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/d27a046d-a1e4-47d1-a95c-fbabe41debb4@iki.fi

diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 3037cf478d1dece60e8f8f065b04c0f4a34c6c20..a64b6a2b303bafa2e4dabc8adc0caa0263478ed2 100644 (file)
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -948,7 +948,7 @@ SnapshotResetXmin(void)
 
        if (pairingheap_is_empty(&RegisteredSnapshots))
        {
-               MyProc->xmin = InvalidTransactionId;
+               MyProc->xmin = TransactionXmin = InvalidTransactionId;
                return;
        }
 
@@ -956,7 +956,7 @@ SnapshotResetXmin(void)
                                                                                pairingheap_first(&RegisteredSnapshots));
 
        if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
-               MyProc->xmin = minSnapshot->xmin;
+               MyProc->xmin = TransactionXmin = minSnapshot->xmin;
 }
 
 /*