/src/postgres/src/backend/replication/syncrep.c

Source
/*-------------------------------------------------------------------------
 *
 * syncrep.c
 *
 * Synchronous replication is new as of PostgreSQL 9.1.
 *
 * If requested, transaction commits wait until their commit LSN are
 * acknowledged by the synchronous standbys.
 *
 * This module contains the code for waiting and release of backends.
 * All code in this module executes on the primary. The core streaming
 * replication transport remains within WALreceiver/WALsender modules.
 *
 * The essence of this design is that it isolates all logic about
 * waiting/releasing onto the primary. The primary defines which standbys
 * it wishes to wait for. The standbys are completely unaware of the
 * durability requirements of transactions on the primary, reducing the
 * complexity of the code and streamlining both standby operations and
 * network bandwidth because there is no requirement to ship
 * per-transaction state information.
 *
 * Replication is either synchronous or not synchronous (async). If it is
 * async, we just fastpath out of here. If it is sync, then we wait for
 * the write, flush or apply location on the standby before releasing
 * the waiting backend. Further complexity in that interaction is
 * expected in later releases.
 *
 * The best performing way to manage the waiting backends is to have a
 * single ordered queue of waiting backends, so that we can avoid
 * searching the through all waiters each time we receive a reply.
 *
 * In 9.5 or before only a single standby could be considered as
 * synchronous. In 9.6 we support a priority-based multiple synchronous
 * standbys. In 10.0 a quorum-based multiple synchronous standbys is also
 * supported. The number of synchronous standbys that transactions
 * must wait for replies from is specified in synchronous_standby_names.
 * This parameter also specifies a list of standby names and the method
 * (FIRST and ANY) to choose synchronous standbys from the listed ones.
 *
 * The method FIRST specifies a priority-based synchronous replication
 * and makes transaction commits wait until their WAL records are
 * replicated to the requested number of synchronous standbys chosen based
 * on their priorities. The standbys whose names appear earlier in the list
 * are given higher priority and will be considered as synchronous.
 * Other standby servers appearing later in this list represent potential
 * synchronous standbys. If any of the current synchronous standbys
 * disconnects for whatever reason, it will be replaced immediately with
 * the next-highest-priority standby.
 *
 * The method ANY specifies a quorum-based synchronous replication
 * and makes transaction commits wait until their WAL records are
 * replicated to at least the requested number of synchronous standbys
 * in the list. All the standbys appearing in the list are considered as
 * candidates for quorum synchronous standbys.
 *
 * If neither FIRST nor ANY is specified, FIRST is used as the method.
 * This is for backward compatibility with 9.6 or before where only a
 * priority-based sync replication was supported.
 *
 * Before the standbys chosen from synchronous_standby_names can
 * become the synchronous standbys they must have caught up with
 * the primary; that may take some time. Once caught up,
 * the standbys which are considered as synchronous at that moment
 * will release waiters from the queue.
 *
 * Portions Copyright (c) 2010-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/syncrep.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>

#include "access/xact.h"
#include "common/int.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "replication/syncrep.h"
#include "replication/walsender.h"
#include "replication/walsender_private.h"
#include "storage/proc.h"
#include "tcop/tcopprot.h"
#include "utils/guc_hooks.h"
#include "utils/ps_status.h"
#include "utils/wait_event.h"

/* User-settable parameters for sync rep */
char     *SyncRepStandbyNames;

#define SyncStandbysDefined() \
  (SyncRepStandbyNames != NULL && SyncRepStandbyNames[0] != '\0')

static bool announce_next_takeover = true;

SyncRepConfigData *SyncRepConfig = NULL;
static int  SyncRepWaitMode = SYNC_REP_NO_WAIT;

static void SyncRepQueueInsert(int mode);
static void SyncRepCancelWait(void);
static int  SyncRepWakeQueue(bool all, int mode);

static bool SyncRepGetSyncRecPtr(XLogRecPtr *writePtr,
                 XLogRecPtr *flushPtr,
                 XLogRecPtr *applyPtr,
                 bool *am_sync);
static void SyncRepGetOldestSyncRecPtr(XLogRecPtr *writePtr,
                     XLogRecPtr *flushPtr,
                     XLogRecPtr *applyPtr,
                     SyncRepStandbyData *sync_standbys,
                     int num_standbys);
static void SyncRepGetNthLatestSyncRecPtr(XLogRecPtr *writePtr,
                      XLogRecPtr *flushPtr,
                      XLogRecPtr *applyPtr,
                      SyncRepStandbyData *sync_standbys,
                      int num_standbys,
                      uint8 nth);
static int  SyncRepGetStandbyPriority(void);
static int  standby_priority_comparator(const void *a, const void *b);
static int  cmp_lsn(const void *a, const void *b);

#ifdef USE_ASSERT_CHECKING
static bool SyncRepQueueIsOrderedByLSN(int mode);
#endif

