Flush logical slots to disk during a shutdown checkpoint if required.

It's entirely possible for a logical slot to have a confirmed_flush LSN
higher than the last value saved on disk while not being marked as dirty.
Currently, it is not a major problem but a later patch adding support for
the upgrade of slots relies on that value being properly flushed to disk.

It can also help avoid processing the same transactions again in some
boundary cases after the clean shutdown and restart.  Say, we process
some transactions for which we didn't send anything downstream (the
changes got filtered) but the confirm_flush LSN is updated due to
keepalives.  As we don't flush the latest value of confirm_flush LSN, it
may lead to processing the same changes again without this patch.

The approach taken by this patch has been suggested by Ashutosh Bapat.

Author: Vignesh C, Julien Rouhaud, Kuroda Hayato
Reviewed-by: Amit Kapila, Dilip Kumar, Michael Paquier, Ashutosh Bapat, Peter Smith, Hou Zhijie
Discussion: http://postgr.es/m/CAA4eK1JzJagMmb_E8D4au=GYQkxox0AfNBm1FbP7sy7t4YWXPQ@mail.gmail.com
Discussion: http://postgr.es/m/TYAPR01MB58664C81887B3AF2EB6B16E3F5939@TYAPR01MB5866.jpnprd01.prod.outlook.com

src/backend/access/transam/xlog.c

@@ -7039,7 +7039,7 @@ static void
 CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
 {
 	CheckPointRelationMap();
-	CheckPointReplicationSlots();
+	CheckPointReplicationSlots(flags & CHECKPOINT_IS_SHUTDOWN);
 	CheckPointSnapBuild();
 	CheckPointLogicalRewriteHeap();
 	CheckPointReplicationOrigin();

src/backend/replication/slot.c

@@ -321,6 +321,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->candidate_xmin_lsn = InvalidXLogRecPtr;
 	slot->candidate_restart_valid = InvalidXLogRecPtr;
 	slot->candidate_restart_lsn = InvalidXLogRecPtr;
+	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
@@ -1572,11 +1573,13 @@ restart:
 /*
  * Flush all replication slots to disk.
  *
- * This needn't actually be part of a checkpoint, but it's a convenient
- * location.
+ * It is convenient to flush dirty replication slots at the time of checkpoint.
+ * Additionally, in case of a shutdown checkpoint, we also identify the slots
+ * for which the confirmed_flush LSN has been updated since the last time it
+ * was saved and flush them.
  */
 void
-CheckPointReplicationSlots(void)
+CheckPointReplicationSlots(bool is_shutdown)
 {
 	int			i;
 
@@ -1601,6 +1604,30 @@ CheckPointReplicationSlots(void)
 
 		/* save the slot to disk, locking is handled in SaveSlotToPath() */
 		sprintf(path, "pg_replslot/%s", NameStr(s->data.name));
+
+		/*
+		 * Slot's data is not flushed each time the confirmed_flush LSN is
+		 * updated as that could lead to frequent writes.  However, we decide
+		 * to force a flush of all logical slot's data at the time of shutdown
+		 * if the confirmed_flush LSN is changed since we last flushed it to
+		 * disk.  This helps in avoiding an unnecessary retreat of the
+		 * confirmed_flush LSN after restart.
+		 */
+		if (is_shutdown && SlotIsLogical(s))
+		{
+			SpinLockAcquire(&s->mutex);
+
+			Assert(s->data.confirmed_flush >= s->last_saved_confirmed_flush);
+
+			if (s->data.invalidated == RS_INVAL_NONE &&
+				s->data.confirmed_flush != s->last_saved_confirmed_flush)
+			{
+				s->just_dirtied = true;
+				s->dirty = true;
+			}
+			SpinLockRelease(&s->mutex);
+		}
+
 		SaveSlotToPath(s, path, LOG);
 	}
 	LWLockRelease(ReplicationSlotAllocationLock);
@@ -1873,11 +1900,12 @@ SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel)
 
 	/*
 	 * Successfully wrote, unset dirty bit, unless somebody dirtied again
-	 * already.
+	 * already and remember the confirmed_flush LSN value.
 	 */
 	SpinLockAcquire(&slot->mutex);
 	if (!slot->just_dirtied)
 		slot->dirty = false;
+	slot->last_saved_confirmed_flush = cp.slotdata.confirmed_flush;
 	SpinLockRelease(&slot->mutex);
 
