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Fix race condition in committing a serializable transaction

The finished transaction list can contain XIDs that are older than theserializable global xmin. It's a short-lived state;ClearOldPredicateLocks() removes any such transactions from the list,and it's called whenever the global xmin advances. But if anotherbackend calls SummarizeOldestCommittedSxact() in that window, it willcall SerialAdd() on an XID that's older than the global xmin, or ifthere are no more transactions running, when global xmin isinvalid. That trips the assertion in SerialAdd().Fixes bug #18658 reported by Andrew Bille. Thanks to Alexander Lakhinfor analysis. Backpatch to all versions.Discussion:https://www.postgresql.org/message-id/18658-7dab125ec688c70b%40postgresql.org

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src/backend/storage/lmgr
- predicate.c

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`‎src/backend/storage/lmgr/predicate.c`

Lines changed: 9 additions & 4 deletions

Original file line number	Diff line number	Diff line change
`@@ -877,12 +877,17 @@ SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)`
`877`	`877`	`LWLockAcquire(SerialControlLock,LW_EXCLUSIVE);`
`878`	`878`
`879`	`879`	`/*`
`880`		`- * If no serializable transactions are active, there shouldn't be anything`
`881`		`- * to push out to the SLRU. Hitting this assert would mean there's`
`882`		`- * something wrong with the earlier cleanup logic.`
	`880`	`+ * If 'xid' is older than the global xmin (== tailXid), there's no need to`
	`881`	`+ * store it, after all. This can happen if the oldest transaction holding`
	`882`	`+ * back the global xmin just finished, making 'xid' uninteresting, but`
	`883`	`+ * ClearOldPredicateLocks() has not yet run.`
`883`	`884`	`*/`
`884`	`885`	`tailXid=serialControl->tailXid;`
`885`		`-Assert(TransactionIdIsValid(tailXid));`
	`886`	`+if (!TransactionIdIsValid(tailXid)\|\|TransactionIdPrecedes(xid,tailXid))`
	`887`	`+{`
	`888`	`+LWLockRelease(SerialControlLock);`
	`889`	`+return;`
	`890`	`+}`
`886`	`891`
`887`	`892`	`/*`
`888`	`893`	`* If the SLRU is currently unused, zero out the whole active region from`

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Commit1a43de5

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`‎src/backend/storage/lmgr/predicate.c`

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