BUG: deadlock between autovacuum worker and client backend during removal of orphan temp tables with sequences

Hi pgsql-bugs,

Several customers reported deadlock error raised after accidental 
restart of server.

Problem description
--------------------

Client backend tries to create first temporary table after connection 
and it gets deadlock error.
Here is example of log file records:

2023-12-29 13:48:21.977 MSK [29546] ERROR:  deadlock detected at 
character 24
2023-12-29 13:48:21.977 MSK [29546] DETAIL:  Process 29546 waits for 
AccessExclusiveLock on relation 416811 of database 5; blocked by process 
29513.
         Process 29513 waits for AccessExclusiveLock on relation 416795 
of database 5; blocked by process 29546.
         Process 29546: create temporary table tt1 (c1 bytea, c2 bytea, 
c3 bytea, c4 bytea, c5 numeric(7, 0), c6 serial4 ) without oids;
         Process 29513: <command string not enabled>

29546 is PID of client backend, 29513 is PID of autovacuum worker.

Client backend tries to initialize namespace and cleans up all orphan 
objects into its temporary namespace. Whole cleanup is performed in 
scope of client transaction.
Autovacuum worker tries to remove tables one-by-one, using one 
transaction for each table.

How to reproduce
-----------------

To reproduce it, use pgbench and attached SQL file. Start pgbench with 
following arguments:

  $ pgbench -n -j 1 -c 50 -T 10 -C -f create_temp_tables.sql 
--verbose-errors

Kill postgres processes to make table orphan. For local connections the 
following command can be used:

  $ pkill -9  -f "postgres.*local"

Wait a bit for recovery completion and start pgbench again:

  $ pgbench -n -j 1 -c 50 -T 10 -C -f create_temp_tables.sql 
--verbose-errors

And check log files for deadlock. As autovacuum start is hardly 
predictable, the launcher nap time is better to set to 1:

  # alter system set autovacuum_naptime=1;

I've noticed that preliminary "vacuum database" helps to reproduce it.

Here is all my custom parameters:
alter system set autovacuum_naptime=1;
alter system set max_connections to 1000;
alter system set backtrace_functions to 'DeadLockReport';
alter system set logging_collector to on;
alter system set log_filename to 'postgresql-%Y-%m-%d.log';
alter system set autovacuum_max_workers to 1;

Stack traces
--------------

Client backend has following stack trace (thanks to backtrace_functions:
         0xafbe7e <DeadLockReport+0x27e> at /opt/pgpro/mybuild/bin/postgres
         0xb00dd0 <GrantAwaitedLock> at /opt/pgpro/mybuild/bin/postgres
         0xaff983 <LockAcquireExtended+0x603> at 
/opt/pgpro/mybuild/bin/postgres
         0xafd2f9 <LockRelationOid+0x69> at /opt/pgpro/mybuild/bin/postgres
         0x742a07 <findDependentObjects+0x627> at 
/opt/pgpro/mybuild/bin/postgres
         0x7422a4 <performDeletion+0xf4> at /opt/pgpro/mybuild/bin/postgres
         0x754624 <AccessTempTableNamespace+0x134> at 
/opt/pgpro/mybuild/bin/postgres
         0x754424 <RangeVarGetCreationNamespace+0xa4> at 
/opt/pgpro/mybuild/bin/postgres
         0x754f6c <RangeVarGetAndCheckCreationNamespace+0x5c> at 
/opt/pgpro/mybuild/bin/postgres
         0x7d36b0 <transformCreateStmt+0x50> at 
/opt/pgpro/mybuild/bin/postgres
         0xb2a05f <ProcessUtilitySlow+0xdf> at 
/opt/pgpro/mybuild/bin/postgres
         0xb28450 <standard_ProcessUtility+0x530> at 
/opt/pgpro/mybuild/bin/postgres
         0xb27ea8 <ProcessUtility+0x68> at /opt/pgpro/mybuild/bin/postgres
         0xb2745b <PortalRunUtility+0xab> at /opt/pgpro/mybuild/bin/postgres
         0xb2681c <PortalRunMulti+0x1fc> at /opt/pgpro/mybuild/bin/postgres
         0xb25eb8 <PortalRun+0x1e8> at /opt/pgpro/mybuild/bin/postgres
         0xb24127 <exec_simple_query+0x5c7> at 
/opt/pgpro/mybuild/bin/postgres
         0xb2170e <PostgresMain+0x119e> at /opt/pgpro/mybuild/bin/postgres
         0xa4e4f8 <BackendRun+0x38> at /opt/pgpro/mybuild/bin/postgres
         0xa4dabf <ServerLoop+0xdef> at /opt/pgpro/mybuild/bin/postgres
         0xa4a824 <PostmasterMain+0x1594> at /opt/pgpro/mybuild/bin/postgres
         0x93a58a <main+0x33a> at /opt/pgpro/mybuild/bin/postgres
         0x82b77d36a <__libc_start1+0x12a> at /lib/libc.so.7

Autovacuum worker (got by increase deadlock timeout and debugger):

         WaitEventSetWaitBlock() at latch.c:1,649 0xae6ffb
         WaitEventSetWait() at latch.c:1,435 0xae6ffb
         WaitLatch() at latch.c:497 0xae6cc4
         ProcSleep() at proc.c:1,341 0xb135de
         WaitOnLock() at lock.c:1,859 0xb00d7d
         LockAcquireExtended() at lock.c:1,101 0xaffa03
         LockRelationOid() at lmgr.c:117 0xafd379
         AcquireDeletionLock() at dependency.c:1,552 0x742a37
         findDependentObjects() at dependency.c:894 0x742a37
         performDeletion() at dependency.c:346 0x7422d4
         do_autovacuum() at autovacuum.c:2,274 0xa41f5d
         AutoVacWorkerMain() at autovacuum.c:1,716 0xa40200
         StartAutoVacWorker() at autovacuum.c:1,494 0xa3fe26
         StartAutovacuumWorker() at postmaster.c:5,463 0xa4b5e1
         sigusr1_handler() at postmaster.c:5,172 0xa4b5e1
         handle_signal() at thr_sig.c:301 0x82980755f
         thr_sighandler() at thr_sig.c:244 0x829806b1b

Reason
-------

Table contains columns with serial type, i.e. with autogenerated 
sequences. When backend tries to clean namespace,
it removes tables and sequences in random way. When autovacuum worker 
tries to remove table, at first it locks table,
then tries to remove sequence by dependency.

So it may happen that:
  - Backend locks and removes sequence
  - Autovacuum worker locks table to remove, finds sequence and tries to 
remove sequence and get locked.
  - Backend tries to lock table locked by autovacuum worker and get locked.

Idea how to fix it
------------------

As of now, autovacuum worker tries to avoid concurrency by preliminary 
conditional lock of table. It provides guarantee to
avoid concurrency with other workers. To avoid concurrency with client 
backend during orphan table removal, it may be worth
to conditionally lock namespace before removal attempt.

If backend starts namespace removal, it locks namespace exclusively and 
autovacuum worker can identify it by conditional locking and can skip 
removal.
On another hand, if worker gets namespace lock, then there is no backend 
removing orphan tables.

Also it may worth to mention that worker can try to get AccessShare lock 
to allow concurrency between workers.

Please find attached patch for this idea.

Please feel free to ask any questions!

Thank you!

-- 
Michael Zhilin
Postgres Professional
https://www.postgrespro.ru