From c7b4f5dd22a2d629d88c7e34469bac7547c0e2fc Mon Sep 17 00:00:00 2001
From: Nisha Moond <nisha.moond412@gmail.com>
Date: Fri, 9 May 2025 11:09:23 +0530
Subject: [PATCH v14] PG17 Approach 3 Fix slot synchronization for two_phase
 enables 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 prevent the risk of losing prepared transactions, we disallow enabling
both failover and twophase during slot creation, but permits altering
failover to true once ensured that slot's restart_lsn > two_phase_at.

The fix enforces the following conditions:
1) Always disallow creating slots with two_phase=true and failover=true.
2) Always disallow creating subscriptions with (two_phase=true, failover=true).
3) Prevent altering the slot's failover to true if two_phase=true and restart_lsn
   is less than two_phase_at. Otherwise, allow changing failover to true.
4) Disallow altering slot's failover to true when two_phase state is 'pending'.
   User can try altering failover again when two_phase state is moved beyond 'pending'.
---
 contrib/test_decoding/expected/slot.out       |   2 +
 contrib/test_decoding/sql/slot.sql            |   1 +
 doc/src/sgml/func.sgml                        |  13 ++
 doc/src/sgml/protocol.sgml                    |  17 +++
 doc/src/sgml/ref/alter_subscription.sgml      |  12 ++
 doc/src/sgml/ref/create_subscription.sgml     |  17 +++
 src/backend/commands/subscriptioncmds.c       |  25 ++++
 src/backend/replication/logical/logical.c     |  11 ++
 src/backend/replication/logical/slotsync.c    |  10 ++
 src/backend/replication/slot.c                |  27 +++++
 src/bin/pg_upgrade/t/003_logical_slots.pl     |  50 ++++++--
 .../t/040_standby_failover_slots_sync.pl      | 112 ++++++++++++++++++
 src/test/regress/expected/subscription.out    |   4 +
 src/test/regress/sql/subscription.sql         |   4 +
 14 files changed, 294 insertions(+), 11 deletions(-)

diff --git a/contrib/test_decoding/expected/slot.out b/contrib/test_decoding/expected/slot.out
index 7de03c79f6f..87b28ad8d55 100644
--- a/contrib/test_decoding/expected/slot.out
+++ b/contrib/test_decoding/expected/slot.out
@@ -427,6 +427,8 @@ SELECT 'init' FROM pg_create_logical_replication_slot('failover_default_slot', '
 
 SELECT 'init' FROM pg_create_logical_replication_slot('failover_true_temp_slot', 'test_decoding', true, false, true);
 ERROR:  cannot enable failover for a temporary replication slot
+SELECT 'init' FROM pg_create_logical_replication_slot('failover_twophase_true_slot', 'test_decoding', false, true, true);
+ERROR:  cannot enable both "failover" and "two_phase" options during replication slot creation
 SELECT 'init' FROM pg_create_physical_replication_slot('physical_slot');
  ?column? 
 ----------
diff --git a/contrib/test_decoding/sql/slot.sql b/contrib/test_decoding/sql/slot.sql
index 580e3ae3bef..a89fe712ff6 100644
--- a/contrib/test_decoding/sql/slot.sql
+++ b/contrib/test_decoding/sql/slot.sql
@@ -182,6 +182,7 @@ SELECT 'init' FROM pg_create_logical_replication_slot('failover_true_slot', 'tes
 SELECT 'init' FROM pg_create_logical_replication_slot('failover_false_slot', 'test_decoding', false, false, false);
 SELECT 'init' FROM pg_create_logical_replication_slot('failover_default_slot', 'test_decoding', false, false);
 SELECT 'init' FROM pg_create_logical_replication_slot('failover_true_temp_slot', 'test_decoding', true, false, true);
+SELECT 'init' FROM pg_create_logical_replication_slot('failover_twophase_true_slot', 'test_decoding', false, true, true);
 SELECT 'init' FROM pg_create_physical_replication_slot('physical_slot');
 
