Fix 020_messages.pl test.

We were not waiting for a publisher to catch up with the subscriber after
creating a subscription. Now, it can happen that apply worker starts
replication even after we have disabled the subscription in the test. This
will make the test expect that there is no active slot whereas there
exists one. Fix this symptom by allowing the publisher to wait for
catching up with the subscription.

It is not a good idea to ensure if the slot is still active by checking
for walsender existence as we release the slot after we clean up the
walsender related memory. Fix that by checking the slot status in
pg_replication_slots.

Also, it is better to avoid repeated enabling/disabling of the
subscription.

Finally, we make autovacuum off for this test to avoid any empty
transaction appearing in the test while consuming changes.

Reported-by: as per buildfarm
Author: Vignesh C
Reviewed-by: Amit Kapila, Michael Paquier
Discussion: https://postgr.es/m/CAA4eK1+uW1UGDHDz-HWMHMen76mKP7NJebOTZN4uwbyMjaYVww@mail.gmail.com

src/test/subscription/t/020_messages.pl

@@ -11,6 +11,8 @@ use Test::More tests => 5;
 # Create publisher node
 my $node_publisher = get_new_node('publisher');
 $node_publisher->init(allows_streaming => 'logical');
+$node_publisher->append_conf('postgresql.conf',
+	'autovacuum = off');
 $node_publisher->start;
 
 # Create subscriber node
@@ -35,12 +37,14 @@ $node_subscriber->safe_psql('postgres',
 	"CREATE SUBSCRIPTION tap_sub CONNECTION '$publisher_connstr' PUBLICATION tap_pub"
 );
 
+$node_publisher->wait_for_catchup('tap_sub');
+
 # Ensure a transactional logical decoding message shows up on the slot
 $node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub DISABLE");
 
-# wait for the replication connection to drop from the publisher
+# wait for the replication slot to become inactive in the publisher
 $node_publisher->poll_query_until('postgres',
-	'SELECT COUNT(*) FROM pg_catalog.pg_stat_replication', 0);
+	"SELECT COUNT(*) FROM pg_catalog.pg_replication_slots WHERE slot_name = 'tap_sub' AND active='f'", 1);
 
 $node_publisher->safe_psql('postgres',
 	"SELECT pg_logical_emit_message(true, 'pgoutput', 'a transactional message')"
@@ -77,7 +81,7 @@ is($result, qq(1|pgoutput),
 $result = $node_publisher->safe_psql(
 	'postgres', qq(
 		SELECT get_byte(data, 0)
-		FROM pg_logical_slot_peek_binary_changes('tap_sub', NULL, NULL,
+		FROM pg_logical_slot_get_binary_changes('tap_sub', NULL, NULL,
 			'proto_version', '1',
 			'publication_names', 'tap_pub')
 ));
@@ -88,16 +92,6 @@ is( $result, qq(66
 	'option messages defaults to false so message (M) is not available on slot'
 );
 
-$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub ENABLE");
-$node_publisher->wait_for_catchup('tap_sub');
-
-# ensure a non-transactional logical decoding message shows up on the slot
-$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub DISABLE");
-
-# wait for the replication connection to drop from the publisher
-$node_publisher->poll_query_until('postgres',
-	'SELECT COUNT(*) FROM pg_catalog.pg_stat_replication', 0);
-
 $node_publisher->safe_psql('postgres', "INSERT INTO tab_test VALUES (1)");
 
 my $message_lsn = $node_publisher->safe_psql('postgres',
@@ -109,7 +103,7 @@ $node_publisher->safe_psql('postgres', "INSERT INTO tab_test VALUES (2)");
 $result = $node_publisher->safe_psql(
 	'postgres', qq(
 		SELECT get_byte(data, 0), get_byte(data, 1)
-		FROM pg_logical_slot_peek_binary_changes('tap_sub', NULL, NULL,
+		FROM pg_logical_slot_get_binary_changes('tap_sub', NULL, NULL,
 			'proto_version', '1',
 			'publication_names', 'tap_pub',
 			'messages', 'true')
@@ -118,15 +112,6 @@ $result = $node_publisher->safe_psql(
 
 is($result, qq(77|0), 'non-transactional message on slot is M');
 
-$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub ENABLE");
-$node_publisher->wait_for_catchup('tap_sub');
-
-$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub DISABLE");
-
-# wait for the replication connection to drop from the publisher
-$node_publisher->poll_query_until('postgres',
-	'SELECT COUNT(*) FROM pg_catalog.pg_stat_replication', 0);
-
 # Ensure a non-transactional logical decoding message shows up on the slot when
 # it is emitted within an aborted transaction. The message won't emit until
 # something advances the LSN, hence, we intentionally forces the server to