LCOV - code coverage report
Current view: top level - src/backend/access/transam - clog.c (source / functions) Hit Total Coverage
Test: PostgreSQL 19devel Lines: 183 235 77.9 %
Date: 2025-07-27 06:17:33 Functions: 19 22 86.4 %
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) 1996-2025, 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     2616072 : TransactionIdToPage(TransactionId xid)
      83             : {
      84     2616072 :     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 bool CLOGPagePrecedes(int64 page1, int64 page2);
     114             : static void WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact,
     115             :                                  Oid oldestXactDb);
     116             : static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
     117             :                                        TransactionId *subxids, XidStatus status,
     118             :                                        XLogRecPtr lsn, int64 pageno,
     119             :                                        bool all_xact_same_page);
     120             : static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status,
     121             :                                       XLogRecPtr lsn, int slotno);
     122             : static void set_status_by_pages(int nsubxids, TransactionId *subxids,
     123             :                                 XidStatus status, XLogRecPtr lsn);
     124             : static bool TransactionGroupUpdateXidStatus(TransactionId xid,
     125             :                                             XidStatus status, XLogRecPtr lsn, int64 pageno);
     126             : static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
     127             :                                                TransactionId *subxids, XidStatus status,
     128             :                                                XLogRecPtr lsn, int64 pageno);
     129             : 
     130             : 
     131             : /*
     132             :  * TransactionIdSetTreeStatus
     133             :  *
     134             :  * Record the final state of transaction entries in the commit log for
     135             :  * a transaction and its subtransaction tree. Take care to ensure this is
     136             :  * efficient, and as atomic as possible.
     137             :  *
     138             :  * xid is a single xid to set status for. This will typically be
     139             :  * the top level transactionid for a top level commit or abort. It can
     140             :  * also be a subtransaction when we record transaction aborts.
     141             :  *
     142             :  * subxids is an array of xids of length nsubxids, representing subtransactions
     143             :  * in the tree of xid. In various cases nsubxids may be zero.
     144             :  *
     145             :  * lsn must be the WAL location of the commit record when recording an async
     146             :  * commit.  For a synchronous commit it can be InvalidXLogRecPtr, since the
     147             :  * caller guarantees the commit record is already flushed in that case.  It
     148             :  * should be InvalidXLogRecPtr for abort cases, too.
     149             :  *
     150             :  * In the commit case, atomicity is limited by whether all the subxids are in
     151             :  * the same CLOG page as xid.  If they all are, then the lock will be grabbed
     152             :  * only once, and the status will be set to committed directly.  Otherwise
     153             :  * we must
     154             :  *   1. set sub-committed all subxids that are not on the same page as the
     155             :  *      main xid
     156             :  *   2. atomically set committed the main xid and the subxids on the same page
     157             :  *   3. go over the first bunch again and set them committed
     158             :  * Note that as far as concurrent checkers are concerned, main transaction
     159             :  * commit as a whole is still atomic.
     160             :  *
     161             :  * Example:
     162             :  *      TransactionId t commits and has subxids t1, t2, t3, t4
     163             :  *      t is on page p1, t1 is also on p1, t2 and t3 are on p2, t4 is on p3
     164             :  *      1. update pages2-3:
     165             :  *                  page2: set t2,t3 as sub-committed
     166             :  *                  page3: set t4 as sub-committed
     167             :  *      2. update page1:
     168             :  *                  page1: set t,t1 as committed
     169             :  *      3. update pages2-3:
     170             :  *                  page2: set t2,t3 as committed
     171             :  *                  page3: set t4 as committed
     172             :  *
     173             :  * NB: this is a low-level routine and is NOT the preferred entry point
     174             :  * for most uses; functions in transam.c are the intended callers.
     175             :  *
     176             :  * XXX Think about issuing POSIX_FADV_WILLNEED on pages that we will need,
     177             :  * but aren't yet in cache, as well as hinting pages not to fall out of
     178             :  * cache yet.
     179             :  */
     180             : void
     181      313078 : TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
     182             :                            TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
     183             : {
     184      313078 :     int64       pageno = TransactionIdToPage(xid);  /* get page of parent */
     185             :     int         i;
     186             : 
     187             :     Assert(status == TRANSACTION_STATUS_COMMITTED ||
     188             :            status == TRANSACTION_STATUS_ABORTED);
     189             : 
     190             :     /*
     191             :      * See how many subxids, if any, are on the same page as the parent, if
     192             :      * any.
     193             :      */
     194      323084 :     for (i = 0; i < nsubxids; i++)
     195             :     {
     196       10006 :         if (TransactionIdToPage(subxids[i]) != pageno)
     197           0 :             break;
     198             :     }
     199             : 
     200             :     /*
     201             :      * Do all items fit on a single page?
     202             :      */
     203      313078 :     if (i == nsubxids)
     204             :     {
     205             :         /*
     206             :          * Set the parent and all subtransactions in a single call
     207             :          */
     208      313078 :         TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
     209             :                                    pageno, true);
     210             :     }
     211             :     else
     212             :     {
     213           0 :         int         nsubxids_on_first_page = i;
     214             : 
     215             :         /*
     216             :          * If this is a commit then we care about doing this correctly (i.e.
     217             :          * using the subcommitted intermediate status).  By here, we know
     218             :          * we're updating more than one page of clog, so we must mark entries
     219             :          * that are *not* on the first page so that they show as subcommitted
     220             :          * before we then return to update the status to fully committed.
     221             :          *
     222             :          * To avoid touching the first page twice, skip marking subcommitted
     223             :          * for the subxids on that first page.
