Fix rare assertion failure in standby, if primary is restarted

hlinnaka · hlinnaka · commit 302ce5bd93b4 · 2025-03-23T20:41:52.000+02:00
During hot standby, ExpireAllKnownAssignedTransactionIds() and ExpireOldKnownAssignedTransactionIds() functions mark old transactions as no-longer running, but they failed to update xactCompletionCount and latestCompletedXid. AFAICS it would not lead to incorrect query results, because those functions effectively turn in-progress transactions into aborted transactions and an MVCC snapshot considers both as "not visible". But it could surprise GetSnapshotDataReuse() and trigger the "TransactionIdPrecedesOrEquals(TransactionXmin, RecentXmin))" assertion in it, if the apparent xmin in a backend would move backwards. We saw this happen when GetCatalogSnapshot() would reuse an older catalog snapshot, when GetTransactionSnapshot() had already advanced TransactionXmin. The bug goes back all the way to commit 623a9ba in v14 that introduced the snapshot reuse mechanism, but it started to happen more frequently with commit 952365c which removed a GetTransactionSnapshot() call from backend startup. That made it more likely for ExpireOldKnownAssignedTransactionIds() to be called between GetCatalogSnapshot() and the first GetTransactionSnapshot() in a backend. Andres Freund first spotted this assertion failure on buildfarm member 'skink'. Reproduction and analysis by Tomas Vondra. Backpatch-through: 14 Discussion: https://www.postgresql.org/message-id/oey246mcw43cy4qw2hqjmurbd62lfdpcuxyqiu7botx3typpax%40h7o7mfg5zmdj
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
@@ -4496,9 +4496,23 @@ ExpireTreeKnownAssignedTransactionIds(TransactionId xid, int nsubxids,
 void
 ExpireAllKnownAssignedTransactionIds(void)
 {
+	FullTransactionId latestXid;
+
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 	KnownAssignedXidsRemovePreceding(InvalidTransactionId);
 
+	/* Reset latestCompletedXid to nextXid - 1 */
+	Assert(FullTransactionIdIsValid(TransamVariables->nextXid));
+	latestXid = TransamVariables->nextXid;
+	FullTransactionIdRetreat(&latestXid);
+	TransamVariables->latestCompletedXid = latestXid;
+
+	/*
+	 * Any transactions that were in-progress were effectively aborted, so
+	 * advance xactCompletionCount.
+	 */
+	TransamVariables->xactCompletionCount++;
+
 	/*
 	 * Reset lastOverflowedXid.  Currently, lastOverflowedXid has no use after
 	 * the call of this function.  But do this for unification with what
@@ -4516,8 +4530,18 @@ ExpireAllKnownAssignedTransactionIds(void)
 void
 ExpireOldKnownAssignedTransactionIds(TransactionId xid)
 {
+	TransactionId latestXid;
+
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
+	/* As in ProcArrayEndTransaction, advance latestCompletedXid */
+	latestXid = xid;
+	TransactionIdRetreat(latestXid);
+	MaintainLatestCompletedXidRecovery(latestXid);
+
+	/* ... and xactCompletionCount */
+	TransamVariables->xactCompletionCount++;
+
 	/*
 	 * Reset lastOverflowedXid if we know all transactions that have been
 	 * possibly running are being gone.  Not doing so could cause an incorrect
