pgstat: fix subscription stats entry leak.

Commit 7b64e4b3 taught DropSubscription() to drop stats entry of
subscription that is not associated with a replication slot for apply
worker at DROP SUBSCRIPTION but missed covering the case where the
subscription is not associated with replication slots for both apply
worker and tablesync worker.

Also add a test to verify that the stats for slot-less subscription is
removed at DROP SUBSCRIPTION time.

Backpatch down to 15.

Author: Masahiko Sawada
Reviewed-by: Nathan Bossart, Hayato Kuroda, Melih Mutlu, Amit Kapila
Discussion: https://postgr.es/m/CAD21AoB71zkP7uPT7JDPsZcvp0749ExEQnOJxeNKPDFisHar+w@mail.gmail.com
Backpatch-through: 15

src/backend/commands/subscriptioncmds.c

@@ -1646,6 +1646,12 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 	ReplicationOriginNameForLogicalRep(subid, InvalidOid, originname, sizeof(originname));
 	replorigin_drop_by_name(originname, true, false);
 
+	/*
+	 * Tell the cumulative stats system that the subscription is getting
+	 * dropped.
+	 */
+	pgstat_drop_subscription(subid);
+
 	/*
 	 * If there is no slot associated with the subscription, we can finish
 	 * here.
@@ -1734,12 +1740,6 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 	}
 	PG_END_TRY();
 
-	/*
-	 * Tell the cumulative stats system that the subscription is getting
-	 * dropped.
-	 */
-	pgstat_drop_subscription(subid);
-
 	table_close(rel, NoLock);
 }
 

src/test/subscription/t/026_stats.pl

@@ -267,6 +267,26 @@ is( $node_subscriber->safe_psql(
 	qq(f),
 	qq(Subscription stats for subscription '$sub1_name' should be removed.));
 
+# Get subscription 2 oid
+my $sub2_oid = $node_subscriber->safe_psql($db,
+	qq(SELECT oid FROM pg_subscription WHERE subname = '$sub2_name'));
+
+# Diassociate the subscription 2 from its replication slot and drop it
+$node_subscriber->safe_psql(
+	$db,
+	qq(
+ALTER SUBSCRIPTION $sub2_name DISABLE;
+ALTER SUBSCRIPTION $sub2_name SET (slot_name = NONE);
+DROP SUBSCRIPTION $sub2_name;
+			    ));
+
+# Subscription stats for sub2 should be gone
+is( $node_subscriber->safe_psql(
+		$db, qq(SELECT pg_stat_have_stats('subscription', 0, $sub2_oid))),
+	qq(f),
+	qq(Subscription stats for subscription '$sub2_name' should be removed.));
+$node_publisher->safe_psql($db,
+	qq(SELECT pg_drop_replication_slot('$sub2_name')));
 
 $node_subscriber->stop('fast');
 $node_publisher->stop('fast');