 SELECT slot_name, slot_type, failover FROM pg_replication_slots;
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 697c1a02891..45504cf14d5 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -29076,6 +29076,19 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
         failover. A call to this function has the same effect as
         the replication protocol command
         <literal>CREATE_REPLICATION_SLOT ... LOGICAL</literal>.
+
+        <note>
+         <para>
+          The parameters <parameter>twophase</parameter> and
+          <parameter>failover</parameter> cannot be enabled together when
+          creating a replication slot. However, a slot created with
+          <parameter>twophase</parameter> enabled can later have
+          <parameter>failover</parameter> set to true either using
+          <xref linkend="protocol-replication-alter-replication-slot"/> or via
+          <xref linkend="sql-altersubscription"/> if a subscription is using
+          this slot.
+         </para>
+        </note>
        </para></entry>
       </row>
 
diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index 0396d3a9e98..2c2d66fad35 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -2062,6 +2062,15 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
           <literal>PREPARE TRANSACTION</literal> time.
           The default is false.
          </para>
+
+         <para>
+          This option cannot be set together with <literal>failover</literal>
+          when creating a logical replication slot. However, once the slot is
+          created with <literal>two_phase = true</literal>,
+          <literal>failover</literal> can be set to true after the slot has
+          consumed all the changes up to the point where two-phase decoding
+          was enabled.
+         </para>
         </listitem>
        </varlistentry>
 
@@ -2103,6 +2112,14 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
           so that logical replication can be resumed after failover.
           The default is false.
          </para>
+
+         <para>
+          This option cannot be set together with <literal>two_phase</literal>
+          when creating the slot. However, once the slot is created with
+          <literal>two_phase = true</literal>, <literal>failover</literal> can
+          be set to true after the slot has consumed all the changes up to the
+          point where two-phase decoding was enabled.
+         </para>
         </listitem>
        </varlistentry>
       </variablelist>
diff --git a/doc/src/sgml/ref/alter_subscription.sgml b/doc/src/sgml/ref/alter_subscription.sgml
index 2ccbf5e4897..afa29d4049b 100644
--- a/doc/src/sgml/ref/alter_subscription.sgml
+++ b/doc/src/sgml/ref/alter_subscription.sgml
@@ -257,6 +257,18 @@ ALTER SUBSCRIPTION <replaceable class="parameter">name</replaceable> RENAME TO <
       <link linkend="sql-createsubscription-params-with-failover"><literal>failover</literal></link>
       option is enabled.
      </para>
+
+     <para>
+      When a subscription's
+      <link linkend="sql-createsubscription-params-with-two-phase"><literal>two_phase</literal></link>
+      is in the pending state, setting <literal>failover</literal> to true is not
+      permitted. Once
+      <link linkend="sql-createsubscription-params-with-two-phase"><literal>two_phase</literal></link>
+      is enabled,
+      <link linkend="sql-createsubscription-params-with-failover"><literal>failover</literal></link>
+      can be set to true only after the slot has consumed all the changes up to the
+      point where two-phase decoding was enabled.
+     </para>
     </listitem>
    </varlistentry>
 
diff --git a/doc/src/sgml/ref/create_subscription.sgml b/doc/src/sgml/ref/create_subscription.sgml
index c9c8dd440dc..55b20893e80 100644
--- a/doc/src/sgml/ref/create_subscription.sgml
+++ b/doc/src/sgml/ref/create_subscription.sgml
@@ -354,6 +354,14 @@ CREATE SUBSCRIPTION <replaceable class="parameter">subscription_name</replaceabl
           to know the actual two-phase state.
          </para>
 
+         <para>
+          This option cannot be enabled together with
+          <literal>failover</literal> when creating a subscription. However, a
+          <literal>two_phase</literal> enabled subscription can later have
+          <literal>failover</literal> set to true using
+          <xref linkend="sql-altersubscription"/>, once the slot has consumed
+          all the changes up to the point where two-phase decoding was enabled.
+         </para>
         </listitem>
        </varlistentry>
 