     224             :          */
     225           0 :         if (status == TRANSACTION_STATUS_COMMITTED)
     226           0 :             set_status_by_pages(nsubxids - nsubxids_on_first_page,
     227           0 :                                 subxids + nsubxids_on_first_page,
     228             :                                 TRANSACTION_STATUS_SUB_COMMITTED, lsn);
     229             : 
     230             :         /*
     231             :          * Now set the parent and subtransactions on same page as the parent,
     232             :          * if any
     233             :          */
     234           0 :         pageno = TransactionIdToPage(xid);
     235           0 :         TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
     236             :                                    lsn, pageno, false);
     237             : 
     238             :         /*
     239             :          * Now work through the rest of the subxids one clog page at a time,
     240             :          * starting from the second page onwards, like we did above.
     241             :          */
     242           0 :         set_status_by_pages(nsubxids - nsubxids_on_first_page,
     243           0 :                             subxids + nsubxids_on_first_page,
     244             :                             status, lsn);
     245             :     }
     246      313078 : }
     247             : 
     248             : /*
     249             :  * Helper for TransactionIdSetTreeStatus: set the status for a bunch of
     250             :  * transactions, chunking in the separate CLOG pages involved. We never
     251             :  * pass the whole transaction tree to this function, only subtransactions
     252             :  * that are on different pages to the top level transaction id.
     253             :  */
     254             : static void
     255           0 : set_status_by_pages(int nsubxids, TransactionId *subxids,
     256             :                     XidStatus status, XLogRecPtr lsn)
     257             : {
     258           0 :     int64       pageno = TransactionIdToPage(subxids[0]);
     259           0 :     int         offset = 0;
     260           0 :     int         i = 0;
     261             : 
     262             :     Assert(nsubxids > 0);        /* else the pageno fetch above is unsafe */
     263             : 
     264           0 :     while (i < nsubxids)
     265             :     {
     266           0 :         int         num_on_page = 0;
     267             :         int64       nextpageno;
     268             : 
     269             :         do
     270             :         {
     271           0 :             nextpageno = TransactionIdToPage(subxids[i]);
     272           0 :             if (nextpageno != pageno)
     273           0 :                 break;
     274           0 :             num_on_page++;
     275           0 :             i++;
     276           0 :         } while (i < nsubxids);
     277             : 
     278           0 :         TransactionIdSetPageStatus(InvalidTransactionId,
     279           0 :                                    num_on_page, subxids + offset,
     280             :                                    status, lsn, pageno, false);
     281           0 :         offset = i;
     282           0 :         pageno = nextpageno;
     283             :     }
     284           0 : }
     285             : 
     286             : /*
     287             :  * Record the final state of transaction entries in the commit log for all
     288             :  * entries on a single page.  Atomic only on this page.
     289             :  */
     290             : static void
     291      313078 : TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
     292             :                            TransactionId *subxids, XidStatus status,
     293             :                            XLogRecPtr lsn, int64 pageno,
     294             :                            bool all_xact_same_page)
     295             : {
     296             :     LWLock     *lock;
     297             : 
     298             :     /* Can't use group update when PGPROC overflows. */
     299             :     StaticAssertDecl(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS,
     300             :                      "group clog threshold less than PGPROC cached subxids");
     301             : 
     302             :     /* Get the SLRU bank lock for the page we are going to access. */
     303      313078 :     lock = SimpleLruGetBankLock(XactCtl, pageno);
     304             : 
     305             :     /*
     306             :      * When there is contention on the SLRU bank lock we need, we try to group
     307             :      * multiple updates; a single leader process will perform transaction
     308             :      * status updates for multiple backends so that the number of times the
     309             :      * bank lock needs to be acquired is reduced.
     310             :      *
     311             :      * For this optimization to be safe, the XID and subxids in MyProc must be
     312             :      * the same as the ones for which we're setting the status.  Check that
     313             :      * this is the case.
     314             :      *
     315             :      * For this optimization to be efficient, we shouldn't have too many
     316             :      * sub-XIDs and all of the XIDs for which we're adjusting clog should be
     317             :      * on the same page.  Check those conditions, too.
     318             :      */
     319      313078 :     if (all_xact_same_page && xid == MyProc->xid &&
     320      264170 :         nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
     321      264170 :         nsubxids == MyProc->subxidStatus.count &&
     322         888 :         (nsubxids == 0 ||
     323         888 :          memcmp(subxids, MyProc->subxids.xids,
     324             :                 nsubxids * sizeof(TransactionId)) == 0))
     325             :     {
     326             :         /*
     327             :          * If we can immediately acquire the lock, we update the status of our
     328             :          * own XID and release the lock.  If not, try use group XID update. If
     329             :          * that doesn't work out, fall back to waiting for the lock to perform
     330             :          * an update for this transaction only.
     331             :          */
     332      263940 :         if (LWLockConditionalAcquire(lock, LW_EXCLUSIVE))
     333             :         {
     334             :             /* Got the lock without waiting!  Do the update. */
     335      263754 :             TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
     336             :                                                lsn, pageno);
     337      263754 :             LWLockRelease(lock);
     338      263754 :             return;
     339             :         }
     340         186 :         else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
     341             :         {
     342             :             /* Group update mechanism has done the work. */
     343         186 :             return;
     344             :         }
     345             : 
     346             :         /* Fall through only if update isn't done yet. */
     347             :     }
     348             : 
     349             :     /* Group update not applicable, or couldn't accept this page number. */
     350       49138 :     LWLockAcquire(lock, LW_EXCLUSIVE);
     351       49138 :     TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
     352             :                                        lsn, pageno);
     353       49138 :     LWLockRelease(lock);
     354             : }
     355             : 
     356             : /*
     357             :  * Record the final state of transaction entry in the commit log
     358             :  *
     359             :  * We don't do any locking here; caller must handle that.
