock, LW_EXCLUSIVE);
     976             : 
     977             :     /* Zero the page and make an XLOG entry about it */
     978      863394 :     ZeroCLOGPage(pageno, true);
     979             : 
     980      863394 :     LWLockRelease(lock);
     981             : }
     982             : 
     983             : 
     984             : /*
     985             :  * Remove all CLOG segments before the one holding the passed transaction ID
     986             :  *
     987             :  * Before removing any CLOG data, we must flush XLOG to disk, to ensure
     988             :  * that any recently-emitted FREEZE_PAGE records have reached disk; otherwise
     989             :  * a crash and restart might leave us with some unfrozen tuples referencing
     990             :  * removed CLOG data.  We choose to emit a special TRUNCATE XLOG record too.
     991             :  * Replaying the deletion from XLOG is not critical, since the files could
     992             :  * just as well be removed later, but doing so prevents a long-running hot
     993             :  * standby server from acquiring an unreasonably bloated CLOG directory.
     994             :  *
     995             :  * Since CLOG segments hold a large number of transactions, the opportunity to
     996             :  * actually remove a segment is fairly rare, and so it seems best not to do
     997             :  * the XLOG flush unless we have confirmed that there is a removable segment.
     998             :  */
     999             : void
    1000         996 : TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
    1001             : {
    1002             :     int64       cutoffPage;
    1003             : 
    1004             :     /*
    1005             :      * The cutoff point is the start of the segment containing oldestXact. We
    1006             :      * pass the *page* containing oldestXact to SimpleLruTruncate.
    1007             :      */
    1008         996 :     cutoffPage = TransactionIdToPage(oldestXact);
    1009             : 
    1010             :     /* Check to see if there's any files that could be removed */
    1011         996 :     if (!SlruScanDirectory(XactCtl, SlruScanDirCbReportPresence, &cutoffPage))
    1012         802 :         return;                 /* nothing to remove */
    1013             : 
    1014             :     /*
    1015             :      * Advance oldestClogXid before truncating clog, so concurrent xact status
    1016             :      * lookups can ensure they don't attempt to access truncated-away clog.
    1017             :      *
    1018             :      * It's only necessary to do this if we will actually truncate away clog
    1019             :      * pages.
    1020             :      */
    1021         194 :     AdvanceOldestClogXid(oldestXact);
    1022             : 
    1023             :     /*
    1024             :      * Write XLOG record and flush XLOG to disk. We record the oldest xid
    1025             :      * we're keeping information about here so we can ensure that it's always
    1026             :      * ahead of clog truncation in case we crash, and so a standby finds out
    1027             :      * the new valid xid before the next checkpoint.
    1028             :      */
    1029         194 :     WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
    1030             : 
    1031             :     /* Now we can remove the old CLOG segment(s) */
    1032         194 :     SimpleLruTruncate(XactCtl, cutoffPage);
    1033             : }
    1034             : 
    1035             : 
    1036             : /*
    1037             :  * Decide whether a CLOG page number is "older" for truncation purposes.
    1038             :  *
    1039             :  * We need to use comparison of TransactionIds here in order to do the right
    1040             :  * thing with wraparound XID arithmetic.  However, TransactionIdPrecedes()
    1041             :  * would get weird about permanent xact IDs.  So, offset both such that xid1,
    1042             :  * xid2, and xid2 + CLOG_XACTS_PER_PAGE - 1 are all normal XIDs; this offset
    1043             :  * is relevant to page 0 and to the page preceding page 0.
    1044             :  *
    1045             :  * The page containing oldestXact-2^31 is the important edge case.  The
    1046             :  * portion of that page equaling or following oldestXact-2^31 is expendable,
    1047             :  * but the portion preceding oldestXact-2^31 is not.  When oldestXact-2^31 is
    1048             :  * the first XID of a page and segment, the entire page and segment is
    1049             :  * expendable, and we could truncate the segment.  Recognizing that case would
    1050             :  * require making oldestXact, not just the page containing oldestXact,
    1051             :  * available to this callback.  The benefit would be rare and small, so we
    1052             :  * don't optimize that edge case.
