Fix slot synchronization for two_phase enabled slots.

The issue is that the transactions prepared before two-phase decoding is
enabled can fail to replicate to the subscriber after being committed on a
promoted standby following a failover. This is because the two_phase_at
field of a slot, which tracks the LSN from which two-phase decoding
starts, is not synchronized to standby servers. Without two_phase_at, the
logical decoding might incorrectly identify prepared transaction as
already replicated to the subscriber after promotion of standby server,
causing them to be skipped.

To address the issue on HEAD, the two_phase_at field of the slot is
exposed by the pg_replication_slots view and allows the slot
synchronization to copy this value to the corresponding synced slot on the
standby server.

This bug is likely to occur if the user toggles the two_phase option to
true after initial slot creation. Given that altering the two_phase option
of a replication slot is not allowed in PostgreSQL 17, this bug is less
likely to occur. We can't change the view/function definition in
backbranch so we can't push the same fix but we are brainstorming an
appropriate solution for PG17.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/TYAPR01MB5724CC7C288535BBCEEE65DA94A72@TYAPR01MB5724.jpnprd01.prod.outlook.com

doc/src/sgml/system-views.sgml

@@ -2854,6 +2854,17 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>two_phase_at</structfield> <type>pg_lsn</type>
+      </para>
+      <para>
+       The address (<literal>LSN</literal>) from which the decoding of prepared
+       transactions is enabled. <literal>NULL</literal> for logical slots
+       where <structfield>two_phase</structfield> is false and for physical slots.
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>inactive_since</structfield> <type>timestamptz</type>

src/backend/catalog/system_views.sql

@@ -1025,6 +1025,7 @@ CREATE VIEW pg_replication_slots AS
             L.wal_status,
             L.safe_wal_size,
             L.two_phase,
+            L.two_phase_at,
             L.inactive_since,
             L.conflicting,
             L.invalidation_reason,

src/backend/replication/logical/slotsync.c

@@ -139,6 +139,7 @@ typedef struct RemoteSlot
 	bool		failover;
 	XLogRecPtr	restart_lsn;
 	XLogRecPtr	confirmed_lsn;
+	XLogRecPtr	two_phase_at;
 	TransactionId catalog_xmin;
 
 	/* RS_INVAL_NONE if valid, or the reason of invalidation */
@@ -276,7 +277,8 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
-		strcmp(remote_slot->plugin, NameStr(slot->data.plugin)) != 0)
+		strcmp(remote_slot->plugin, NameStr(slot->data.plugin)) != 0 ||
+		remote_slot->two_phase_at != slot->data.two_phase_at)
 	{
 		NameData	plugin_name;
 
@@ -287,6 +289,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 		slot->data.plugin = plugin_name;
 		slot->data.database = remote_dbid;
 		slot->data.two_phase = remote_slot->two_phase;
+		slot->data.two_phase_at = remote_slot->two_phase_at;
 		slot->data.failover = remote_slot->failover;
 		SpinLockRelease(&slot->mutex);
 
@@ -788,9 +791,9 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 static bool
 synchronize_slots(WalReceiverConn *wrconn)
 {
-#define SLOTSYNC_COLUMN_COUNT 9
+#define SLOTSYNC_COLUMN_COUNT 10
 	Oid			slotRow[SLOTSYNC_COLUMN_COUNT] = {TEXTOID, TEXTOID, LSNOID,
-	LSNOID, XIDOID, BOOLOID, BOOLOID, TEXTOID, TEXTOID};
+	LSNOID, XIDOID, BOOLOID, LSNOID, BOOLOID, TEXTOID, TEXTOID};
 
 	WalRcvExecResult *res;
 	TupleTableSlot *tupslot;
@@ -798,7 +801,7 @@ synchronize_slots(WalReceiverConn *wrconn)
 	bool		some_slot_updated = false;
 	bool		started_tx = false;
 	const char *query = "SELECT slot_name, plugin, confirmed_flush_lsn,"
-		" restart_lsn, catalog_xmin, two_phase, failover,"
+		" restart_lsn, catalog_xmin, two_phase, two_phase_at, failover,"
 		" database, invalidation_reason"
 		" FROM pg_catalog.pg_replication_slots"
 		" WHERE failover and NOT temporary";
@@ -853,6 +856,9 @@ synchronize_slots(WalReceiverConn *wrconn)
 														   &isnull));
 		Assert(!isnull);
 