     360             :  */
     361             : static void
     362      313078 : TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
     363             :                                    TransactionId *subxids, XidStatus status,
     364             :                                    XLogRecPtr lsn, int64 pageno)
     365             : {
     366             :     int         slotno;
     367             :     int         i;
     368             : 
     369             :     Assert(status == TRANSACTION_STATUS_COMMITTED ||
     370             :            status == TRANSACTION_STATUS_ABORTED ||
     371             :            (status == TRANSACTION_STATUS_SUB_COMMITTED && !TransactionIdIsValid(xid)));
     372             :     Assert(LWLockHeldByMeInMode(SimpleLruGetBankLock(XactCtl, pageno),
     373             :                                 LW_EXCLUSIVE));
     374             : 
     375             :     /*
     376             :      * If we're doing an async commit (ie, lsn is valid), then we must wait
     377             :      * for any active write on the page slot to complete.  Otherwise our
     378             :      * update could reach disk in that write, which will not do since we
     379             :      * mustn't let it reach disk until we've done the appropriate WAL flush.
     380             :      * But when lsn is invalid, it's OK to scribble on a page while it is
     381             :      * write-busy, since we don't care if the update reaches disk sooner than
     382             :      * we think.
     383             :      */
     384      313078 :     slotno = SimpleLruReadPage(XactCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
     385             : 
     386             :     /*
     387             :      * Set the main transaction id, if any.
     388             :      *
     389             :      * If we update more than one xid on this page while it is being written
     390             :      * out, we might find that some of the bits go to disk and others don't.
     391             :      * If we are updating commits on the page with the top-level xid that
     392             :      * could break atomicity, so we subcommit the subxids first before we mark
     393             :      * the top-level commit.
     394             :      */
     395      313078 :     if (TransactionIdIsValid(xid))
     396             :     {
     397             :         /* Subtransactions first, if needed ... */
     398      313078 :         if (status == TRANSACTION_STATUS_COMMITTED)
     399             :         {
     400      306936 :             for (i = 0; i < nsubxids; i++)
     401             :             {
     402             :                 Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
     403        9366 :                 TransactionIdSetStatusBit(subxids[i],
     404             :                                           TRANSACTION_STATUS_SUB_COMMITTED,
     405             :                                           lsn, slotno);
     406             :             }
     407             :         }
     408             : 
     409             :         /* ... then the main transaction */
     410      313078 :         TransactionIdSetStatusBit(xid, status, lsn, slotno);
     411             :     }
     412             : 
     413             :     /* Set the subtransactions */
     414      323084 :     for (i = 0; i < nsubxids; i++)
     415             :     {
     416             :         Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
     417       10006 :         TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
     418             :     }
     419             : 
     420      313078 :     XactCtl->shared->page_dirty[slotno] = true;
     421      313078 : }
     422             : 
     423             : /*
     424             :  * Subroutine for TransactionIdSetPageStatus, q.v.
     425             :  *
     426             :  * When we cannot immediately acquire the SLRU bank lock in exclusive mode at
     427             :  * commit time, add ourselves to a list of processes that need their XIDs
     428             :  * status update.  The first process to add itself to the list will acquire
     429             :  * the lock in exclusive mode and set transaction status as required on behalf
     430             :  * of all group members.  This avoids a great deal of contention when many
     431             :  * processes are trying to commit at once, since the lock need not be
     432             :  * repeatedly handed off from one committing process to the next.
     433             :  *
     434             :  * Returns true when transaction status has been updated in clog; returns
     435             :  * false if we decided against applying the optimization because the page
     436             :  * number we need to update differs from those processes already waiting.
     437             :  */
     438             : static bool
     439         186 : TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
     440             :                                 XLogRecPtr lsn, int64 pageno)
     441             : {
     442         186 :     volatile PROC_HDR *procglobal = ProcGlobal;
     443         186 :     PGPROC     *proc = MyProc;
     444             :     uint32      nextidx;
     445             :     uint32      wakeidx;
     446             :     int64       prevpageno;
     447         186 :     LWLock     *prevlock = NULL;
     448             : 
     449             :     /* We should definitely have an XID whose status needs to be updated. */
     450             :     Assert(TransactionIdIsValid(xid));
     451             : 
     452             :     /*
     453             :      * Prepare to add ourselves to the list of processes needing a group XID
     454             :      * status update.
     455             :      */
     456         186 :     proc->clogGroupMember = true;
     457         186 :     proc->clogGroupMemberXid = xid;
     458         186 :     proc->clogGroupMemberXidStatus = status;
     459         186 :     proc->clogGroupMemberPage = pageno;
     460         186 :     proc->clogGroupMemberLsn = lsn;
     461             : 
     462             :     /*
     463             :      * We put ourselves in the queue by writing MyProcNumber to
     464             :      * ProcGlobal->clogGroupFirst.  However, if there's already a process
     465             :      * listed there, we compare our pageno with that of that process; if it
     466             :      * differs, we cannot participate in the group, so we return for caller to
     467             :      * update pg_xact in the normal way.
     468             :      *
     469             :      * If we're not the first process in the list, we must follow the leader.
     470             :      * We do this by storing the data we want updated in our PGPROC entry
     471             :      * where the leader can find it, then going to sleep.
     472             :      *
     473             :      * If no process is already in the list, we're the leader; our first step
     474             :      * is to lock the SLRU bank to which our page belongs, then we close out
     475             :      * the group by resetting the list pointer from ProcGlobal->clogGroupFirst
     476             :      * (this lets other processes set up other groups later); finally we do
     477             :      * the SLRU updates, release the SLRU bank lock, and wake up the sleeping
     478             :      * processes.
