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Fix race with timeline selection in logical decoding during promotion

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During promotion, there is a window where RecoveryInProgress() returns
true but the WAL segments of the old timeline have already been removed.
A logical decoding could pick up the old timeline in this window when
reading a page, failing with the following error:
ERROR: requested WAL segment ... has already been removed

This issue does not lead to any data correctness issue, as retrying to
decode the data works in follow-up decoding attempts.  It impacts
availability, though.  Other WAL page read callbacks have a similar
issue, this commit takes care of what should be the noisiest code path:
logical decoding with START_REPLICATION in a WAL sender.

A TAP test, based on an injection point waiting in the startup process
after the segments have been removed/recycled, is added.  This part is
backpatched down to v17.

This issue has been causing sporadic failures in the buildfarm, and
was reproducible manually.  This issue happens since logical decoding on
standbys exists, down to v16.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/7daef094-abf3-4672-bc23-3df4763b16a3@gmail.com
Backpatch-through: 16
This commit is contained in:
Michael Paquier 2026-06-11 17:28:57 +09:00
parent 987440b33a
commit eb4e7224a1
3 changed files with 94 additions and 1 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
src/backend/access/transam/xlog.c
View file

@ -6571,6 +6571,8 @@ StartupXLOG(void)
if (ArchiveRecoveryRequested)
CleanupAfterArchiveRecovery(EndOfLogTLI, EndOfLog, newTLI);
INJECTION_POINT("promotion-after-wal-segment-cleanup", NULL);
/*
* Local WAL inserts enabled, so it's time to finish initialization of
* commit timestamp.

24
src/backend/replication/walsender.c
View file

@ -1104,7 +1104,29 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
am_cascading_walsender = RecoveryInProgress();
if (am_cascading_walsender)
GetXLogReplayRecPtr(&currTLI);
{
TimeLineID insertTLI;
/*
* If the insertion timeline has already been set, use it.
* InsertTimeLineID is set before the WAL segments of the old timeline
* are removed, before SharedRecoveryState switches to
* RECOVERY_STATE_DONE.
*
* There is a window where RecoveryInProgress() still returns true but
* the old timeline's WAL segments have already been removed or
* recycled. Using the WAL insertion timeline avoids attempting to
* read from those removed segments, improving availability, and is a
* safe thing to do as promotion copies the contents in the last
* segment of the old timeline to the first segment of the new
* timeline, up to the switchpoint.
*/
insertTLI = GetWALInsertionTimeLineIfSet();
if (insertTLI != 0)
currTLI = insertTLI;
else
GetXLogReplayRecPtr(&currTLI);
}
else
currTLI = GetWALInsertionTimeLine();

69
src/test/recovery/t/035_standby_logical_decoding.pl
View file

@ -1060,4 +1060,73 @@ is($cascading_stdout, $expected,
'got same expected output from pg_recvlogical decoding session on cascading standby'
);
##################################################
# Test that logical decoding on standby correctly handles a timeline
# change during promotion. This relies on an injection point that
# waits between the moment the segments of the old timeline are removed
# and the moment RecoveryInProgress() would set, catching that a WAL
# sender is still able to decode changes across a promotion.
##################################################
# Create a logical slot on the cascading standby for this test.
$node_cascading_standby->create_logical_slot_on_standby($node_standby,
'race_slot', 'testdb');
$node_standby->safe_psql('testdb',
qq[INSERT INTO decoding_test(x,y) SELECT s, s::text FROM generate_series(10,13) s;]
);
$node_standby->wait_for_replay_catchup($node_cascading_standby);
$expected = q{BEGIN
table public.decoding_test: INSERT: x[integer]:10 y[text]:'10'
table public.decoding_test: INSERT: x[integer]:11 y[text]:'11'
table public.decoding_test: INSERT: x[integer]:12 y[text]:'12'
table public.decoding_test: INSERT: x[integer]:13 y[text]:'13'
COMMIT};
$node_standby->safe_psql('testdb', 'CREATE EXTENSION injection_points;');
$node_standby->wait_for_replay_catchup($node_cascading_standby);
# Attach injection point to pause startup after WAL segment cleanup
# but before RecoveryInProgress() flips to false.
$node_cascading_standby->safe_psql('testdb',
"SELECT injection_points_attach('promotion-after-wal-segment-cleanup', 'wait');"
);
# Promote, wait for the removal of the segments on the old timeline.
$node_cascading_standby->safe_psql('testdb', "SELECT pg_promote(false)");
$node_cascading_standby->wait_for_event('startup',
'promotion-after-wal-segment-cleanup');
# Start pg_recvlogical.
my ($stdout2, $stderr2);
my $handle2 = IPC::Run::start(
[
'pg_recvlogical',
'--dbname' => $node_cascading_standby->connstr('testdb'),
'--slot' => 'race_slot',
'--option' => 'include-xids=0',
'--option' => 'skip-empty-xacts=1',
'--file' => '-',
'--no-loop',
'--start',
],
'>' => \$stdout2,
'2>' => \$stderr2,
IPC::Run::timeout($default_timeout));
# Verify that pg_recvlogical successfully decodes the data while startup
# is still paused in the injection point.
$pump_timeout = IPC::Run::timer($default_timeout);
ok( pump_until($handle2, $pump_timeout, \$stdout2, qr/COMMIT/s),
'pg_recvlogical works during promotion timeline switch');
chomp($stdout2);
is($stdout2, $expected,
'got expected output from pg_recvlogical during promotion timeline switch'
);
# Resume promotion.
$node_cascading_standby->safe_psql('testdb',
"SELECT injection_points_wakeup('promotion-after-wal-segment-cleanup');");
done_testing();