@@ -426,6 +434,15 @@ CREATE SUBSCRIPTION <replaceable class="parameter">subscription_name</replaceabl
           replication can be resumed from the new primary after failover.
           The default is <literal>false</literal>.
          </para>
+
+         <para>
+          This option cannot be enabled together with
+          <literal>two_phase</literal> when creating a subscription. However, a
+          <literal>two_phase</literal> enabled subscription can later have
+          <literal>failover</literal> set to true using
+          <xref linkend="sql-altersubscription"/>, once the slot has consumed
+          all the changes up to the point where two-phase decoding was enabled.
+         </para>
         </listitem>
        </varlistentry>
       </variablelist></para>
diff --git a/src/backend/commands/subscriptioncmds.c b/src/backend/commands/subscriptioncmds.c
index 9467f58a23d..54afb3f2e42 100644
--- a/src/backend/commands/subscriptioncmds.c
+++ b/src/backend/commands/subscriptioncmds.c
@@ -648,6 +648,18 @@ CreateSubscription(ParseState *pstate, CreateSubscriptionStmt *stmt,
 				 errmsg("password_required=false is superuser-only"),
 				 errhint("Subscriptions with the password_required option set to false may only be created or modified by the superuser.")));
 
+	/*
+	 * Do not allow users to enable the failover and two_phase options
+	 * together. See comments atop slotsync.c for details.
+	 */
+	if (opts.twophase && opts.failover)
+		ereport(ERROR,
+				errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				errmsg("cannot enable both \"%s\" and \"%s\" options at CREATE SUBSCRIPTION",
+					   "failover", "two_phase"),
+				errhint("Use ALTER SUBSCRIPTION ... SET (failover = true) to enable \"%s\" after two_phase state is ready",
+						"failover"));
+
 	/*
 	 * If built with appropriate switch, whine when regression-testing
 	 * conventions for subscription names are violated.
@@ -1245,6 +1257,19 @@ AlterSubscription(ParseState *pstate, AlterSubscriptionStmt *stmt,
 								 errmsg("cannot set %s for enabled subscription",
 										"failover")));
 
+					/*
+					 * Do not allow users to enable the failover until
+					 * two_phase state reaches READY state. See comments atop
+					 * slotsync.c for details.
+					 */
+					if (sub->twophasestate == LOGICALREP_TWOPHASE_STATE_PENDING &&
+						opts.failover)
+						ereport(ERROR,
+								errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+								errmsg("cannot enable failover for a subscription with a pending two_phase state"),
+								errhint("The \"%s\" option can be enabled after \"%s\" state is ready.",
+										"failover", "two_phase"));
+
 					/*
 					 * The changed failover option of the slot can't be rolled
 					 * back.
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index e941bb491d8..c33024dad19 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -613,6 +613,17 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 	/* Mark slot to allow two_phase decoding if not already marked */
 	if (ctx->twophase && !slot->data.two_phase)
 	{
+		/*
+		 * Do not allow two-phase decoding for failover enabled slots.
+		 *
+		 * See comments atop slotsync.c for details.
+		 */
+		if (slot->data.failover)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("cannot enable two-phase decoding for failover enabled slot \"%s\"",
+							NameStr(slot->data.name))));
+
 		SpinLockAcquire(&slot->mutex);
 		slot->data.two_phase = true;
 		slot->data.two_phase_at = start_lsn;
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 73fe3f56af2..011c244ac35 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -42,6 +42,16 @@
  * Any standby synchronized slots will be dropped if they no longer need
  * to be synchronized. See comment atop drop_local_obsolete_slots() for more
  * details.
+ *
+ * Regarding two-phase enabled slots, we need to note that the two_phase_at
+ * field of a slot is not synchronized. In the absence of two_phase_at,
+ * logical decoding could incorrectly identify prepared transactions as
+ * having been replicated to the subscriber after promotion, resulting in
+ * their omission. Consequently, both two_phase and failover features
+ * cannot be simultaneously enabled during slot creation. Failover can only
+ * be enabled once we can ensure that no transactions were prepared prior
+ * to two_phase_at on the primary. These restrictions are enforced through
+ * checks in ReplicationSlotCreate() and ReplicationSlotAlter().
  *---------------------------------------------------------------------------
  */
 
