Avoid backend hang during temp table cleanup in deferrable transactions

Previously, a session that had created temporary objects could hang during
exit if its default transaction mode was SERIALIZABLE READ ONLY DEFERRABLE
and a concurrent serializable transaction was prepared. During backend
exit, temporary-relation cleanup pushed a regular transaction snapshot,
which honored the session's default settings and could wait for a safe
serializable snapshot. So, if the conflicting transaction was prepared,
this wait could last indefinitely.

The wait occurred while the backend was already exiting and interrupts
were held off, so even pg_terminate_backend() could not cancel it. The
backend therefore remained until the prepared transaction was resolved.

Temporary-relation cleanup only needs an active MVCC snapshot for fetching
TOAST data from catalog tuples while dropping temporary objects. It does
not need a transaction snapshot affected by the user's default isolation
settings. So, use a catalog snapshot instead, which provides the needed
protection without entering the deferrable safe-snapshot wait.

Backpatch to all supported versions.

Bug: #19441
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Andrey Rachitskiy <pl0h0yp1@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/19441-ec29f3b1363b4a68@postgresql.org
Backpatch-through: 14

3 files changed, 68 insertions, 1 deletions

diff --git a/src/backend/catalog/namespace.c b/src/backend/catalog/namespace.c
index a0e4ce24a7c..8e0c02d2dd5 100644
--- a/src/backend/catalog/namespace.c
+++ b/src/backend/catalog/namespace.c
@@ -4631,7 +4631,15 @@ RemoveTempRelationsCallback(int code, Datum arg)
 		/* Need to ensure we have a usable transaction. */
 		AbortOutOfAnyTransaction();
 		StartTransactionCommand();
-		PushActiveSnapshot(GetTransactionSnapshot());
+
+		/*
+		 * Need an active snapshot for toast fetches during deletion.  Do not
+		 * use GetTransactionSnapshot(): under SERIALIZABLE READ ONLY
+		 * DEFERRABLE it may wait in GetSafeSnapshot(), and proc_exit holds
+		 * off interrupts so that wait cannot be cancelled.  A catalog
+		 * snapshot is enough and avoids that path.
+		 */
+		PushActiveSnapshot(GetCatalogSnapshot(RelationRelationId));
 
 		RemoveTempRelations(myTempNamespace);
 
diff --git a/src/test/modules/test_misc/meson.build b/src/test/modules/test_misc/meson.build
index 216c38016ae..23939fab2a8 100644
--- a/src/test/modules/test_misc/meson.build
+++ b/src/test/modules/test_misc/meson.build
@@ -18,6 +18,7 @@ tests += {
       't/007_catcache_inval.pl',
       't/008_replslot_single_user.pl',
       't/013_temp_obj_multisession.pl',
+      't/015_temp_schema_exit_deferrable.pl',
     ],
   },
 }
diff --git a/src/test/modules/test_misc/t/015_temp_schema_exit_deferrable.pl b/src/test/modules/test_misc/t/015_temp_schema_exit_deferrable.pl
new file mode 100644
index 00000000000..07de36630a6
--- /dev/null
+++ b/src/test/modules/test_misc/t/015_temp_schema_exit_deferrable.pl
@@ -0,0 +1,58 @@
+# Copyright (c) 2026, PostgreSQL Global Development Group
+
+# Backend exit must finish temp-schema cleanup even when the session default
+# is SERIALIZABLE READ ONLY DEFERRABLE and a prepared serializable transaction
+# is still around.
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('deferrable_temp_exit');
+$node->init;
+$node->append_conf('postgresql.conf', 'max_prepared_transactions = 1');
+$node->start;
+
+my $psql1 = $node->background_psql('postgres');
+
+$psql1->query_safe(
+	q{
+CREATE TEMPORARY TABLE tt (i int);
+SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;
+});
+
+my $pid = $psql1->query_safe(q{SELECT pg_backend_pid();});
+chomp $pid;
+like($pid, qr/^\d+$/, "backend pid $pid");
+
+# Overlapping read/write serializable xact that outlives session 1.
+$node->safe_psql(
+	'postgres',
+	q{
+CREATE TABLE t (i int);
+BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+INSERT INTO t VALUES (1);
+PREPARE TRANSACTION 'pt';
+});
+
+# Disconnect: RemoveTempRelationsCallback runs during backend exit.
+$psql1->quit;
+
+ok( $node->poll_query_until(
+		'postgres',
+		"SELECT count(*) = 0 FROM pg_stat_activity WHERE pid = $pid"),
+	'backend exited despite prepared serializable xact');
+
+is( $node->safe_psql(
+		'postgres',
+		q{SELECT count(*) FROM pg_class WHERE relname = 'tt' AND relpersistence = 't'}
+	),
+	'0',
+	'temporary table cleaned up on exit');
+
+$node->safe_psql('postgres', q{ROLLBACK PREPARED 'pt';});
+$node->safe_psql('postgres', q{DROP TABLE t;});
+
+done_testing();