+		d = slot_getattr(tupslot, ++col, &isnull);
+		remote_slot->two_phase_at = isnull ? InvalidXLogRecPtr : DatumGetLSN(d);
+
 		remote_slot->failover = DatumGetBool(slot_getattr(tupslot, ++col,
 														  &isnull));
 		Assert(!isnull);

src/backend/replication/slotfuncs.c

@@ -235,7 +235,7 @@ pg_drop_replication_slot(PG_FUNCTION_ARGS)
 Datum
 pg_get_replication_slots(PG_FUNCTION_ARGS)
 {
-#define PG_GET_REPLICATION_SLOTS_COLS 19
+#define PG_GET_REPLICATION_SLOTS_COLS 20
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 	XLogRecPtr	currlsn;
 	int			slotno;
@@ -406,6 +406,12 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.two_phase);
 
+		if (slot_contents.data.two_phase &&
+			!XLogRecPtrIsInvalid(slot_contents.data.two_phase_at))
+			values[i++] = LSNGetDatum(slot_contents.data.two_phase_at);
+		else
+			nulls[i++] = true;
+
 		if (slot_contents.inactive_since > 0)
 			values[i++] = TimestampTzGetDatum(slot_contents.inactive_since);
 		else

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202504021
+#define CATALOG_VERSION_NO	202504031
 
 #endif

src/include/catalog/pg_proc.dat

@@ -11424,9 +11424,9 @@
   proname => 'pg_get_replication_slots', prorows => '10', proisstrict => 'f',
   proretset => 't', provolatile => 's', prorettype => 'record',
   proargtypes => '',
-  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8,bool,timestamptz,bool,text,bool,bool}',
+  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8,bool,pg_lsn,timestamptz,bool,text,bool,bool}',
-  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
+  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,safe_wal_size,two_phase,inactive_since,conflicting,invalidation_reason,failover,synced}',
+  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,safe_wal_size,two_phase,two_phase_at,inactive_since,conflicting,invalidation_reason,failover,synced}',
   prosrc => 'pg_get_replication_slots' },
 { oid => '3786', descr => 'set up a logical replication slot',
   proname => 'pg_create_logical_replication_slot', provolatile => 'v',

src/test/recovery/t/040_standby_failover_slots_sync.pl

@@ -22,7 +22,11 @@ $publisher->init(
 # Disable autovacuum to avoid generating xid during stats update as otherwise
 # the new XID could then be replicated to standby at some random point making
 # slots at primary lag behind standby during slot sync.
-$publisher->append_conf('postgresql.conf', 'autovacuum = off');
+$publisher->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+max_prepared_transactions = 1
+});
 $publisher->start;
 
 $publisher->safe_psql('postgres',
@@ -33,6 +37,7 @@ my $publisher_connstr = $publisher->connstr . ' dbname=postgres';
 # Create a subscriber node, wait for sync to complete
 my $subscriber1 = PostgreSQL::Test::Cluster->new('subscriber1');
 $subscriber1->init;
+$subscriber1->append_conf('postgresql.conf', 'max_prepared_transactions = 1');
 $subscriber1->start;
 
 # Capture the time before the logical failover slot is created on the
@@ -830,13 +835,72 @@ $primary->adjust_conf('postgresql.conf', 'synchronized_standby_slots',
 	"'sb1_slot'");
 $primary->reload;
 
