Avoid accumulating relation locks during sequence synchronization.

While collecting the sequences to synchronize, the sequence sync worker
opened each INIT sequence with RowExclusiveLock and held it until the
transaction committed. With many such sequences, this could exhaust the
shared lock table and fail with "out of shared memory".

The worker only reads each sequence's identity (namespace and name) here
and needs it to stay stable while read, for which AccessShareLock is
enough, as it conflicts with the AccessExclusiveLock taken by DROP,
RENAME, and SET SCHEMA. Take that lock instead and release it as soon as
the identity is read. The later synchronization re-opens each sequence, so
it does not rely on the lock being retained.

Reported-by: Noah Misch <noah@leadboat.com>
Author: vignesh C <vignesh21@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 19, where it was introduced
Discussion: https://postgr.es/m/20260710045217.f0.noahmisch@microsoft.com

diff --git a/src/backend/replication/logical/sequencesync.c b/src/backend/replication/logical/sequencesync.c
index 40f0f1c69733213361e85e67b7f4424850740267..bd74a17d28bbdc141b17f90734e93195e2ee6239 100644 (file)
--- a/src/backend/replication/logical/sequencesync.c
+++ b/src/backend/replication/logical/sequencesync.c
@@ -752,7 +752,16 @@ LogicalRepSyncSequences(void)
 
                subrel = (Form_pg_subscription_rel) GETSTRUCT(tup);
 
-               sequence_rel = try_table_open(subrel->srrelid, RowExclusiveLock);
+               /*
+                * Lock the sequence so its identity (namespace and name) cannot
+                * change under us via a concurrent DROP, RENAME or SET SCHEMA. The
+                * lock is released immediately rathen than at the transaction end.
+                * The later synchronization does not depend on this captured identity
+                * remaining valid, as it re-opens the sequence and tolerates
+                * concurrent changes. Releasing early also avoids holding one lock
+                * per sequence, which could exhaust the lock table.
+                */
+               sequence_rel = try_table_open(subrel->srrelid, AccessShareLock);
 
                /* Skip if sequence was dropped concurrently */
                if (!sequence_rel)
@@ -761,7 +770,7 @@ LogicalRepSyncSequences(void)
                /* Skip if the relation is not a sequence */
                if (sequence_rel->rd_rel->relkind != RELKIND_SEQUENCE)
                {
-                       table_close(sequence_rel, NoLock);
+                       table_close(sequence_rel, AccessShareLock);
                        continue;
                }
 
@@ -779,7 +788,7 @@ LogicalRepSyncSequences(void)
 
                MemoryContextSwitchTo(oldctx);
 
-               table_close(sequence_rel, NoLock);
+               table_close(sequence_rel, AccessShareLock);
        }
 
        /* Cleanup */