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index a1d4768623f..f5f2bcc708b 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -343,6 +343,20 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 			ereport(ERROR,
 					errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 					errmsg("cannot enable failover for a temporary replication slot"));
+
+		/*
+		 * Do not allow users to enable both failover and two_phase for slots.
+		 * Please see the comments atop slotsync.c for details.
+		 *
+		 * However, both failover and two_phase enabled slots can be created
+		 * during slot synchronization because we need to retain the same
+		 * values as the remote slot.
+		 */
+		if (two_phase && !IsSyncingReplicationSlots())
+			ereport(ERROR,
+					errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					errmsg("cannot enable both \"%s\" and \"%s\" options during replication slot creation",
+						   "failover", "two_phase"));
 	}
 
 	/*
@@ -848,6 +862,19 @@ ReplicationSlotAlter(const char *name, bool failover)
 				errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				errmsg("cannot enable failover for a temporary replication slot"));
 
+	/*
+	 * Do not allow users to enable failover for a two_phase enabled slot if
+	 * there are potentially un-decoded transactions that are prepared before
+	 * two_phase_at. See comments atop slotsync.c for details.
+	 */
+	if (failover && MyReplicationSlot->data.two_phase &&
+		MyReplicationSlot->data.restart_lsn < MyReplicationSlot->data.two_phase_at)
+		ereport(ERROR,
+				errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				errmsg("cannot enable failover for a two-phase enabled replication slot due to unconsumed changes"),
+				errdetail("The slot need to consume change upto %X/%X to enable failover.",
+						  LSN_FORMAT_ARGS(MyReplicationSlot->data.two_phase_at)));
+
 	if (MyReplicationSlot->data.failover != failover)
 	{
 		SpinLockAcquire(&MyReplicationSlot->mutex);
diff --git a/src/bin/pg_upgrade/t/003_logical_slots.pl b/src/bin/pg_upgrade/t/003_logical_slots.pl
index 0a2483d3dfc..36c3b73677e 100644
--- a/src/bin/pg_upgrade/t/003_logical_slots.pl
+++ b/src/bin/pg_upgrade/t/003_logical_slots.pl
@@ -106,6 +106,7 @@ $oldpub->start;
 $oldpub->safe_psql(
 	'postgres', qq[
 		CREATE TABLE tbl AS SELECT generate_series(1, 10) AS a;
+		CREATE TABLE tbl2 AS SELECT generate_series(1, 10) AS a;
 		SELECT pg_replication_slot_advance('test_slot2', pg_current_wal_lsn());
 		SELECT count(*) FROM pg_logical_emit_message('false', 'prefix', 'This is a non-transactional message');
 ]);
@@ -162,7 +163,8 @@ $oldpub->safe_psql(
 	'postgres', qq[
 	SELECT * FROM pg_drop_replication_slot('test_slot1');
 	SELECT * FROM pg_drop_replication_slot('test_slot2');
-	CREATE PUBLICATION regress_pub FOR ALL TABLES;
+	CREATE PUBLICATION regress_pub FOR TABLE tbl;
+	CREATE PUBLICATION regress_pub2 FOR TABLE tbl2;
 ]);
 
 # Initialize subscriber cluster
@@ -173,18 +175,23 @@ $sub->start;
 $sub->safe_psql(
 	'postgres', qq[
 	CREATE TABLE tbl (a int);
-	CREATE SUBSCRIPTION regress_sub CONNECTION '$old_connstr' PUBLICATION regress_pub WITH (two_phase = 'true', failover = 'true')
+	CREATE TABLE tbl2 (a int);
+	CREATE SUBSCRIPTION regress_sub CONNECTION '$old_connstr' PUBLICATION regress_pub WITH (two_phase = 'true');
+	CREATE SUBSCRIPTION regress_sub2 CONNECTION '$old_connstr' PUBLICATION regress_pub2 WITH (failover = 'true');
 ]);
 $sub->wait_for_subscription_sync($oldpub, 'regress_sub');
+$sub->wait_for_subscription_sync($oldpub, 'regress_sub2');
 
