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Update TransactionXmin when MyProc->xmin is updated

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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
This commit is contained in:
Heikki Linnakangas 2024-12-21 23:42:39 +02:00
parent 0350b876b0
commit 7cfdb4d1e7
1 changed files with 2 additions and 2 deletions
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4
src/backend/utils/time/snapmgr.c
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@ -920,7 +920,7 @@ SnapshotResetXmin(void)
if (pairingheap_is_empty(&RegisteredSnapshots)) if (pairingheap_is_empty(&RegisteredSnapshots))
{ {
MyProc->xmin = InvalidTransactionId; MyProc->xmin = TransactionXmin = InvalidTransactionId;
return; return;
} }
@ -928,7 +928,7 @@ SnapshotResetXmin(void)
pairingheap_first(&RegisteredSnapshots)); pairingheap_first(&RegisteredSnapshots));
if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin)) if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
MyProc->xmin = minSnapshot->xmin; MyProc->xmin = TransactionXmin = minSnapshot->xmin;
} }
/* /*