/*
 * ===========================================================
 * Synchronous Replication functions for normal user backends
 * ===========================================================
 */

/*
 * Wait for synchronous replication, if requested by user.
 *
 * Initially backends start in state SYNC_REP_NOT_WAITING and then
 * change that state to SYNC_REP_WAITING before adding ourselves
 * to the wait queue. During SyncRepWakeQueue() a WALSender changes
 * the state to SYNC_REP_WAIT_COMPLETE once replication is confirmed.
 * This backend then resets its state to SYNC_REP_NOT_WAITING.
 *
 * 'lsn' represents the LSN to wait for.  'commit' indicates whether this LSN
 * represents a commit record.  If it doesn't, then we wait only for the WAL
 * to be flushed if synchronous_commit is set to the higher level of
 * remote_apply, because only commit records provide apply feedback.
 */
void
SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
{
  int     mode;

  /*
   * This should be called while holding interrupts during a transaction
   * commit to prevent the follow-up shared memory queue cleanups to be
   * influenced by external interruptions.
   */
  Assert(InterruptHoldoffCount > 0);

  /*
   * Fast exit if user has not requested sync replication, or there are no
   * sync replication standby names defined.
   *
   * Since this routine gets called every commit time, it's important to
   * exit quickly if sync replication is not requested.
   *
   * We check WalSndCtl->sync_standbys_status flag without the lock and exit
   * immediately if SYNC_STANDBY_INIT is set (the checkpointer has
   * initialized this data) but SYNC_STANDBY_DEFINED is missing (no sync
   * replication requested).
   *
   * If SYNC_STANDBY_DEFINED is set, we need to check the status again later
   * while holding the lock, to check the flag and operate the sync rep
   * queue atomically.  This is necessary to avoid the race condition
   * described in SyncRepUpdateSyncStandbysDefined().  On the other hand, if
   * SYNC_STANDBY_DEFINED is not set, the lock is not necessary because we
   * don't touch the queue.
   */
  if (!SyncRepRequested() ||
    ((((volatile WalSndCtlData *) WalSndCtl)->sync_standbys_status) &
     (SYNC_STANDBY_INIT | SYNC_STANDBY_DEFINED)) == SYNC_STANDBY_INIT)
    return;

  /* Cap the level for anything other than commit to remote flush only. */
  if (commit)
    mode = SyncRepWaitMode;
  else
    mode = Min(SyncRepWaitMode, SYNC_REP_WAIT_FLUSH);

  Assert(dlist_node_is_detached(&MyProc->syncRepLinks));
  Assert(WalSndCtl != NULL);

  LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
  Assert(MyProc->syncRepState == SYNC_REP_NOT_WAITING);

  /*
   * We don't wait for sync rep if SYNC_STANDBY_DEFINED is not set.  See
   * SyncRepUpdateSyncStandbysDefined().
   *
   * Also check that the standby hasn't already replied. Unlikely race
   * condition but we'll be fetching that cache line anyway so it's likely
   * to be a low cost check.
   *
   * If the sync standby data has not been initialized yet
   * (SYNC_STANDBY_INIT is not set), fall back to a check based on the LSN,
   * then do a direct GUC check.
   */
  if (WalSndCtl->sync_standbys_status & SYNC_STANDBY_INIT)
  {
    if ((WalSndCtl->sync_standbys_status & SYNC_STANDBY_DEFINED) == 0 ||
      lsn <= WalSndCtl->lsn[mode])
    {
      LWLockRelease(SyncRepLock);
      return;
    }
  }
  else if (lsn <= WalSndCtl->lsn[mode])
  {
    /*
     * The LSN is older than what we need to wait for.  The sync standby
     * data has not been initialized yet, but we are OK to not wait
     * because we know that there is no point in doing so based on the
     * LSN.
     */
    LWLockRelease(SyncRepLock);
    return;
  }
  else if (!SyncStandbysDefined())
  {
    /*
     * If we are here, the sync standby data has not been initialized yet,
     * and the LSN is newer than what need to wait for, so we have fallen
     * back to the best thing we could do in this case: a check on
     * SyncStandbysDefined() to see if the GUC is set or not.
     *
     * When the GUC has a value, we wait until the checkpointer updates
     * the status data because we cannot be sure yet if we should wait or
     * not. Here, the GUC has *no* value, we are sure that there is no
     * point to wait; this matters for example when initializing a
     * cluster, where we should never wait, and no sync standbys is the
     * default behavior.
     */
    LWLockRelease(SyncRepLock);
    return;
  }

  /*
   * Set our waitLSN so WALSender will know when to wake us, and add
   * ourselves to the queue.
   */
  MyProc->waitLSN = lsn;
  MyProc->syncRepState = SYNC_REP_WAITING;
  SyncRepQueueInsert(mode);
  Assert(SyncRepQueueIsOrderedByLSN(mode));
  LWLockRelease(SyncRepLock);