     479             :      *
     480             :      * If another group starts to update a page in a different SLRU bank, they
     481             :      * can proceed concurrently, since the bank lock they're going to use is
     482             :      * different from ours.  If another group starts to update a page in the
     483             :      * same bank as ours, they wait until we release the lock.
     484             :      */
     485         186 :     nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
     486             : 
     487             :     while (true)
     488             :     {
     489             :         /*
     490             :          * Add the proc to list, if the clog page where we need to update the
     491             :          * current transaction status is same as group leader's clog page.
     492             :          *
     493             :          * There is a race condition here, which is that after doing the below
     494             :          * check and before adding this proc's clog update to a group, the
     495             :          * group leader might have already finished the group update for this
     496             :          * page and becomes group leader of another group, updating a
     497             :          * different page.  This will lead to a situation where a single group
     498             :          * can have different clog page updates.  This isn't likely and will
     499             :          * still work, just less efficiently -- we handle this case by
     500             :          * switching to a different bank lock in the loop below.
     501             :          */
     502         186 :         if (nextidx != INVALID_PROC_NUMBER &&
     503           6 :             GetPGProcByNumber(nextidx)->clogGroupMemberPage != proc->clogGroupMemberPage)
     504             :         {
     505             :             /*
     506             :              * Ensure that this proc is not a member of any clog group that
     507             :              * needs an XID status update.
     508             :              */
     509           0 :             proc->clogGroupMember = false;
     510           0 :             pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PROC_NUMBER);
     511           0 :             return false;
     512             :         }
     513             : 
     514         186 :         pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
     515             : 
     516         186 :         if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst,
     517             :                                            &nextidx,
     518             :                                            (uint32) MyProcNumber))
     519         186 :             break;
     520             :     }
     521             : 
     522             :     /*
     523             :      * If the list was not empty, the leader will update the status of our
     524             :      * XID. It is impossible to have followers without a leader because the
     525             :      * first process that has added itself to the list will always have
     526             :      * nextidx as INVALID_PROC_NUMBER.
     527             :      */
     528         186 :     if (nextidx != INVALID_PROC_NUMBER)
     529             :     {
     530           6 :         int         extraWaits = 0;
     531             : 
     532             :         /* Sleep until the leader updates our XID status. */
     533           6 :         pgstat_report_wait_start(WAIT_EVENT_XACT_GROUP_UPDATE);
     534             :         for (;;)
     535             :         {
     536             :             /* acts as a read barrier */
     537           6 :             PGSemaphoreLock(proc->sem);
     538           6 :             if (!proc->clogGroupMember)
     539           6 :                 break;
     540           0 :             extraWaits++;
     541             :         }
     542           6 :         pgstat_report_wait_end();
     543             : 
     544             :         Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PROC_NUMBER);
     545             : 
     546             :         /* Fix semaphore count for any absorbed wakeups */
     547           6 :         while (extraWaits-- > 0)
     548           0 :             PGSemaphoreUnlock(proc->sem);
     549           6 :         return true;
     550             :     }
     551             : 
     552             :     /*
     553             :      * By here, we know we're the leader process.  Acquire the SLRU bank lock
     554             :      * that corresponds to the page we originally wanted to modify.
     555             :      */
     556         180 :     prevpageno = proc->clogGroupMemberPage;
     557         180 :     prevlock = SimpleLruGetBankLock(XactCtl, prevpageno);
     558         180 :     LWLockAcquire(prevlock, LW_EXCLUSIVE);
     559             : 
     560             :     /*
     561             :      * Now that we've got the lock, clear the list of processes waiting for
     562             :      * group XID status update, saving a pointer to the head of the list.
     563             :      * (Trying to pop elements one at a time could lead to an ABA problem.)
     564             :      *
     565             :      * At this point, any processes trying to do this would create a separate
     566             :      * group.
     567             :      */
     568         180 :     nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
     569             :                                      INVALID_PROC_NUMBER);
     570             : 
     571             :     /* Remember head of list so we can perform wakeups after dropping lock. */
     572         180 :     wakeidx = nextidx;
     573             : 
     574             :     /* Walk the list and update the status of all XIDs. */
     575         366 :     while (nextidx != INVALID_PROC_NUMBER)
     576             :     {
     577         186 :         PGPROC     *nextproc = &ProcGlobal->allProcs[nextidx];
     578         186 :         int64       thispageno = nextproc->clogGroupMemberPage;
     579             : 
     580             :         /*
     581             :          * If the page to update belongs to a different bank than the previous
     582             :          * one, exchange bank lock to the new one.  This should be quite rare,
     583             :          * as described above.
     584             :          *
     585             :          * (We could try to optimize this by waking up the processes for which
     586             :          * we have already updated the status while we exchange the lock, but
     587             :          * the code doesn't do that at present.  I think it'd require
     588             :          * additional bookkeeping, making the common path slower in order to
     589             :          * improve an infrequent case.)
     590             :          */
     591         186 :         if (thispageno != prevpageno)
     592             :         {
     593           0 :             LWLock     *lock = SimpleLruGetBankLock(XactCtl, thispageno);
     594             : 
     595           0 :             if (prevlock != lock)
     596             :             {
     597           0 :                 LWLockRelease(prevlock);
     598           0 :                 LWLockAcquire(lock, LW_EXCLUSIVE);
     599             :             }
     600           0 :             prevlock = lock;
     601           0 :             prevpageno = thispageno;
     602             :         }
     603             : 
     604             :         /*
     605             :          * Transactions with more than THRESHOLD_SUBTRANS_CLOG_OPT sub-XIDs
     606             :          * should not use group XID status update mechanism.