 # Also wait for two-phase to be enabled
-my $twophase_query =
-  "SELECT count(1) = 0 FROM pg_subscription WHERE subtwophasestate NOT IN ('e');";
-$sub->poll_query_until('postgres', $twophase_query)
-  or die "Timed out while waiting for subscriber to enable twophase";
+ok( $sub->poll_query_until(
+		'postgres',
+		qq[SELECT subtwophasestate FROM pg_subscription WHERE subname = 'regress_sub';],
+		'e',),
+	"Timed out while waiting for subscriber to enable twophase");
 
 # 2. Temporarily disable the subscription
 $sub->safe_psql('postgres', "ALTER SUBSCRIPTION regress_sub DISABLE");
+$sub->safe_psql('postgres', "ALTER SUBSCRIPTION regress_sub2 DISABLE");
 $oldpub->stop;
 
 # pg_upgrade should be successful
@@ -193,10 +200,17 @@ command_ok([@pg_upgrade_cmd], 'run of pg_upgrade of old cluster');
 # Check that the slot 'regress_sub' has migrated to the new cluster
 $newpub->start;
 my $result = $newpub->safe_psql('postgres',
-	"SELECT slot_name, two_phase, failover FROM pg_replication_slots");
-is($result, qq(regress_sub|t|t), 'check the slot exists on new cluster');
+	"SELECT slot_name, two_phase FROM pg_replication_slots where slot_name = 'regress_sub'"
+);
+is($result, qq(regress_sub|t), 'check the slot exists on new cluster');
+
+# Check that the slot 'regress_sub2' has migrated to the new cluster
+$result = $newpub->safe_psql('postgres',
+	"SELECT slot_name, failover FROM pg_replication_slots where slot_name = 'regress_sub2'"
+);
+is($result, qq(regress_sub2|t), 'check the slot exists on new cluster');
 
-# Update the connection
+# Update the connection for regress_sub
 my $new_connstr = $newpub->connstr . ' dbname=postgres';
 $sub->safe_psql(
 	'postgres', qq[
@@ -204,13 +218,27 @@ $sub->safe_psql(
 	ALTER SUBSCRIPTION regress_sub ENABLE;
 ]);
 
+# Update the connection for regress_sub2
+$sub->safe_psql(
+	'postgres', qq[
+	ALTER SUBSCRIPTION regress_sub2 CONNECTION '$new_connstr';
+	ALTER SUBSCRIPTION regress_sub2 ENABLE;
+]);
+
 # Check whether changes on the new publisher get replicated to the subscriber
-$newpub->safe_psql('postgres',
-	"INSERT INTO tbl VALUES (generate_series(11, 20))");
+$newpub->safe_psql(
+	'postgres',
+	"INSERT INTO tbl VALUES (generate_series(11, 20));
+	INSERT INTO tbl2 VALUES (generate_series(11, 20));");
+
 $newpub->wait_for_catchup('regress_sub');
 $result = $sub->safe_psql('postgres', "SELECT count(*) FROM tbl");
 is($result, qq(20), 'check changes are replicated to the sub');
 
+$newpub->wait_for_catchup('regress_sub2');
+$result = $sub->safe_psql('postgres', "SELECT count(*) FROM tbl2");
+is($result, qq(20), 'check changes are replicated to the sub2');
+
 # Clean up
 $sub->stop();
 $newpub->stop();
diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index 823857bb329..a78c4d2bd95 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -25,6 +25,8 @@ $publisher->init(
 $publisher->append_conf('postgresql.conf', 'autovacuum = off');
 $publisher->start;
 