  /* Alter ps display to show waiting for sync rep. */
  if (update_process_title)
  {
    char    buffer[32];

    sprintf(buffer, "waiting for %X/%08X", LSN_FORMAT_ARGS(lsn));
    set_ps_display_suffix(buffer);
  }

  /*
   * Wait for specified LSN to be confirmed.
   *
   * Each proc has its own wait latch, so we perform a normal latch
   * check/wait loop here.
   */
  for (;;)
  {
    int     rc;

    /* Must reset the latch before testing state. */
    ResetLatch(MyLatch);

    /*
     * Acquiring the lock is not needed, the latch ensures proper
     * barriers. If it looks like we're done, we must really be done,
     * because once walsender changes the state to SYNC_REP_WAIT_COMPLETE,
     * it will never update it again, so we can't be seeing a stale value
     * in that case.
     */
    if (MyProc->syncRepState == SYNC_REP_WAIT_COMPLETE)
      break;

    /*
     * If a wait for synchronous replication is pending, we can neither
     * acknowledge the commit nor raise ERROR or FATAL.  The latter would
     * lead the client to believe that the transaction aborted, which is
     * not true: it's already committed locally. The former is no good
     * either: the client has requested synchronous replication, and is
     * entitled to assume that an acknowledged commit is also replicated,
     * which might not be true. So in this case we issue a WARNING (which
     * some clients may be able to interpret) and shut off further output.
     * We do NOT reset ProcDiePending, so that the process will die after
     * the commit is cleaned up.
     */
    if (ProcDiePending)
    {
      if (ProcDieSenderPid != 0)
        ereport(WARNING,
            (errcode(ERRCODE_ADMIN_SHUTDOWN),
             errmsg("canceling the wait for synchronous replication and terminating connection due to administrator command"),
             errdetail("The transaction has already committed locally, but might not have been replicated to the standby."),
             errdetail_log("The transaction has already committed locally, but might not have been replicated to the standby.  Signal sent by PID %d, UID %d.",
                     (int) ProcDieSenderPid,
                     (int) ProcDieSenderUid)));
      else
        ereport(WARNING,
            (errcode(ERRCODE_ADMIN_SHUTDOWN),
             errmsg("canceling the wait for synchronous replication and terminating connection due to administrator command"),
             errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
      whereToSendOutput = DestNone;
      SyncRepCancelWait();
      break;
    }

    /*
     * It's unclear what to do if a query cancel interrupt arrives.  We
     * can't actually abort at this point, but ignoring the interrupt
     * altogether is not helpful, so we just terminate the wait with a
     * suitable warning.
     */
    if (QueryCancelPending)
    {
      QueryCancelPending = false;
      ereport(WARNING,
          (errmsg("canceling wait for synchronous replication due to user request"),
           errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
      SyncRepCancelWait();
      break;
    }

    /*
     * Wait on latch.  Any condition that should wake us up will set the
     * latch, so no need for timeout.
     */
    rc = WaitLatch(MyLatch, WL_LATCH_SET | WL_POSTMASTER_DEATH, -1,
             WAIT_EVENT_SYNC_REP);

    /*
     * If the postmaster dies, we'll probably never get an acknowledgment,
     * because all the wal sender processes will exit. So just bail out.
     */
    if (rc & WL_POSTMASTER_DEATH)
    {
      ProcDiePending = true;
      whereToSendOutput = DestNone;
      SyncRepCancelWait();
      break;
    }
  }

  /*
   * WalSender has checked our LSN and has removed us from queue. Clean up
   * state and leave.  It's OK to reset these shared memory fields without
   * holding SyncRepLock, because any walsenders will ignore us anyway when
   * we're not on the queue.  We need a read barrier to make sure we see the
   * changes to the queue link (this might be unnecessary without
   * assertions, but better safe than sorry).
   */
  pg_read_barrier();
  Assert(dlist_node_is_detached(&MyProc->syncRepLinks));
  MyProc->syncRepState = SYNC_REP_NOT_WAITING;
  MyProc->waitLSN = InvalidXLogRecPtr;

  /* reset ps display to remove the suffix */
  if (update_process_title)
    set_ps_display_remove_suffix();
}

/*
 * Insert MyProc into the specified SyncRepQueue, maintaining sorted invariant.
 *
 * Usually we will go at tail of queue, though it's possible that we arrive
 * here out of order, so start at tail and work back to insertion point.
 */
static void
SyncRepQueueInsert(int mode)
{
  dlist_head *queue;
  dlist_iter  iter;

  Assert(mode >= 0 && mode < NUM_SYNC_REP_WAIT_MODE);
  queue = &WalSndCtl->SyncRepQueue[mode];

  dlist_reverse_foreach(iter, queue)
  {
    PGPROC     *proc = dlist_container(PGPROC, syncRepLinks, iter.cur);