     607             :          */
     608             :         Assert(nextproc->subxidStatus.count <= THRESHOLD_SUBTRANS_CLOG_OPT);
     609             : 
     610         186 :         TransactionIdSetPageStatusInternal(nextproc->clogGroupMemberXid,
     611         186 :                                            nextproc->subxidStatus.count,
     612         186 :                                            nextproc->subxids.xids,
     613             :                                            nextproc->clogGroupMemberXidStatus,
     614             :                                            nextproc->clogGroupMemberLsn,
     615             :                                            nextproc->clogGroupMemberPage);
     616             : 
     617             :         /* Move to next proc in list. */
     618         186 :         nextidx = pg_atomic_read_u32(&nextproc->clogGroupNext);
     619             :     }
     620             : 
     621             :     /* We're done with the lock now. */
     622         180 :     if (prevlock != NULL)
     623         180 :         LWLockRelease(prevlock);
     624             : 
     625             :     /*
     626             :      * Now that we've released the lock, go back and wake everybody up.  We
     627             :      * don't do this under the lock so as to keep lock hold times to a
     628             :      * minimum.
     629             :      *
     630             :      * (Perhaps we could do this in two passes, the first setting
     631             :      * clogGroupNext to invalid while saving the semaphores to an array, then
     632             :      * a single write barrier, then another pass unlocking the semaphores.)
     633             :      */
     634         366 :     while (wakeidx != INVALID_PROC_NUMBER)
     635             :     {
     636         186 :         PGPROC     *wakeproc = &ProcGlobal->allProcs[wakeidx];
     637             : 
     638         186 :         wakeidx = pg_atomic_read_u32(&wakeproc->clogGroupNext);
     639         186 :         pg_atomic_write_u32(&wakeproc->clogGroupNext, INVALID_PROC_NUMBER);
     640             : 
     641             :         /* ensure all previous writes are visible before follower continues. */
     642         186 :         pg_write_barrier();
     643             : 
     644         186 :         wakeproc->clogGroupMember = false;
     645             : 
     646         186 :         if (wakeproc != MyProc)
     647           6 :             PGSemaphoreUnlock(wakeproc->sem);
     648             :     }
     649             : 
     650         180 :     return true;
     651             : }
     652             : 
     653             : /*
     654             :  * Sets the commit status of a single transaction.
     655             :  *
     656             :  * Caller must hold the corresponding SLRU bank lock, will be held at exit.
     657             :  */
     658             : static void
     659      332450 : TransactionIdSetStatusBit(TransactionId xid, XidStatus status, XLogRecPtr lsn, int slotno)
     660             : {
     661      332450 :     int         byteno = TransactionIdToByte(xid);
     662      332450 :     int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
     663             :     char       *byteptr;
     664             :     char        byteval;
     665             :     char        curval;
     666             : 
     667             :     Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(xid));
     668             :     Assert(LWLockHeldByMeInMode(SimpleLruGetBankLock(XactCtl,
     669             :                                                      XactCtl->shared->page_number[slotno]),
     670             :                                 LW_EXCLUSIVE));
     671             : 
     672      332450 :     byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
     673      332450 :     curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
     674             : 
     675             :     /*
     676             :      * When replaying transactions during recovery we still need to perform
     677             :      * the two phases of subcommit and then commit. However, some transactions
     678             :      * are already correctly marked, so we just treat those as a no-op which
     679             :      * allows us to keep the following Assert as restrictive as possible.
     680             :      */
     681      332450 :     if (InRecovery && status == TRANSACTION_STATUS_SUB_COMMITTED &&
     682             :         curval == TRANSACTION_STATUS_COMMITTED)
     683           0 :         return;
     684             : 
     685             :     /*
     686             :      * Current state change should be from 0 or subcommitted to target state
     687             :      * or we should already be there when replaying changes during recovery.
     688             :      */
     689             :     Assert(curval == 0 ||
     690             :            (curval == TRANSACTION_STATUS_SUB_COMMITTED &&
     691             :             status != TRANSACTION_STATUS_IN_PROGRESS) ||
     692             :            curval == status);
     693             : 
     694             :     /* note this assumes exclusive access to the clog page */
     695      332450 :     byteval = *byteptr;
     696      332450 :     byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
     697      332450 :     byteval |= (status << bshift);
     698      332450 :     *byteptr = byteval;
     699             : 
     700             :     /*
     701             :      * Update the group LSN if the transaction completion LSN is higher.
     702             :      *
     703             :      * Note: lsn will be invalid when supplied during InRecovery processing,
     704             :      * so we don't need to do anything special to avoid LSN updates during
     705             :      * recovery. After recovery completes the next clog change will set the
     706             :      * LSN correctly.
     707             :      */
     708      332450 :     if (!XLogRecPtrIsInvalid(lsn))
     709             :     {
     710       56482 :         int         lsnindex = GetLSNIndex(slotno, xid);
     711             : 
     712       56482 :         if (XactCtl->shared->group_lsn[lsnindex] < lsn)
     713       51254 :             XactCtl->shared->group_lsn[lsnindex] = lsn;
     714             :     }
     715             : }
     716             : 
     717             : /*
     718             :  * Interrogate the state of a transaction in the commit log.
     719             :  *
     720             :  * Aside from the actual commit status, this function returns (into *lsn)
     721             :  * an LSN that is late enough to be able to guarantee that if we flush up to
     722             :  * that LSN then we will have flushed the transaction's commit record to disk.
     723             :  * The result is not necessarily the exact LSN of the transaction's commit
     724             :  * record!  For example, for long-past transactions (those whose clog pages
     725             :  * already migrated to disk), we'll return InvalidXLogRecPtr.  Also, because
     726             :  * we group transactions on the same clog page to conserve storage, we might
     727             :  * return the LSN of a later transaction that falls into the same group.