+$publisher->safe_psql('postgres', "CREATE TABLE tab_full (a int)");
+
 $publisher->safe_psql('postgres',
 	"CREATE PUBLICATION regress_mypub FOR ALL TABLES;");
 
@@ -42,6 +44,8 @@ my $slot_creation_time_on_primary = $publisher->safe_psql(
     SELECT current_timestamp;
 ]);
 
+$subscriber1->safe_psql('postgres', "CREATE TABLE tab_full (a int)");
+
 # Create a subscription that enables failover.
 $subscriber1->safe_psql('postgres',
 	"CREATE SUBSCRIPTION regress_mysub1 CONNECTION '$publisher_connstr' PUBLICATION regress_mypub WITH (slot_name = lsub1_slot, copy_data = false, failover = true, enabled = false);"
@@ -98,6 +102,114 @@ my ($result, $stdout, $stderr) = $subscriber1->psql('postgres',
 ok( $stderr =~ /ERROR:  cannot set failover for enabled subscription/,
 	"altering failover is not allowed for enabled subscription");
 
+##################################################
+# Test that the failover option can be enabled for a two_phase enabled
+# subscription only through Alter Subscription (failover=true).
+##################################################
+
+# Test that the failover option cannot be set for a slot with
+# restart_lsn < two_phase_at
+
+$publisher->psql('postgres',
+	"SELECT pg_create_logical_replication_slot('regress_mysub2', 'pgoutput', false, true, false);"
+);
+
+# Try to enable failover of the slot
+($result, $stdout, $stderr) = $publisher->psql(
+	'postgres',
+	qq[ALTER_REPLICATION_SLOT regress_mysub2 (failover);],
+	replication => 'database');
+ok( $stderr =~
+	  qr/ERROR:  cannot enable failover for a two-phase enabled replication slot due to unconsumed changes/,
+	"failover can't be enabled if restart_lsn < two_phase_at on a two_phase subscription."
+);
+
+# Create a subscription with two_phase enabled, using above slot
+$subscriber1->safe_psql('postgres',
+	"CREATE SUBSCRIPTION regress_mysub2 CONNECTION '$publisher_connstr' PUBLICATION regress_mypub WITH (create_slot = false, enabled = false, two_phase = true);"
+);
+
+# Try to enable failover for the subscription with two_phase in pending state
+($result, $stdout, $stderr) = $subscriber1->psql('postgres',
+	"ALTER SUBSCRIPTION regress_mysub2 SET (failover = true)");
+ok( $stderr =~
+	  /ERROR:  cannot enable failover for a subscription with a pending two_phase state
+.*HINT:  The "failover" option can be enabled after "two_phase" state is ready./,
+	"enabling failover is not allowed for a two_phase pending subscription");
+
+# Enable the subscription
+$subscriber1->safe_psql('postgres',
+	"ALTER SUBSCRIPTION regress_mysub2 ENABLE;");
+
+# Wait for the subscription to be in sync
+$subscriber1->wait_for_subscription_sync($publisher, 'regress_mysub2');
+
+# Also wait for two-phase to be enabled
+ok( $subscriber1->poll_query_until(
+		'postgres',
+		qq[SELECT subtwophasestate FROM pg_subscription WHERE subname = 'regress_mysub2';],
+		'e',),
+	"Timed out while waiting for subscriber to enable twophase");
+
+# Advance the slot's restart_lsn to allow enabling the failover option
+# on a two_phase-enabled subscription using ALTER SUBSCRIPTION.
+$publisher->safe_psql(
+	'postgres', qq(
+		BEGIN;
+		SELECT txid_current();
+		SELECT pg_log_standby_snapshot();
+		COMMIT;
+		BEGIN;
+		SELECT txid_current();
+		SELECT pg_log_standby_snapshot();
+		COMMIT;
+));
+
+# Alter subscription to enable failover
+$subscriber1->psql(
+	'postgres',
