pgsql: Fix race condition in committing a serializable transaction

Fix race condition in committing a serializable transaction

The finished transaction list can contain XIDs that are older than the
serializable global xmin. It's a short-lived state;
ClearOldPredicateLocks() removes any such transactions from the list,
and it's called whenever the global xmin advances. But if another
backend calls SummarizeOldestCommittedSxact() in that window, it will
call SerialAdd() on an XID that's older than the global xmin, or if
there are no more transactions running, when global xmin is
invalid. That trips the assertion in SerialAdd().

Fixes bug #18658 reported by Andrew Bille. Thanks to Alexander Lakhin
for analysis. Backpatch to all versions.

Discussion: https://www.postgresql.org/message-id/18658-7dab125ec688c70b%40postgresql.org

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/22665f2106248793cfd8dd0a49fe3c5f7a4bfdfe

Modified Files
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src/backend/storage/lmgr/predicate.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)