     728             :  *
     729             :  * NB: this is a low-level routine and is NOT the preferred entry point
     730             :  * for most uses; TransactionLogFetch() in transam.c is the intended caller.
     731             :  */
     732             : XidStatus
     733     1424086 : TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
     734             : {
     735     1424086 :     int64       pageno = TransactionIdToPage(xid);
     736     1424086 :     int         byteno = TransactionIdToByte(xid);
     737     1424086 :     int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
     738             :     int         slotno;
     739             :     int         lsnindex;
     740             :     char       *byteptr;
     741             :     XidStatus   status;
     742             : 
     743             :     /* lock is acquired by SimpleLruReadPage_ReadOnly */
     744             : 
     745     1424086 :     slotno = SimpleLruReadPage_ReadOnly(XactCtl, pageno, xid);
     746     1424086 :     byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
     747             : 
     748     1424086 :     status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
     749             : 
     750     1424086 :     lsnindex = GetLSNIndex(slotno, xid);
     751     1424086 :     *lsn = XactCtl->shared->group_lsn[lsnindex];
     752             : 
     753     1424086 :     LWLockRelease(SimpleLruGetBankLock(XactCtl, pageno));
     754             : 
     755     1424086 :     return status;
     756             : }
     757             : 
     758             : /*
     759             :  * Number of shared CLOG buffers.
     760             :  *
     761             :  * If asked to autotune, use 2MB for every 1GB of shared buffers, up to 8MB.
     762             :  * Otherwise just cap the configured amount to be between 16 and the maximum
     763             :  * allowed.
     764             :  */
     765             : static int
     766        8292 : CLOGShmemBuffers(void)
     767             : {
     768             :     /* auto-tune based on shared buffers */
     769        8292 :     if (transaction_buffers == 0)
     770        6130 :         return SimpleLruAutotuneBuffers(512, 1024);
     771             : 
     772        2162 :     return Min(Max(16, transaction_buffers), CLOG_MAX_ALLOWED_BUFFERS);
     773             : }
     774             : 
     775             : /*
     776             :  * Initialization of shared memory for CLOG
     777             :  */
     778             : Size
     779        3998 : CLOGShmemSize(void)
     780             : {
     781        3998 :     return SimpleLruShmemSize(CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE);
     782             : }
     783             : 
     784             : void
     785        2152 : CLOGShmemInit(void)
     786             : {
     787             :     /* If auto-tuning is requested, now is the time to do it */
     788        2152 :     if (transaction_buffers == 0)
     789             :     {
     790             :         char        buf[32];
     791             : 
     792        2142 :         snprintf(buf, sizeof(buf), "%d", CLOGShmemBuffers());
     793        2142 :         SetConfigOption("transaction_buffers", buf, PGC_POSTMASTER,
     794             :                         PGC_S_DYNAMIC_DEFAULT);
     795             : 
     796             :         /*
     797             :          * We prefer to report this value's source as PGC_S_DYNAMIC_DEFAULT.
     798             :          * However, if the DBA explicitly set transaction_buffers = 0 in the
     799             :          * config file, then PGC_S_DYNAMIC_DEFAULT will fail to override that
     800             :          * and we must force the matter with PGC_S_OVERRIDE.
     801             :          */
     802        2142 :         if (transaction_buffers == 0)   /* failed to apply it? */
     803           0 :             SetConfigOption("transaction_buffers", buf, PGC_POSTMASTER,
     804             :                             PGC_S_OVERRIDE);
     805             :     }
     806             :     Assert(transaction_buffers != 0);
     807             : 
     808        2152 :     XactCtl->PagePrecedes = CLOGPagePrecedes;
     809        2152 :     SimpleLruInit(XactCtl, "transaction", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
     810             :                   "pg_xact", LWTRANCHE_XACT_BUFFER,
     811             :                   LWTRANCHE_XACT_SLRU, SYNC_HANDLER_CLOG, false);
     812             :     SlruPagePrecedesUnitTests(XactCtl, CLOG_XACTS_PER_PAGE);
     813        2152 : }
     814             : 
     815             : /*
     816             :  * GUC check_hook for transaction_buffers
     817             :  */
     818             : bool
     819        4370 : check_transaction_buffers(int *newval, void **extra, GucSource source)
     820             : {
     821        4370 :     return check_slru_buffers("transaction_buffers", newval);
     822             : }
     823             : 
     824             : /*
     825             :  * This func must be called ONCE on system install.  It creates
     826             :  * the initial CLOG segment.  (The CLOG directory is assumed to
     827             :  * have been created by initdb, and CLOGShmemInit must have been
     828             :  * called already.)
     829             :  */
     830             : void
     831         102 : BootStrapCLOG(void)
     832             : {
     833             :     /* Zero the initial page and flush it to disk */
     834         102 :     SimpleLruZeroAndWritePage(XactCtl, 0);
     835         102 : }
     836             : 
     837             : /*
     838             :  * This must be called ONCE during postmaster or standalone-backend startup,
     839             :  * after StartupXLOG has initialized TransamVariables->nextXid.
     840             :  */
     841             : void
     842        1862 : StartupCLOG(void)
     843             : {
     844        1862 :     TransactionId xid = XidFromFullTransactionId(TransamVariables->nextXid);
     845        1862 :     int64       pageno = TransactionIdToPage(xid);
     846             : 
     847             :     /*
     848             :      * Initialize our idea of the latest page number.
