Update TransactionXmin when MyProc->xmin is updated

hlinnaka · hlinnaka · commit 7cfdb4d1e778 · 2024-12-21T23:42:52.000+02:00
GetSnapshotData() set TransactionXmin = MyProc->xmin, but when SnapshotResetXmin() advanced MyProc->xmin, it did not advance TransactionXmin correspondingly. That meant that TransactionXmin could be older than MyProc->xmin, and XIDs between than TransactionXmin and the real MyProc->xmin could be vacuumed away. One known consequence is in pg_subtrans lookups: we might try to look up the status of an XID that was already truncated away. Back-patch to all supported versions. Reviewed-by: Andres Freund Discussion: https://www.postgresql.org/message-id/d27a046d-a1e4-47d1-a95c-fbabe41debb4@iki.fi
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
@@ -920,15 +920,15 @@ SnapshotResetXmin(void)
 
 	if (pairingheap_is_empty(&RegisteredSnapshots))
 	{
-		MyProc->xmin = InvalidTransactionId;
+		MyProc->xmin = TransactionXmin = InvalidTransactionId;
 		return;
 	}
 
 	minSnapshot = pairingheap_container(SnapshotData, ph_node,
 										pairingheap_first(&RegisteredSnapshots));
 
 	if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
-		MyProc->xmin = minSnapshot->xmin;
+		MyProc->xmin = TransactionXmin = minSnapshot->xmin;
 }
 
 /*

Original file line number	Diff line number	Diff line change
`@@ -920,15 +920,15 @@ SnapshotResetXmin(void)`
`920`	`920`
`921`	`921`	`if (pairingheap_is_empty(&RegisteredSnapshots))`
`922`	`922`	`{`
`923`		`- MyProc->xmin = InvalidTransactionId;`
	`923`	`+ MyProc->xmin = TransactionXmin = InvalidTransactionId;`
`924`	`924`	`return;`
`925`	`925`	`}`
`926`	`926`
`927`	`927`	`minSnapshot = pairingheap_container(SnapshotData, ph_node,`
`928`	`928`	`pairingheap_first(&RegisteredSnapshots));`
`929`	`929`
`930`	`930`	`if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))`
`931`		`- MyProc->xmin = minSnapshot->xmin;`
	`931`	`+ MyProc->xmin = TransactionXmin = minSnapshot->xmin;`
`932`	`932`	`}`
`933`	`933`
`934`	`934`	`/*`