Fix race with timeline selection in logical decoding during promotion

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

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index d69d03b2ef368997975681e78818b0a629ea2b4c..6c2304fef33f80ff9f8d84bac351aa9c440cf635 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -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.

diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 04aa770d981cdef96b302c568f36fce6ad10e883..c931d9b4fa80a9737369bc10d018682d3a23d544 100644 (file)
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -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();
 

diff --git a/src/test/recovery/t/035_standby_logical_decoding.pl b/src/test/recovery/t/035_standby_logical_decoding.pl
index 4421059f100eccdf600fef057becc2d17123f6ec..b3a5bb2694c3bbbd83d3708e10d7eb6d46caf8a6 100644 (file)
--- a/src/test/recovery/t/035_standby_logical_decoding.pl
+++ b/src/test/recovery/t/035_standby_logical_decoding.pl
@@ -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();