     849             :      */
     850        1862 :     pg_atomic_write_u64(&XactCtl->shared->latest_page_number, pageno);
     851        1862 : }
     852             : 
     853             : /*
     854             :  * This must be called ONCE at the end of startup/recovery.
     855             :  */
     856             : void
     857        1746 : TrimCLOG(void)
     858             : {
     859        1746 :     TransactionId xid = XidFromFullTransactionId(TransamVariables->nextXid);
     860        1746 :     int64       pageno = TransactionIdToPage(xid);
     861        1746 :     LWLock     *lock = SimpleLruGetBankLock(XactCtl, pageno);
     862             : 
     863        1746 :     LWLockAcquire(lock, LW_EXCLUSIVE);
     864             : 
     865             :     /*
     866             :      * Zero out the remainder of the current clog page.  Under normal
     867             :      * circumstances it should be zeroes already, but it seems at least
     868             :      * theoretically possible that XLOG replay will have settled on a nextXID
     869             :      * value that is less than the last XID actually used and marked by the
     870             :      * previous database lifecycle (since subtransaction commit writes clog
     871             :      * but makes no WAL entry).  Let's just be safe. (We need not worry about
     872             :      * pages beyond the current one, since those will be zeroed when first
     873             :      * used.  For the same reason, there is no need to do anything when
     874             :      * nextXid is exactly at a page boundary; and it's likely that the
     875             :      * "current" page doesn't exist yet in that case.)
     876             :      */
     877        1746 :     if (TransactionIdToPgIndex(xid) != 0)
     878             :     {
     879        1744 :         int         byteno = TransactionIdToByte(xid);
     880        1744 :         int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
     881             :         int         slotno;
     882             :         char       *byteptr;
     883             : 
     884        1744 :         slotno = SimpleLruReadPage(XactCtl, pageno, false, xid);
     885        1744 :         byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
     886             : 
     887             :         /* Zero so-far-unused positions in the current byte */
     888        1744 :         *byteptr &= (1 << bshift) - 1;
     889             :         /* Zero the rest of the page */
     890        1744 :         MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
     891             : 
     892        1744 :         XactCtl->shared->page_dirty[slotno] = true;
     893             :     }
     894             : 
     895        1746 :     LWLockRelease(lock);
     896        1746 : }
     897             : 
     898             : /*
     899             :  * Perform a checkpoint --- either during shutdown, or on-the-fly
     900             :  */
     901             : void
     902        3382 : CheckPointCLOG(void)
     903             : {
     904             :     /*
     905             :      * Write dirty CLOG pages to disk.  This may result in sync requests
     906             :      * queued for later handling by ProcessSyncRequests(), as part of the
     907             :      * checkpoint.
     908             :      */
     909             :     TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
     910        3382 :     SimpleLruWriteAll(XactCtl, true);
     911             :     TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
     912        3382 : }
     913             : 
     914             : 
     915             : /*
     916             :  * Make sure that CLOG has room for a newly-allocated XID.
     917             :  *
     918             :  * NB: this is called while holding XidGenLock.  We want it to be very fast
     919             :  * most of the time; even when it's not so fast, no actual I/O need happen
     920             :  * unless we're forced to write out a dirty clog or xlog page to make room
     921             :  * in shared memory.
     922             :  */
     923             : void
     924    49000618 : ExtendCLOG(TransactionId newestXact)
     925             : {
     926             :     int64       pageno;
     927             :     LWLock     *lock;
     928             : 
     929             :     /*
     930             :      * No work except at first XID of a page.  But beware: just after
     931             :      * wraparound, the first XID of page zero is FirstNormalTransactionId.
     932             :      */
     933    49000618 :     if (TransactionIdToPgIndex(newestXact) != 0 &&
     934             :         !TransactionIdEquals(newestXact, FirstNormalTransactionId))
     935    48137212 :         return;
     936             : 
     937      863406 :     pageno = TransactionIdToPage(newestXact);
     938      863406 :     lock = SimpleLruGetBankLock(XactCtl, pageno);
     939             : 
     940      863406 :     LWLockAcquire(lock, LW_EXCLUSIVE);
     941             : 
     942             :     /* Zero the page and make a WAL entry about it */
     943      863406 :     SimpleLruZeroPage(XactCtl, pageno);
     944      863406 :     XLogSimpleInsertInt64(RM_CLOG_ID, CLOG_ZEROPAGE, pageno);
     945             : 
     946      863406 :     LWLockRelease(lock);
     947             : }
     948             : 
     949             : 
     950             : /*
     951             :  * Remove all CLOG segments before the one holding the passed transaction ID
     952             :  *
     953             :  * Before removing any CLOG data, we must flush XLOG to disk, to ensure that
     954             :  * any recently-emitted records with freeze plans have reached disk; otherwise
     955             :  * a crash and restart might leave us with some unfrozen tuples referencing
     956             :  * removed CLOG data.  We choose to emit a special TRUNCATE XLOG record too.
     957             :  * Replaying the deletion from XLOG is not critical, since the files could
     958             :  * just as well be removed later, but doing so prevents a long-running hot
     959             :  * standby server from acquiring an unreasonably bloated CLOG directory.
     960             :  *
     961             :  * Since CLOG segments hold a large number of transactions, the opportunity to
     962             :  * actually remove a segment is fairly rare, and so it seems best not to do
     963             :  * the XLOG flush unless we have confirmed that there is a removable segment.
     964             :  */
     965             : void
     966        1888 : TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
     967             : {
     968             :     int64       cutoffPage;
     969             : 
     970             :     /*
     971             :      * The cutoff point is the start of the segment containing oldestXact. We
     972             :      * pass the *page* containing oldestXact to SimpleLruTruncate.