+##################################################
+# Test the synchronization of the two_phase setting for a subscription with the
+# standby. Additionally, prepare a transaction before enabling the two_phase
+# option; subsequent tests will verify if it can be correctly replicated to the
+# subscriber after committing it on the promoted standby.
+##################################################
+
+$standby1->start;
+
+# Prepare a transaction
+$primary->safe_psql(
+	'postgres', qq[
+	BEGIN;
+	INSERT INTO tab_int values(0);
+	PREPARE TRANSACTION 'test_twophase_slotsync';
+]);
+
+$primary->wait_for_replay_catchup($standby1);
+$primary->wait_for_catchup('regress_mysub1');
+
+# Disable the subscription to allow changing the two_phase option.
+$subscriber1->safe_psql('postgres',
+	"ALTER SUBSCRIPTION regress_mysub1 DISABLE");
+
+# Wait for the replication slot to become inactive on the publisher
+$primary->poll_query_until(
+	'postgres',
+	"SELECT COUNT(*) FROM pg_catalog.pg_replication_slots WHERE slot_name = 'lsub1_slot' AND active='f'",
+	1);
+
+# Set two_phase to true and enable the subscription
+$subscriber1->safe_psql(
+	'postgres', qq[
+	ALTER SUBSCRIPTION regress_mysub1 SET (two_phase = true);
+	ALTER SUBSCRIPTION regress_mysub1 ENABLE;
+]);
+
+$primary->wait_for_catchup('regress_mysub1');
+
+my $two_phase_at = $primary->safe_psql('postgres',
+	"SELECT two_phase_at from pg_replication_slots WHERE slot_name = 'lsub1_slot';"
+);
+
+# Confirm that two_phase setting of lsub1_slot slot is synced to the standby
+ok( $standby1->poll_query_until(
+		'postgres',
+		"SELECT two_phase AND '$two_phase_at' = two_phase_at from pg_replication_slots WHERE slot_name = 'lsub1_slot' AND synced AND NOT temporary;"
+	),
+	'two_phase setting of slot lsub1_slot synced to standby');
+
+# Confirm that the prepared transaction is not yet replicated to the
+# subscriber.
+$result = $subscriber1->safe_psql('postgres',
+	"SELECT count(*) = 0 FROM pg_prepared_xacts;");
+is($result, 't',
+	"the prepared transaction is not replicated to the subscriber");
+
 ##################################################
 # Promote the standby1 to primary. Confirm that:
 # a) the slot 'lsub1_slot' and 'snap_test_slot' are retained on the new primary
 # b) logical replication for regress_mysub1 is resumed successfully after failover
-# c) changes can be consumed from the synced slot 'snap_test_slot'
+# c) changes from the transaction prepared 'test_twophase_slotsync' can be
+#    consumed from the synced slot 'snap_test_slot' once committed on the new
+#    primary.
+# d) changes can be consumed from the synced slot 'snap_test_slot'
 ##################################################
-$standby1->start;
 $primary->wait_for_replay_catchup($standby1);
 
 # Capture the time before the standby is promoted
@@ -876,6 +940,15 @@ is( $standby1->safe_psql(
 	't',
 	'synced slot retained on the new primary');
 
+# Commit the prepared transaction
+$standby1->safe_psql('postgres',
+	"COMMIT PREPARED 'test_twophase_slotsync';");
+$standby1->wait_for_catchup('regress_mysub1');
+
+# Confirm that the prepared transaction is replicated to the subscriber
+is($subscriber1->safe_psql('postgres', q{SELECT count(*) FROM tab_int;}),
+	"11", 'prepared data replicated from the new primary');
+
 # Insert data on the new primary
 $standby1->safe_psql('postgres',
 	"INSERT INTO tab_int SELECT generate_series(11, 20);");
@@ -883,7 +956,7 @@ $standby1->wait_for_catchup('regress_mysub1');
 
 # Confirm that data in tab_int replicated on the subscriber
 is($subscriber1->safe_psql('postgres', q{SELECT count(*) FROM tab_int;}),
-	"20", 'data replicated from the new primary');
+	"21", 'data replicated from the new primary');
 
 # Consume the data from the snap_test_slot. The synced slot should reach a
 # consistent point by restoring the snapshot at the restart_lsn serialized

src/test/regress/expected/rules.out

@@ -1490,12 +1490,13 @@ pg_replication_slots| SELECT l.slot_name,
     l.wal_status,
     l.safe_wal_size,
     l.two_phase,
+    l.two_phase_at,
     l.inactive_since,
     l.conflicting,
     l.invalidation_reason,
     l.failover,
     l.synced
-   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, safe_wal_size, two_phase, inactive_since, conflicting, invalidation_reason, failover, synced)
+   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, safe_wal_size, two_phase, two_phase_at, inactive_since, conflicting, invalidation_reason, failover, synced)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,