sync_standbys_defined read/write race on startup - Mailing list pgsql-hackers

Hi,
On my windows machine I've found unstable test, it fails very rarely, but anyway, sometimes it fails.

src/test/recovery/t/009_twophase.pl

#   Failed test 'Committed prepared transaction is visible to new snapshot in standby'
#   at t/009_twophase.pl line 360.
#          got: '0'
#     expected: '2'
# Looks like you failed 1 test of 30.
t/009_twophase.pl ....................
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/30 subtests

cat src/test/recovery/t/009_twophase.pl

....
$cur_primary->psql(
    'postgres', "
    SET synchronous_commit='remote_apply'; -- To ensure the standby is caught up
    CREATE TABLE t_009_tbl_standby_mvcc (id int, msg text);
    BEGIN;
    INSERT INTO t_009_tbl_standby_mvcc VALUES (1, 'issued to ${cur_primary_name}');
    SAVEPOINT s1;
    INSERT INTO t_009_tbl_standby_mvcc VALUES (2, 'issued to ${cur_primary_name}');
    PREPARE TRANSACTION 'xact_009_standby_mvcc';
    ");
$cur_primary->stop;
$cur_standby->restart;

# Acquire a snapshot in standby, before we commit the prepared transaction
my $standby_session =
  $cur_standby->background_psql('postgres', on_error_die => 1);
$standby_session->query_safe("BEGIN ISOLATION LEVEL REPEATABLE READ");
$psql_out =
  $standby_session->query_safe("SELECT count(*) FROM t_009_tbl_standby_mvcc");
is($psql_out, '0',
    "Prepared transaction not visible in standby before commit");

# Commit the transaction in primary
$cur_primary->start;
$cur_primary->psql(
    'postgres', "
SET synchronous_commit='remote_apply'; -- To ensure the standby is caught up
COMMIT PREPARED 'xact_009_standby_mvcc';
");

# Still not visible to the old snapshot
$psql_out =
  $standby_session->query_safe("SELECT count(*) FROM t_009_tbl_standby_mvcc");
is($psql_out, '0',
    "Committed prepared transaction not visible to old snapshot in standby");

# Is visible to a new snapshot
$standby_session->query_safe("COMMIT");
$psql_out =
  $standby_session->query_safe("SELECT count(*) FROM t_009_tbl_standby_mvcc");
is($psql_out, '2',
    "Committed prepared transaction is visible to new snapshot in standby");
$standby_session->quit;
.....



Test check visibility of prepared transactions in standby after a restart while primary is down.
Failed assert check that changes, commited on transaction on primary node, were synchronously replicated to standby node.
In test flow we have primary server stop/start events.
Assert failure is hard to reproduce, but I found how to do it synthetically, can do it by injecting in code synthetical sleep call

--- a/src/backend/replication/syncrep.c
+++ b/src/backend/replication/syncrep.c
@@ -924,6 +924,7 @@ SyncRepUpdateSyncStandbysDefined(void)
 
        if (sync_standbys_defined != WalSndCtl->sync_standbys_defined)
        {
+               sleep(2);
                LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
 
                /*
                

The root cause is WalSndCtl->sync_standbys_defined, it is located in shared memory and can be updated by checkpointer process only,
in backends it can be read in case of sync replication. So we have race on postgres startup between backend, processing commit request, and checkpointer,
setting WalSndCtl->sync_standbys_defined.

Synchronous replication occurred in SyncRepWaitForLSN(syncrep.c)

void
SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
{
.....

    /*
     * Fast exit if user has not requested sync replication, or there are no
     * sync replication standby names defined.
     *
     * Since this routine gets called every commit time, it's important to
     * exit quickly if sync replication is not requested. So we check
     * WalSndCtl->sync_standbys_defined flag without the lock and exit
     * immediately if it's false. If it's true, we need to check it again
     * later while holding the lock, to check the flag and operate the sync
     * rep queue atomically. This is necessary to avoid the race condition
     * described in SyncRepUpdateSyncStandbysDefined(). On the other hand, if
     * it's false, the lock is not necessary because we don't touch the queue.
     */
    if (!SyncRepRequested() ||
        !((volatile WalSndCtlData *) WalSndCtl)->sync_standbys_defined)
        return;

.....

    if (!WalSndCtl->sync_standbys_defined ||
        lsn <= WalSndCtl->lsn[mode])
    {
        LWLockRelease(SyncRepLock);
        return;
    }
    
...........


Checkpointer set WalSndCtl->sync_standbys_defined parameter in src/backend/replication/syncrep.c,
SyncRepUpdateSyncStandbysDefined method

void
SyncRepUpdateSyncStandbysDefined(void)
{
    bool        sync_standbys_defined = SyncStandbysDefined();

    if (sync_standbys_defined != WalSndCtl->sync_standbys_defined)
    {
        LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
..................
        WalSndCtl->sync_standbys_defined = sync_standbys_defined;

        LWLockRelease(SyncRepLock);
    }
}

So my question is
can we accept such behavior as bug of postgres codebase, or, on other way, it is bug of unstable test?
P.S.
For me it is looking strange, that postgres start processing workload before full config initializing.
Attached possible patch for codebase.

Best regards
Melnikov Maksim