    /*
     * Stop at the queue element that we should insert after to ensure the
     * queue is ordered by LSN.
     */
    if (proc->waitLSN < MyProc->waitLSN)
    {
      dlist_insert_after(&proc->syncRepLinks, &MyProc->syncRepLinks);
      return;
    }
  }

  /*
   * If we get here, the list was either empty, or this process needs to be
   * at the head.
   */
  dlist_push_head(queue, &MyProc->syncRepLinks);
}

/*
 * Acquire SyncRepLock and cancel any wait currently in progress.
 */
static void
SyncRepCancelWait(void)
{
  LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
  if (!dlist_node_is_detached(&MyProc->syncRepLinks))
    dlist_delete_thoroughly(&MyProc->syncRepLinks);
  MyProc->syncRepState = SYNC_REP_NOT_WAITING;
  LWLockRelease(SyncRepLock);
}

void
SyncRepCleanupAtProcExit(void)
{
  /*
   * First check if we are removed from the queue without the lock to not
   * slow down backend exit.
   */
  if (!dlist_node_is_detached(&MyProc->syncRepLinks))
  {
    LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);

    /* maybe we have just been removed, so recheck */
    if (!dlist_node_is_detached(&MyProc->syncRepLinks))
      dlist_delete_thoroughly(&MyProc->syncRepLinks);

    LWLockRelease(SyncRepLock);
  }
}

/*
 * ===========================================================
 * Synchronous Replication functions for wal sender processes
 * ===========================================================
 */

/*
 * Take any action required to initialise sync rep state from config
 * data. Called at WALSender startup and after each SIGHUP.
 */
void
SyncRepInitConfig(void)
{
  int     priority;

  /*
   * Determine if we are a potential sync standby and remember the result
   * for handling replies from standby.
   */
  priority = SyncRepGetStandbyPriority();
  if (MyWalSnd->sync_standby_priority != priority)
  {
    SpinLockAcquire(&MyWalSnd->mutex);
    MyWalSnd->sync_standby_priority = priority;
    SpinLockRelease(&MyWalSnd->mutex);

    ereport(DEBUG1,
        (errmsg_internal("standby \"%s\" now has synchronous standby priority %d",
                 application_name, priority)));
  }
}

/*
 * Update the LSNs on each queue based upon our latest state. This
 * implements a simple policy of first-valid-sync-standby-releases-waiter.
 *
 * Other policies are possible, which would change what we do here and
 * perhaps also which information we store as well.
 */
void
SyncRepReleaseWaiters(void)
{
  XLogRecPtr  writePtr;
  XLogRecPtr  flushPtr;
  XLogRecPtr  applyPtr;
  bool    got_recptr;
  bool    am_sync;
  int     numwrite = 0;
  int     numflush = 0;
  int     numapply = 0;

  /*
   * If this WALSender is serving a standby that is not on the list of
   * potential sync standbys then we have nothing to do. If we are still
   * starting up, still running base backup or the current flush position is
   * still invalid, then leave quickly also.  Streaming or stopping WAL
   * senders are allowed to release waiters.
   */
  if (MyWalSnd->sync_standby_priority == 0 ||
    (MyWalSnd->state != WALSNDSTATE_STREAMING &&
     MyWalSnd->state != WALSNDSTATE_STOPPING) ||
    !XLogRecPtrIsValid(MyWalSnd->flush))
  {
    announce_next_takeover = true;
    return;
  }

  /*
   * We're a potential sync standby. Release waiters if there are enough
   * sync standbys and we are considered as sync.
   */
  LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);

  /*
   * Check whether we are a sync standby or not, and calculate the synced
   * positions among all sync standbys.  (Note: although this step does not
   * of itself require holding SyncRepLock, it seems like a good idea to do
   * it after acquiring the lock.  This ensures that the WAL pointers we use
   * to release waiters are newer than any previous execution of this
   * routine used.)
   */
  got_recptr = SyncRepGetSyncRecPtr(&writePtr, &flushPtr, &applyPtr, &am_sync);

  /*
   * If we are managing a sync standby, though we weren't prior to this,
   * then announce we are now a sync standby.
   */
  if (announce_next_takeover && am_sync)
  {
    announce_next_takeover = false;

    if (SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY)
      ereport(LOG,
          (errmsg("standby \"%s\" is now a synchronous standby with priority %d",
              application_name, MyWalSnd->sync_standby_priority)));
    else
      ereport(LOG,
          (errmsg("standby \"%s\" is now a candidate for quorum synchronous standby",
              application_name)));
  }

  /*
   * If the number of sync standbys is less than requested or we aren't
   * managing a sync standby then just leave.
   */
  if (!got_recptr || !am_sync)
  {
    LWLockRelease(SyncRepLock);
    announce_next_takeover = !am_sync;
    return;
  }