+	"ALTER SUBSCRIPTION regress_mysub2 DISABLE;
+	 ALTER SUBSCRIPTION regress_mysub2 SET (failover = true);");
+
+# Confirm that the failover flag on the slot has been turned on
+is( $publisher->safe_psql(
+		'postgres',
+		q{SELECT failover from pg_replication_slots WHERE slot_name = 'regress_mysub2';}
+	),
+	"t",
+	'logical slot has failover true on the publisher');
+
+# Drop the subscription
+$subscriber1->safe_psql('postgres', "DROP SUBSCRIPTION regress_mysub2;");
+
+# Test that subscription creation with two_phase=true results in error when
+# using a slot with failover set to true.
+
+# Create a slot with failover enabled
+$publisher->psql('postgres',
+	"SELECT pg_create_logical_replication_slot('regress_mysub3', 'pgoutput', false, false, true);"
+);
+
+my $logoffset = -s $subscriber1->logfile;
+
+# Create a subscription with two_phase enabled
+$subscriber1->safe_psql('postgres',
+	"CREATE SUBSCRIPTION regress_mysub3 CONNECTION '$publisher_connstr' PUBLICATION regress_mypub WITH (create_slot = false, two_phase = true);"
+);
+
+ok( $subscriber1->wait_for_log(
+		qr/cannot enable two-phase decoding for failover enabled slot "regress_mysub3"/,
+		$logoffset),
+	"creating subscription with two_phase=true is not allowed when the slot has failover set to true"
+);
+
+# Drop the subscription
+$subscriber1->safe_psql('postgres', "DROP SUBSCRIPTION regress_mysub3;");
+
+# Clean up the tables created
+$publisher->safe_psql('postgres', "DROP TABLE tab_full;");
+$subscriber1->safe_psql('postgres', "DROP TABLE tab_full;");
+
 ##################################################
 # Test that pg_sync_replication_slots() cannot be executed on a non-standby server.
 ##################################################
diff --git a/src/test/regress/expected/subscription.out b/src/test/regress/expected/subscription.out
index 0f2a25cdc19..8d4d55c802d 100644
--- a/src/test/regress/expected/subscription.out
+++ b/src/test/regress/expected/subscription.out
@@ -479,6 +479,10 @@ COMMIT;
 ALTER SUBSCRIPTION regress_testsub SET (slot_name = NONE);
 DROP SUBSCRIPTION regress_testsub;
 RESET SESSION AUTHORIZATION;
+-- fail - cannot enable two_phase and failover together
+CREATE SUBSCRIPTION regress_testsub CONNECTION 'dbname=regress_doesnotexist' PUBLICATION testpub WITH (connect = false, two_phase = true, failover = true);
+ERROR:  cannot enable both "failover" and "two_phase" options at CREATE SUBSCRIPTION
+HINT:  Use ALTER SUBSCRIPTION ... SET (failover = true) to enable "failover" after two_phase state is ready
 DROP ROLE regress_subscription_user;
 DROP ROLE regress_subscription_user2;
 DROP ROLE regress_subscription_user3;
diff --git a/src/test/regress/sql/subscription.sql b/src/test/regress/sql/subscription.sql
index 3e5ba4cb8c6..47fc1e5329b 100644
--- a/src/test/regress/sql/subscription.sql
+++ b/src/test/regress/sql/subscription.sql
@@ -342,6 +342,10 @@ ALTER SUBSCRIPTION regress_testsub SET (slot_name = NONE);
 DROP SUBSCRIPTION regress_testsub;
 
 RESET SESSION AUTHORIZATION;
+
+-- fail - cannot enable two_phase and failover together
+CREATE SUBSCRIPTION regress_testsub CONNECTION 'dbname=regress_doesnotexist' PUBLICATION testpub WITH (connect = false, two_phase = true, failover = true);
+
 DROP ROLE regress_subscription_user;
 DROP ROLE regress_subscription_user2;
 DROP ROLE regress_subscription_user3;
-- 
2.34.1

