Hi,
The 021_twophase test has failed on Kestrel at [1] with the following error:
# Failed test 'should be no prepared transactions on subscriber'
# at
/home/bf/bf-build/kestrel/HEAD/pgsql/src/test/subscription/t/021_twophase.pl
line 438.
# got: '1'
# expected: '0'
# Looks like you failed 1 test of 30.
This failure is caused by a prepared transaction that was not properly
committed due to replication lag on one of the subscriptions. The test
involves two subscriptions: tap_sub and tap_sub_copy. After committing
the prepared transaction 'mygid', the test only waits for tap_sub_copy
to catch up:
node_publisher->wait_for_catchup($appname_copy);
However, tap_sub is dropped before ensuring it has replayed the commit
of 'mygid' prepared transaction, leading to a leftover prepared
transaction on the subscriber:
$node_subscriber->safe_psql('postgres', "DROP SUBSCRIPTION tap_sub");
When the test later checks for the number of prepared transactions, it
fails because tap_sub had not finished applying the commit:
# at line 438
# got: '1'
# expected: '0'
This issue can be consistently reproduced by injecting a delay (e.g.,
3 seconds) in tap_sub's walsender while decoding the commit of
'mygid'. A patch to demonstrate this behavior is provided at
021_two_phase_test_failure_reproduce.patch. The test can be fixed by
explicitly waiting for both subscriptions to catch up before dropping
either. A patch implementing this fix is attached.
Thanks Amit for the offline discussion and sharing your thoughts on the same.
[1] -
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=kestrel&dt=2025-05-22%2021%3A19%3A22
Regards,
Vignesh
From 4f75511e9866ef29f79789e5be0a31f0ee5786e7 Mon Sep 17 00:00:00 2001
From: Vignesh <vignes...@gmail.com>
Date: Fri, 23 May 2025 20:40:40 +0530
Subject: [PATCH] Fix random 021_twophase test failure
This issue occurred because the test dropped the subscription
tap_sub before confirming it had replayed the commit for the
previously prepared transaction 'mygid'. The test only waited
for the second subscription, tap_sub_copy, to catch up. As a
result, tap_sub had not yet applied the commit, leaving the
prepared transaction uncommitted on the subscriber.
---
src/test/subscription/t/021_twophase.pl | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/src/test/subscription/t/021_twophase.pl b/src/test/subscription/t/021_twophase.pl
index 61c427aed21..b8e4242d1f1 100644
--- a/src/test/subscription/t/021_twophase.pl
+++ b/src/test/subscription/t/021_twophase.pl
@@ -373,7 +373,14 @@ $result =
$node_publisher->safe_psql('postgres', "SELECT count(*) FROM tab_copy;");
is($result, qq(6), 'publisher inserted data');
+# Wait for both subscribers to catchup
$node_publisher->wait_for_catchup($appname_copy);
+$node_publisher->wait_for_catchup($appname);
+
+# Make sure there are no prepared transactions on the subscriber
+$result = $node_subscriber->safe_psql('postgres',
+ "SELECT count(*) FROM pg_prepared_xacts;");
+is($result, qq(0), 'should be no prepared transactions on subscriber');
$result =
$node_subscriber->safe_psql('postgres', "SELECT count(*) FROM tab_copy;");
--
2.43.0
diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index cc03f0706e9..981a8e45003 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -38,6 +38,7 @@
#include "replication/message.h"
#include "replication/reorderbuffer.h"
#include "replication/snapbuild.h"
+#include "replication/slot.h"
#include "storage/standbydefs.h"
/* individual record(group)'s handlers */
@@ -682,6 +683,10 @@ DecodeCommit(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
RepOriginId origin_id = XLogRecGetOrigin(buf->record);
int i;
+ if (parsed->twophase_xid && strcmp(parsed->twophase_gid, "mygid") == 0 &&
+ strcmp(NameStr(MyReplicationSlot->data.name), "tap_sub") == 0)
+ sleep(3);
+
if (parsed->xinfo & XACT_XINFO_HAS_ORIGIN)
{
origin_lsn = parsed->origin_lsn;