  /*
   * Set the lsn first so that when we wake backends they will release up to
   * this location.
   */
  if (WalSndCtl->lsn[SYNC_REP_WAIT_WRITE] < writePtr)
  {
    WalSndCtl->lsn[SYNC_REP_WAIT_WRITE] = writePtr;
    numwrite = SyncRepWakeQueue(false, SYNC_REP_WAIT_WRITE);
  }
  if (WalSndCtl->lsn[SYNC_REP_WAIT_FLUSH] < flushPtr)
  {
    WalSndCtl->lsn[SYNC_REP_WAIT_FLUSH] = flushPtr;
    numflush = SyncRepWakeQueue(false, SYNC_REP_WAIT_FLUSH);
  }
  if (WalSndCtl->lsn[SYNC_REP_WAIT_APPLY] < applyPtr)
  {
    WalSndCtl->lsn[SYNC_REP_WAIT_APPLY] = applyPtr;
    numapply = SyncRepWakeQueue(false, SYNC_REP_WAIT_APPLY);
  }

  LWLockRelease(SyncRepLock);

  elog(DEBUG3, "released %d procs up to write %X/%08X, %d procs up to flush %X/%08X, %d procs up to apply %X/%08X",
     numwrite, LSN_FORMAT_ARGS(writePtr),
     numflush, LSN_FORMAT_ARGS(flushPtr),
     numapply, LSN_FORMAT_ARGS(applyPtr));
}

/*
 * Calculate the synced Write, Flush and Apply positions among sync standbys.
 *
 * Return false if the number of sync standbys is less than
 * synchronous_standby_names specifies. Otherwise return true and
 * store the positions into *writePtr, *flushPtr and *applyPtr.
 *
 * On return, *am_sync is set to true if this walsender is connecting to
 * sync standby. Otherwise it's set to false.
 */
static bool
SyncRepGetSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
           XLogRecPtr *applyPtr, bool *am_sync)
{
  SyncRepStandbyData *sync_standbys;
  int     num_standbys;
  int     i;

  /* Initialize default results */
  *writePtr = InvalidXLogRecPtr;
  *flushPtr = InvalidXLogRecPtr;
  *applyPtr = InvalidXLogRecPtr;
  *am_sync = false;

  /* Quick out if not even configured to be synchronous */
  if (SyncRepConfig == NULL)
    return false;

  /* Get standbys that are considered as synchronous at this moment */
  num_standbys = SyncRepGetCandidateStandbys(&sync_standbys);

  /* Am I among the candidate sync standbys? */
  for (i = 0; i < num_standbys; i++)
  {
    if (sync_standbys[i].is_me)
    {
      *am_sync = true;
      break;
    }
  }

  /*
   * Nothing more to do if we are not managing a sync standby or there are
   * not enough synchronous standbys.
   */
  if (!(*am_sync) ||
    num_standbys < SyncRepConfig->num_sync)
  {
    pfree(sync_standbys);
    return false;
  }

  /*
   * In a priority-based sync replication, the synced positions are the
   * oldest ones among sync standbys. In a quorum-based, they are the Nth
   * latest ones.
   *
   * SyncRepGetNthLatestSyncRecPtr() also can calculate the oldest
   * positions. But we use SyncRepGetOldestSyncRecPtr() for that calculation
   * because it's a bit more efficient.
   *
   * XXX If the numbers of current and requested sync standbys are the same,
   * we can use SyncRepGetOldestSyncRecPtr() to calculate the synced
   * positions even in a quorum-based sync replication.
   */
  if (SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY)
  {
    SyncRepGetOldestSyncRecPtr(writePtr, flushPtr, applyPtr,
                   sync_standbys, num_standbys);
  }
  else
  {
    SyncRepGetNthLatestSyncRecPtr(writePtr, flushPtr, applyPtr,
                    sync_standbys, num_standbys,
                    SyncRepConfig->num_sync);
  }

  pfree(sync_standbys);
  return true;
}

/*
 * Calculate the oldest Write, Flush and Apply positions among sync standbys.
 */
static void
SyncRepGetOldestSyncRecPtr(XLogRecPtr *writePtr,
               XLogRecPtr *flushPtr,
               XLogRecPtr *applyPtr,
               SyncRepStandbyData *sync_standbys,
               int num_standbys)
{
  int     i;

  /*
   * Scan through all sync standbys and calculate the oldest Write, Flush
   * and Apply positions.  We assume *writePtr et al were initialized to
   * InvalidXLogRecPtr.
   */
  for (i = 0; i < num_standbys; i++)
  {
    XLogRecPtr  write = sync_standbys[i].write;
    XLogRecPtr  flush = sync_standbys[i].flush;
    XLogRecPtr  apply = sync_standbys[i].apply;

    if (!XLogRecPtrIsValid(*writePtr) || *writePtr > write)
      *writePtr = write;
    if (!XLogRecPtrIsValid(*flushPtr) || *flushPtr > flush)
      *flushPtr = flush;
    if (!XLogRecPtrIsValid(*applyPtr) || *applyPtr > apply)
      *applyPtr = apply;
  }
}

