From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Sat, 21 Dec 2024 21:42:39 +0000 (+0200)
Subject: Update TransactionXmin when MyProc->xmin is updated
X-Git-Tag: REL_14_16~54
X-Git-Url: http://git.postgresql.org/gitweb/?a=commitdiff_plain;h=41eafbb494c9578fa04ffa529a10e4cfd6942421;p=postgresql.git

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 3037cf478d1..a64b6a2b303 100644
--- 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;
 }
 
 /*