    1053             :  */
    1054             : static bool
    1055      661134 : CLOGPagePrecedes(int64 page1, int64 page2)
    1056             : {
    1057             :     TransactionId xid1;
    1058             :     TransactionId xid2;
    1059             : 
    1060      661134 :     xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
    1061      661134 :     xid1 += FirstNormalTransactionId + 1;
    1062      661134 :     xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
    1063      661134 :     xid2 += FirstNormalTransactionId + 1;
    1064             : 
    1065      715078 :     return (TransactionIdPrecedes(xid1, xid2) &&
    1066       53944 :             TransactionIdPrecedes(xid1, xid2 + CLOG_XACTS_PER_PAGE - 1));
    1067             : }
    1068             : 
    1069             : 
    1070             : /*
    1071             :  * Write a ZEROPAGE xlog record
    1072             :  */
    1073             : static void
    1074      863394 : WriteZeroPageXlogRec(int64 pageno)
    1075             : {
    1076      863394 :     XLogBeginInsert();
    1077      863394 :     XLogRegisterData((char *) (&pageno), sizeof(pageno));
    1078      863394 :     (void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE);
    1079      863394 : }
    1080             : 
    1081             : /*
    1082             :  * Write a TRUNCATE xlog record
    1083             :  *
    1084             :  * We must flush the xlog record to disk before returning --- see notes
    1085             :  * in TruncateCLOG().
    1086             :  */
    1087             : static void
    1088         194 : WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact, Oid oldestXactDb)
    1089             : {
    1090             :     XLogRecPtr  recptr;
    1091             :     xl_clog_truncate xlrec;
    1092             : 
    1093         194 :     xlrec.pageno = pageno;
    1094         194 :     xlrec.oldestXact = oldestXact;
    1095         194 :     xlrec.oldestXactDb = oldestXactDb;
    1096             : 
    1097         194 :     XLogBeginInsert();
    1098         194 :     XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
    1099         194 :     recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
    1100         194 :     XLogFlush(recptr);
    1101         194 : }
    1102             : 
    1103             : /*
    1104             :  * CLOG resource manager's routines
    1105             :  */
    1106             : void
    1107           0 : clog_redo(XLogReaderState *record)
    1108             : {
    1109           0 :     uint8       info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
    1110             : 
    1111             :     /* Backup blocks are not used in clog records */
    1112             :     Assert(!XLogRecHasAnyBlockRefs(record));
    1113             : 
    1114           0 :     if (info == CLOG_ZEROPAGE)
    1115             :     {
    1116             :         int64       pageno;
    1117             :         int         slotno;
    1118             :         LWLock     *lock;
    1119             : 
    1120           0 :         memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));
    1121             : 
    1122           0 :         lock = SimpleLruGetBankLock(XactCtl, pageno);
    1123           0 :         LWLockAcquire(lock, LW_EXCLUSIVE);
    1124             : 
    1125           0 :         slotno = ZeroCLOGPage(pageno, false);
    1126           0 :         SimpleLruWritePage(XactCtl, slotno);
    1127             :         Assert(!XactCtl->shared->page_dirty[slotno]);
    1128             : 
    1129           0 :         LWLockRelease(lock);
    1130             :     }
    1131           0 :     else if (info == CLOG_TRUNCATE)
    1132             :     {
    1133             :         xl_clog_truncate xlrec;
    1134             : 
    1135           0 :         memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
    1136             : 
    1137           0 :         AdvanceOldestClogXid(xlrec.oldestXact);
    1138             : 
    1139           0 :         SimpleLruTruncate(XactCtl, xlrec.pageno);
    1140             :     }
    1141             :     else
    1142           0 :         elog(PANIC, "clog_redo: unknown op code %u", info);
    1143           0 : }
    1144             : 
    1145             : /*
    1146             :  * Entrypoint for sync.c to sync clog files.
    1147             :  */
    1148             : int
    1149           0 : clogsyncfiletag(const FileTag *ftag, char *path)
    1150             : {
    1151           0 :     return SlruSyncFileTag(XactCtl, ftag, path);
    1152             : }

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