Update TransactionXmin when MyProc->xmin is updated

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/[email protected]

Author: hlinnaka

src/backend/utils/time/snapmgr.c

@@ -1045,15 +1045,15 @@ SnapshotResetXmin(void)
 
 	if (pairingheap_is_empty(&RegisteredSnapshots))
 	{
-		MyPgXact->xmin = InvalidTransactionId;
+		MyPgXact->xmin = TransactionXmin = InvalidTransactionId;
 		return;
 	}
 
 	minSnapshot = pairingheap_container(SnapshotData, ph_node,
 										pairingheap_first(&RegisteredSnapshots));
 
 	if (TransactionIdPrecedes(MyPgXact->xmin, minSnapshot->xmin))
-		MyPgXact->xmin = minSnapshot->xmin;
+		MyPgXact->xmin = TransactionXmin = minSnapshot->xmin;
 }
 
 /*