/*
 * Calculate the Nth latest Write, Flush and Apply positions among sync
 * standbys.
 */
static void
SyncRepGetNthLatestSyncRecPtr(XLogRecPtr *writePtr,
                XLogRecPtr *flushPtr,
                XLogRecPtr *applyPtr,
                SyncRepStandbyData *sync_standbys,
                int num_standbys,
                uint8 nth)
{
  XLogRecPtr *write_array;
  XLogRecPtr *flush_array;
  XLogRecPtr *apply_array;
  int     i;

  /* Should have enough candidates, or somebody messed up */
  Assert(nth > 0 && nth <= num_standbys);

  write_array = palloc_array(XLogRecPtr, num_standbys);
  flush_array = palloc_array(XLogRecPtr, num_standbys);
  apply_array = palloc_array(XLogRecPtr, num_standbys);

  for (i = 0; i < num_standbys; i++)
  {
    write_array[i] = sync_standbys[i].write;
    flush_array[i] = sync_standbys[i].flush;
    apply_array[i] = sync_standbys[i].apply;
  }

  /* Sort each array in descending order */
  qsort(write_array, num_standbys, sizeof(XLogRecPtr), cmp_lsn);
  qsort(flush_array, num_standbys, sizeof(XLogRecPtr), cmp_lsn);
  qsort(apply_array, num_standbys, sizeof(XLogRecPtr), cmp_lsn);

  /* Get Nth latest Write, Flush, Apply positions */
  *writePtr = write_array[nth - 1];
  *flushPtr = flush_array[nth - 1];
  *applyPtr = apply_array[nth - 1];

  pfree(write_array);
  pfree(flush_array);
  pfree(apply_array);
}

/*
 * Compare lsn in order to sort array in descending order.
 */
static int
cmp_lsn(const void *a, const void *b)
{
  XLogRecPtr  lsn1 = *((const XLogRecPtr *) a);
  XLogRecPtr  lsn2 = *((const XLogRecPtr *) b);

  return pg_cmp_u64(lsn2, lsn1);
}

/*
 * Return data about walsenders that are candidates to be sync standbys.
 *
 * *standbys is set to a palloc'd array of structs of per-walsender data,
 * and the number of valid entries (candidate sync senders) is returned.
 * (This might be more or fewer than num_sync; caller must check.)
 */
int
SyncRepGetCandidateStandbys(SyncRepStandbyData **standbys)
{
  int     i;
  int     n;

  /* Create result array */
  *standbys = palloc_array(SyncRepStandbyData, max_wal_senders);

  /* Quick exit if sync replication is not requested */
  if (SyncRepConfig == NULL)
    return 0;

  /* Collect raw data from shared memory */
  n = 0;
  for (i = 0; i < max_wal_senders; i++)
  {
    WalSnd     *walsnd;
    SyncRepStandbyData *stby;
    WalSndState state;    /* not included in SyncRepStandbyData */

    walsnd = &WalSndCtl->walsnds[i];
    stby = *standbys + n;

    SpinLockAcquire(&walsnd->mutex);
    stby->pid = walsnd->pid;
    state = walsnd->state;
    stby->write = walsnd->write;
    stby->flush = walsnd->flush;
    stby->apply = walsnd->apply;
    stby->sync_standby_priority = walsnd->sync_standby_priority;
    SpinLockRelease(&walsnd->mutex);

    /* Must be active */
    if (stby->pid == 0)
      continue;

    /* Must be streaming or stopping */
    if (state != WALSNDSTATE_STREAMING &&
      state != WALSNDSTATE_STOPPING)
      continue;

    /* Must be synchronous */
    if (stby->sync_standby_priority == 0)
      continue;

    /* Must have a valid flush position */
    if (!XLogRecPtrIsValid(stby->flush))
      continue;

    /* OK, it's a candidate */
    stby->walsnd_index = i;
    stby->is_me = (walsnd == MyWalSnd);
    n++;
  }

  /*
   * In quorum mode, we return all the candidates.  In priority mode, if we
   * have too many candidates then return only the num_sync ones of highest
   * priority.
   */
  if (SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY &&
    n > SyncRepConfig->num_sync)
  {
    /* Sort by priority ... */
    qsort(*standbys, n, sizeof(SyncRepStandbyData),
        standby_priority_comparator);
    /* ... then report just the first num_sync ones */
    n = SyncRepConfig->num_sync;
  }

  return n;
}

/*
 * qsort comparator to sort SyncRepStandbyData entries by priority
 */
static int
standby_priority_comparator(const void *a, const void *b)
{
  const SyncRepStandbyData *sa = (const SyncRepStandbyData *) a;
  const SyncRepStandbyData *sb = (const SyncRepStandbyData *) b;