     973             :      */
     974        1888 :     cutoffPage = TransactionIdToPage(oldestXact);
     975             : 
     976             :     /* Check to see if there's any files that could be removed */
     977        1888 :     if (!SlruScanDirectory(XactCtl, SlruScanDirCbReportPresence, &cutoffPage))
     978        1688 :         return;                 /* nothing to remove */
     979             : 
     980             :     /*
     981             :      * Advance oldestClogXid before truncating clog, so concurrent xact status
     982             :      * lookups can ensure they don't attempt to access truncated-away clog.
     983             :      *
     984             :      * It's only necessary to do this if we will actually truncate away clog
     985             :      * pages.
     986             :      */
     987         200 :     AdvanceOldestClogXid(oldestXact);
     988             : 
     989             :     /*
     990             :      * Write XLOG record and flush XLOG to disk. We record the oldest xid
     991             :      * we're keeping information about here so we can ensure that it's always
     992             :      * ahead of clog truncation in case we crash, and so a standby finds out
     993             :      * the new valid xid before the next checkpoint.
     994             :      */
     995         200 :     WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
     996             : 
     997             :     /* Now we can remove the old CLOG segment(s) */
     998         200 :     SimpleLruTruncate(XactCtl, cutoffPage);
     999             : }
    1000             : 
    1001             : 
    1002             : /*
    1003             :  * Decide whether a CLOG page number is "older" for truncation purposes.
    1004             :  *
    1005             :  * We need to use comparison of TransactionIds here in order to do the right
    1006             :  * thing with wraparound XID arithmetic.  However, TransactionIdPrecedes()
    1007             :  * would get weird about permanent xact IDs.  So, offset both such that xid1,
    1008             :  * xid2, and xid2 + CLOG_XACTS_PER_PAGE - 1 are all normal XIDs; this offset
    1009             :  * is relevant to page 0 and to the page preceding page 0.
    1010             :  *
    1011             :  * The page containing oldestXact-2^31 is the important edge case.  The
    1012             :  * portion of that page equaling or following oldestXact-2^31 is expendable,
    1013             :  * but the portion preceding oldestXact-2^31 is not.  When oldestXact-2^31 is
    1014             :  * the first XID of a page and segment, the entire page and segment is
    1015             :  * expendable, and we could truncate the segment.  Recognizing that case would
    1016             :  * require making oldestXact, not just the page containing oldestXact,
    1017             :  * available to this callback.  The benefit would be rare and small, so we
    1018             :  * don't optimize that edge case.
    1019             :  */
    1020             : static bool
    1021     1741300 : CLOGPagePrecedes(int64 page1, int64 page2)
    1022             : {
    1023             :     TransactionId xid1;
    1024             :     TransactionId xid2;
    1025             : 
    1026     1741300 :     xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
    1027     1741300 :     xid1 += FirstNormalTransactionId + 1;
    1028     1741300 :     xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
    1029     1741300 :     xid2 += FirstNormalTransactionId + 1;
    1030             : 
    1031     1795408 :     return (TransactionIdPrecedes(xid1, xid2) &&
    1032       54108 :             TransactionIdPrecedes(xid1, xid2 + CLOG_XACTS_PER_PAGE - 1));
    1033             : }
    1034             : 
    1035             : 
    1036             : /*
    1037             :  * Write a TRUNCATE xlog record
    1038             :  *
    1039             :  * We must flush the xlog record to disk before returning --- see notes
    1040             :  * in TruncateCLOG().
    1041             :  */
    1042             : static void
    1043         200 : WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact, Oid oldestXactDb)
    1044             : {
    1045             :     XLogRecPtr  recptr;
    1046             :     xl_clog_truncate xlrec;
    1047             : 
    1048         200 :     xlrec.pageno = pageno;
    1049         200 :     xlrec.oldestXact = oldestXact;
    1050         200 :     xlrec.oldestXactDb = oldestXactDb;
    1051             : 
    1052         200 :     XLogBeginInsert();
    1053         200 :     XLogRegisterData(&xlrec, sizeof(xl_clog_truncate));
    1054         200 :     recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
    1055         200 :     XLogFlush(recptr);
    1056         200 : }
    1057             : 
    1058             : /*
    1059             :  * CLOG resource manager's routines
    1060             :  */
    1061             : void
    1062           0 : clog_redo(XLogReaderState *record)
    1063             : {
    1064           0 :     uint8       info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
    1065             : 
    1066             :     /* Backup blocks are not used in clog records */
    1067             :     Assert(!XLogRecHasAnyBlockRefs(record));
    1068             : 
    1069           0 :     if (info == CLOG_ZEROPAGE)
    1070             :     {
    1071             :         int64       pageno;
    1072             : 
    1073           0 :         memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));
    1074           0 :         SimpleLruZeroAndWritePage(XactCtl, pageno);
    1075             :     }
    1076           0 :     else if (info == CLOG_TRUNCATE)
    1077             :     {
    1078             :         xl_clog_truncate xlrec;
    1079             : 
    1080           0 :         memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
    1081             : 
    1082           0 :         AdvanceOldestClogXid(xlrec.oldestXact);
    1083             : 
    1084           0 :         SimpleLruTruncate(XactCtl, xlrec.pageno);
    1085             :     }
    1086             :     else
    1087           0 :         elog(PANIC, "clog_redo: unknown op code %u", info);
    1088           0 : }
    1089             : 
    1090             : /*
    1091             :  * Entrypoint for sync.c to sync clog files.
    1092             :  */
    1093             : int
    1094           0 : clogsyncfiletag(const FileTag *ftag, char *path)
    1095             : {
    1096           0 :     return SlruSyncFileTag(XactCtl, ftag, path);
    1097             : }

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