Reset lastOverflowedXid on standby when needed

akorotkov · akorotkov · commit b0f6bd48f35d · 2021-11-06T19:13:53.000+03:00
Currently, lastOverflowedXid is never reset. It's just adjusted on new transactions known to be overflowed. But if there are no overflowed transactions for a long time, snapshots could be mistakenly marked as suboverflowed due to wraparound. This commit fixes this issue by resetting lastOverflowedXid when needed altogether with KnownAssignedXids. Backpatch to all supported versions. Reported-by: Stan Hu Discussion: https://postgr.es/m/CAMBWrQ%3DFp5UAsU_nATY7EMY7NHczG4-DTDU%3DmCvBQZAQ6wa2xQ%40mail.gmail.com Author: Kyotaro Horiguchi, Alexander Korotkov Reviewed-by: Stan Hu, Simon Riggs, Nikolay Samokhvalov, Andrey Borodin, Dmitry Dolgov
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
@@ -4445,24 +4445,41 @@ ExpireTreeKnownAssignedTransactionIds(TransactionId xid, int nsubxids,
 
 /*
  * ExpireAllKnownAssignedTransactionIds
- *		Remove all entries in KnownAssignedXids
+ *		Remove all entries in KnownAssignedXids and reset lastOverflowedXid.
  */
 void
 ExpireAllKnownAssignedTransactionIds(void)
 {
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 	KnownAssignedXidsRemovePreceding(InvalidTransactionId);
+
+	/*
+	 * Reset lastOverflowedXid.  Currently, lastOverflowedXid has no use after
+	 * the call of this function.  But do this for unification with what
+	 * ExpireOldKnownAssignedTransactionIds() do.
+	 */
+	procArray->lastOverflowedXid = InvalidTransactionId;
 	LWLockRelease(ProcArrayLock);
 }
 
 /*
  * ExpireOldKnownAssignedTransactionIds
- *		Remove KnownAssignedXids entries preceding the given XID
+ *		Remove KnownAssignedXids entries preceding the given XID and
+ *		potentially reset lastOverflowedXid.
  */
 void
 ExpireOldKnownAssignedTransactionIds(TransactionId xid)
 {
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+
+	/*
+	 * Reset lastOverflowedXid if we know all transactions that have been
+	 * possibly running are being gone.  Not doing so could cause an incorrect
+	 * lastOverflowedXid value, which makes extra snapshots be marked as
+	 * suboverflowed.
+	 */
+	if (TransactionIdPrecedes(procArray->lastOverflowedXid, xid))
+		procArray->lastOverflowedXid = InvalidTransactionId;
 	KnownAssignedXidsRemovePreceding(xid);
 	LWLockRelease(ProcArrayLock);
 }