  /* First, sort by increasing priority value */
  if (sa->sync_standby_priority != sb->sync_standby_priority)
    return sa->sync_standby_priority - sb->sync_standby_priority;

  /*
   * We might have equal priority values; arbitrarily break ties by position
   * in the WalSnd array.  (This is utterly bogus, since that is arrival
   * order dependent, but there are regression tests that rely on it.)
   */
  return sa->walsnd_index - sb->walsnd_index;
}


/*
 * Check if we are in the list of sync standbys, and if so, determine
 * priority sequence. Return priority if set, or zero to indicate that
 * we are not a potential sync standby.
 *
 * Compare the parameter SyncRepStandbyNames against the application_name
 * for this WALSender, or allow any name if we find a wildcard "*".
 */
static int
SyncRepGetStandbyPriority(void)
{
  const char *standby_name;
  int     priority;
  bool    found = false;

  /*
   * Since synchronous cascade replication is not allowed, we always set the
   * priority of cascading walsender to zero.
   */
  if (am_cascading_walsender)
    return 0;

  if (!SyncStandbysDefined() || SyncRepConfig == NULL)
    return 0;

  standby_name = SyncRepConfig->member_names;
  for (priority = 1; priority <= SyncRepConfig->nmembers; priority++)
  {
    if (pg_strcasecmp(standby_name, application_name) == 0 ||
      strcmp(standby_name, "*") == 0)
    {
      found = true;
      break;
    }
    standby_name += strlen(standby_name) + 1;
  }

  if (!found)
    return 0;

  /*
   * In quorum-based sync replication, all the standbys in the list have the
   * same priority, one.
   */
  return (SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY) ? priority : 1;
}

/*
 * Walk the specified queue from head.  Set the state of any backends that
 * need to be woken, remove them from the queue, and then wake them.
 * Pass all = true to wake whole queue; otherwise, just wake up to
 * the walsender's LSN.
 *
 * The caller must hold SyncRepLock in exclusive mode.
 */
static int
SyncRepWakeQueue(bool all, int mode)
{
  int     numprocs = 0;
  dlist_mutable_iter iter;

  Assert(mode >= 0 && mode < NUM_SYNC_REP_WAIT_MODE);
  Assert(LWLockHeldByMeInMode(SyncRepLock, LW_EXCLUSIVE));
  Assert(SyncRepQueueIsOrderedByLSN(mode));

  dlist_foreach_modify(iter, &WalSndCtl->SyncRepQueue[mode])
  {
    PGPROC     *proc = dlist_container(PGPROC, syncRepLinks, iter.cur);

    /*
     * Assume the queue is ordered by LSN
     */
    if (!all && WalSndCtl->lsn[mode] < proc->waitLSN)
      return numprocs;

    /*
     * Remove from queue.
     */
    dlist_delete_thoroughly(&proc->syncRepLinks);

    /*
     * SyncRepWaitForLSN() reads syncRepState without holding the lock, so
     * make sure that it sees the queue link being removed before the
     * syncRepState change.
     */
    pg_write_barrier();

    /*
     * Set state to complete; see SyncRepWaitForLSN() for discussion of
     * the various states.
     */
    proc->syncRepState = SYNC_REP_WAIT_COMPLETE;

    /*
     * Wake only when we have set state and removed from queue.
     */
    SetLatch(&(proc->procLatch));

    numprocs++;
  }

  return numprocs;
}

/*
 * The checkpointer calls this as needed to update the shared
 * sync_standbys_status flag, so that backends don't remain permanently wedged
 * if synchronous_standby_names is unset.  It's safe to check the current value
 * without the lock, because it's only ever updated by one process.  But we
 * must take the lock to change it.
 */
void
SyncRepUpdateSyncStandbysDefined(void)
{
  bool    sync_standbys_defined = SyncStandbysDefined();

  if (sync_standbys_defined !=
    ((WalSndCtl->sync_standbys_status & SYNC_STANDBY_DEFINED) != 0))
  {
    LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);

    /*
     * If synchronous_standby_names has been reset to empty, it's futile
     * for backends to continue waiting.  Since the user no longer wants
     * synchronous replication, we'd better wake them up.
     */
    if (!sync_standbys_defined)
    {
      int     i;

      for (i = 0; i < NUM_SYNC_REP_WAIT_MODE; i++)
        SyncRepWakeQueue(true, i);
    }

    /*
     * Only allow people to join the queue when there are synchronous
     * standbys defined.  Without this interlock, there's a race
     * condition: we might wake up all the current waiters; then, some
     * backend that hasn't yet reloaded its config might go to sleep on
     * the queue (and never wake up).  This prevents that.
     */
    WalSndCtl->sync_standbys_status = SYNC_STANDBY_INIT |
      (sync_standbys_defined ? SYNC_STANDBY_DEFINED : 0);