 	LWLockRelease(&slot->io_in_progress_lock);
@@ -2074,6 +2102,7 @@ RestoreSlotFromDisk(const char *name)
 		/* initialize in memory state */
 		slot->effective_xmin = cp.slotdata.xmin;
 		slot->effective_catalog_xmin = cp.slotdata.catalog_xmin;
+		slot->last_saved_confirmed_flush = cp.slotdata.confirmed_flush;
 
 		slot->candidate_catalog_xmin = InvalidTransactionId;
 		slot->candidate_xmin_lsn = InvalidXLogRecPtr;

src/include/replication/slot.h

@@ -178,6 +178,13 @@ typedef struct ReplicationSlot
 	XLogRecPtr	candidate_xmin_lsn;
 	XLogRecPtr	candidate_restart_valid;
 	XLogRecPtr	candidate_restart_lsn;
+
+	/*
+	 * This value tracks the last confirmed_flush LSN flushed which is used
+	 * during a shutdown checkpoint to decide if logical's slot data should be
+	 * forcibly flushed or not.
+	 */
+	XLogRecPtr	last_saved_confirmed_flush;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
@@ -241,7 +248,7 @@ extern void ReplicationSlotNameForTablesync(Oid suboid, Oid relid, char *syncslo
 extern void ReplicationSlotDropAtPubNode(WalReceiverConn *wrconn, char *slotname, bool missing_ok);
 
 extern void StartupReplicationSlots(void);
-extern void CheckPointReplicationSlots(void);
+extern void CheckPointReplicationSlots(bool is_shutdown);
 
 extern void CheckSlotRequirements(void);
 extern void CheckSlotPermissions(void);

src/test/recovery/meson.build

@@ -43,6 +43,7 @@ tests += {
       't/035_standby_logical_decoding.pl',
       't/036_truncated_dropped.pl',
       't/037_invalid_database.pl',
+      't/038_save_logical_slots_shutdown.pl',
     ],
   },
 }

src/test/recovery/t/038_save_logical_slots_shutdown.pl

@@ -0,0 +1,102 @@
+
+# Copyright (c) 2023, PostgreSQL Global Development Group
+
+# Test logical replication slots are always flushed to disk during a shutdown
+# checkpoint.
+
+use strict;
+use warnings;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+sub compare_confirmed_flush
+{
+	my ($node, $confirmed_flush_from_log) = @_;
+
+	# Fetch Latest checkpoint location from the control file
+	my ($stdout, $stderr) =
+	  run_command([ 'pg_controldata', $node->data_dir ]);
+	my @control_data = split("\n", $stdout);
+	my $latest_checkpoint = undef;
+	foreach (@control_data)
+	{
+		if ($_ =~ /^Latest checkpoint location:\s*(.*)$/mg)
+		{
+			$latest_checkpoint = $1;
+			last;
+		}
+	}
+	die "Latest checkpoint location not found in control file\n"
+	  unless defined($latest_checkpoint);
+
+	# Is it same as the value read from log?
+	ok( $latest_checkpoint eq $confirmed_flush_from_log,
+		"Check that the slot's confirmed_flush LSN is the same as the latest_checkpoint location"
+	);
+
+	return;
+}
+
+# Initialize publisher node
+my $node_publisher = PostgreSQL::Test::Cluster->new('pub');
+$node_publisher->init(allows_streaming => 'logical');
+# Avoid checkpoint during the test, otherwise, the latest checkpoint location
+# will change.
+$node_publisher->append_conf(
+	'postgresql.conf', q{
+checkpoint_timeout = 1h
+autovacuum = off
+});
+$node_publisher->start;
+
+# Create subscriber node
+my $node_subscriber = PostgreSQL::Test::Cluster->new('sub');
+$node_subscriber->init(allows_streaming => 'logical');
+$node_subscriber->start;
+
+# Create tables
+$node_publisher->safe_psql('postgres', "CREATE TABLE test_tbl (id int)");
+$node_subscriber->safe_psql('postgres', "CREATE TABLE test_tbl (id int)");
+
+# Insert some data
+$node_publisher->safe_psql('postgres',
+	"INSERT INTO test_tbl VALUES (generate_series(1, 5));");
+
+# Setup logical replication
+my $publisher_connstr = $node_publisher->connstr . ' dbname=postgres';
+$node_publisher->safe_psql('postgres',
+	"CREATE PUBLICATION pub FOR ALL TABLES");
+$node_subscriber->safe_psql('postgres',
+	"CREATE SUBSCRIPTION sub CONNECTION '$publisher_connstr' PUBLICATION pub"
+);
+
+$node_subscriber->wait_for_subscription_sync($node_publisher, 'sub');
+
+my $result =
+  $node_subscriber->safe_psql('postgres', "SELECT count(*) FROM test_tbl");
+
+is($result, qq(5), "check initial copy was done");
+
+my $offset = -s $node_publisher->logfile;
+
+# Restart the publisher to ensure that the slot will be flushed if required
+$node_publisher->restart();
+
+# Wait until the walsender creates decoding context
+$node_publisher->wait_for_log(
+	qr/Streaming transactions committing after ([A-F0-9]+\/[A-F0-9]+), reading WAL from ([A-F0-9]+\/[A-F0-9]+)./,
+	$offset);
+
+# Extract confirmed_flush from the logfile
+my $log_contents = slurp_file($node_publisher->logfile, $offset);
+$log_contents =~
+  qr/Streaming transactions committing after ([A-F0-9]+\/[A-F0-9]+), reading WAL from ([A-F0-9]+\/[A-F0-9]+)./
+  or die "could not get confirmed_flush_lsn";
+
+# Ensure that the slot's confirmed_flush LSN is the same as the
+# latest_checkpoint location.
+compare_confirmed_flush($node_publisher, $1);
+
+done_testing();