1 files changed, 67 insertions, 7 deletions

diff --git a/src/backend/replication/syncrep.c b/src/backend/replication/syncrep.c
index df1e341c764..3fa5fa42ee4 100644
--- a/src/backend/replication/syncrep.c
+++ b/src/backend/replication/syncrep.c
@@ -177,16 +177,52 @@ SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
 	Assert(MyProc->syncRepState == SYNC_REP_NOT_WAITING);
 
 	/*
-	 * We don't wait for sync rep if WalSndCtl->sync_standbys_defined is not
-	 * set.  See SyncRepUpdateSyncStandbysDefined.
+	 * 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_defined ||
-		lsn <= WalSndCtl->lsn[mode])
+	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;
 	}
@@ -927,7 +963,7 @@ SyncRepWakeQueue(bool all, int mode)
 
 /*
  * The checkpointer calls this as needed to update the shared
- * sync_standbys_defined flag, so that backends don't remain permanently wedged
+ * 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.
@@ -937,7 +973,8 @@ SyncRepUpdateSyncStandbysDefined(void)
 {
 	bool		sync_standbys_defined = SyncStandbysDefined();
 
-	if (sync_standbys_defined != WalSndCtl->sync_standbys_defined)
+	if (sync_standbys_defined !=
+		((WalSndCtl->sync_standbys_status & SYNC_STANDBY_DEFINED) != 0))
 	{
 		LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
 
@@ -961,7 +998,30 @@ SyncRepUpdateSyncStandbysDefined(void)
 		 * 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_defined = sync_standbys_defined;
+		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);
 	}