pgsql: Update TransactionXmin when MyProc->xmin is updated

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

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/acd5c28db5d0cdb9446c5cb09996629f78287b82

Modified Files
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src/backend/utils/time/snapmgr.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)