Re: Assertion failure in SnapBuildInitialSnapshot()

Hello,

01.02.2024 21:20, vignesh C wrote:
> The patch which you submitted has been awaiting your attention for
> quite some time now. As such, we have moved it to "Returned with
> Feedback" and removed it from the reviewing queue. Depending on
> timing, this may be reversible. Kindly address the feedback you have
> received, and resubmit the patch to the next CommitFest.

While analyzing buildfarm failures, I found [1], which demonstrates the
assertion failure discussed here:
---
031_column_list_publisher.log
TRAP: FailedAssertion("TransactionIdPrecedesOrEquals(safeXid, snap->xmin)", File:
"/home/bf/bf-build/skink/REL_15_STABLE/pgsql.build/../pgsql/src/backend/replication/logical/snapbuild.c", Line: 614,
PID: 1882382)
---

I've managed to reproduce the assertion failure on REL_15_STABLE with the
following modification:
@@ -3928,6 +3928,7 @@ ProcArraySetReplicationSlotXmin(TransactionId xmin, TransactionId catalog_xmin,
 {
     Assert(!already_locked || LWLockHeldByMe(ProcArrayLock));

+pg_usleep(1000);
     if (!already_locked)
         LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

using the script:
numjobs=100
createdb db
export PGDATABASE=db

for ((i=1;i<=100;i++)); do
echo "iteration $i"

for ((j=1;j<=numjobs;j++)); do
echo "
SELECT pg_create_logical_replication_slot('s$j', 'test_decoding');
SELECT txid_current();
" | psql >>/dev/null 2>&1 &

echo "
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
CREATE_REPLICATION_SLOT slot$j LOGICAL test_decoding USE_SNAPSHOT;
" | psql -d "dbname=db replication=database" >>/dev/null 2>&1 &
done
wait

for ((j=1;j<=numjobs;j++)); do
echo "
DROP_REPLICATION_SLOT slot$j;
" | psql -d "dbname=db replication=database" >/dev/null

echo "SELECT pg_drop_replication_slot('s$j');" | psql >/dev/null
done

grep 'TRAP' server.log && break;
done

(with
wal_level = logical
max_replication_slots = 200
max_wal_senders = 200
in postgresql.conf)

iteration 18
ERROR:  replication slot "slot13" is active for PID 538431
TRAP: FailedAssertion("TransactionIdPrecedesOrEquals(safeXid, snap->xmin)", File: "snapbuild.c", Line: 614, PID: 538431)

I've also confirmed that fix_concurrent_slot_xmin_update.patch fixes the
issue.

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2024-05-15%2020%3A55%3A17

Best regards,
Alexander