    LWLockRelease(SyncRepLock);
  }
  else if ((WalSndCtl->sync_standbys_status & SYNC_STANDBY_INIT) == 0)
  {
    LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);

    /*
     * Note that there is no need to wake up the queues here.  We would
     * reach this path only if SyncStandbysDefined() returns false, or it
     * would mean that some backends are waiting with the GUC set.  See
     * SyncRepWaitForLSN().
     */
    Assert(!SyncStandbysDefined());

    /*
     * Even if there is no sync standby defined, let the readers of this
     * information know that the sync standby data has been initialized.
     * This can just be done once, hence the previous check on
     * SYNC_STANDBY_INIT to avoid useless work.
     */
    WalSndCtl->sync_standbys_status |= SYNC_STANDBY_INIT;

    LWLockRelease(SyncRepLock);
  }
}

#ifdef USE_ASSERT_CHECKING
static bool
SyncRepQueueIsOrderedByLSN(int mode)
{
  XLogRecPtr  lastLSN;
  dlist_iter  iter;

  Assert(mode >= 0 && mode < NUM_SYNC_REP_WAIT_MODE);

  lastLSN = InvalidXLogRecPtr;

  dlist_foreach(iter, &WalSndCtl->SyncRepQueue[mode])
  {
    PGPROC     *proc = dlist_container(PGPROC, syncRepLinks, iter.cur);

    /*
     * Check the queue is ordered by LSN and that multiple procs don't
     * have matching LSNs
     */
    if (proc->waitLSN <= lastLSN)
      return false;

    lastLSN = proc->waitLSN;
  }

  return true;
}
#endif

/*
 * ===========================================================
 * Synchronous Replication functions executed by any process
 * ===========================================================
 */

bool
check_synchronous_standby_names(char **newval, void **extra, GucSource source)
{
  if (*newval != NULL && (*newval)[0] != '\0')
  {
    yyscan_t  scanner;
    int     parse_rc;
    SyncRepConfigData *pconf;

    /* Result of parsing is returned in one of these two variables */
    SyncRepConfigData *syncrep_parse_result = NULL;
    char     *syncrep_parse_error_msg = NULL;

    /* Parse the synchronous_standby_names string */
    syncrep_scanner_init(*newval, &scanner);
    parse_rc = syncrep_yyparse(&syncrep_parse_result, &syncrep_parse_error_msg, scanner);
    syncrep_scanner_finish(scanner);

    if (parse_rc != 0 || syncrep_parse_result == NULL)
    {
      GUC_check_errcode(ERRCODE_SYNTAX_ERROR);
      if (syncrep_parse_error_msg)
        GUC_check_errdetail("%s", syncrep_parse_error_msg);
      else
        /* translator: %s is a GUC name */
        GUC_check_errdetail("\"%s\" parser failed.",
                  "synchronous_standby_names");
      return false;
    }

    if (syncrep_parse_result->num_sync <= 0)
    {
      GUC_check_errmsg("number of synchronous standbys (%d) must be greater than zero",
               syncrep_parse_result->num_sync);
      return false;
    }

    /* GUC extra value must be guc_malloc'd, not palloc'd */
    pconf = (SyncRepConfigData *)
      guc_malloc(LOG, syncrep_parse_result->config_size);
    if (pconf == NULL)
      return false;
    memcpy(pconf, syncrep_parse_result, syncrep_parse_result->config_size);

    *extra = pconf;

    /*
     * We need not explicitly clean up syncrep_parse_result.  It, and any
     * other cruft generated during parsing, will be freed when the
     * current memory context is deleted.  (This code is generally run in
     * a short-lived context used for config file processing, so that will
     * not be very long.)
     */
  }
  else
    *extra = NULL;

  return true;
}

void
assign_synchronous_standby_names(const char *newval, void *extra)
{
  SyncRepConfig = (SyncRepConfigData *) extra;
}

void
assign_synchronous_commit(int newval, void *extra)
{
  switch (newval)
  {
    case SYNCHRONOUS_COMMIT_REMOTE_WRITE:
      SyncRepWaitMode = SYNC_REP_WAIT_WRITE;
      break;
    case SYNCHRONOUS_COMMIT_REMOTE_FLUSH:
      SyncRepWaitMode = SYNC_REP_WAIT_FLUSH;
      break;
    case SYNCHRONOUS_COMMIT_REMOTE_APPLY:
      SyncRepWaitMode = SYNC_REP_WAIT_APPLY;
      break;
    default:
      SyncRepWaitMode = SYNC_REP_NO_WAIT;
      break;
  }
}

Coverage Report

Created: 2026-08-13 07:12