Allow pg_recvlogical --drop-slot to work without --dbname.

When pg_recvlogical was introduced in 9.4, the --dbname option was not
required for --drop-slot. Without it, pg_recvlogical --drop-slot connected
using a replication connection (not tied to a specific database) and
was able to drop both physical and logical replication slots, similar to
pg_receivewal --drop-slot.

However, commit 0c013e0 unintentionally changed this behavior in 9.5,
making pg_recvlogical always check whether it's connected to a specific
database and fail if it's not. This change was expected for --create-slot
and --start, which handle logical replication slots and require a database
connection, but it was unnecessary for --drop-slot, which should work with
any replication connection. As a result, --dbname became a required option
for --drop-slot.

This commit removes that restriction, restoring the original behavior and
allowing pg_recvlogical --drop-slot to work without specifying --dbname.

Although this issue originated from an unintended change, it has existed
for a long time without complaints or bug reports, and the documentation
never explicitly stated that --drop-slot should work without --dbname.
Therefore, the change is not treated as a bug fix and is applied only to
master.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/b15ecf4f-e5af-4fbb-82c2-a425f453e0b2@oss.nttdata.com

Author: MasaoFujii

src/bin/pg_basebackup/pg_recvlogical.c

@@ -944,13 +944,16 @@ main(int argc, char **argv)
 #endif
 
 	/*
-	 * Run IDENTIFY_SYSTEM to make sure we connected using a database specific
-	 * replication connection.
+	 * Run IDENTIFY_SYSTEM to check the connection type for each action.
+	 * --create-slot and --start actions require a database-specific
+	 * replication connection because they handle logical replication slots.
+	 * --drop-slot can remove replication slots from any replication
+	 * connection without this restriction.
 	 */
 	if (!RunIdentifySystem(conn, NULL, NULL, NULL, &db_name))
 		exit(1);
 
-	if (db_name == NULL)
+	if (!do_drop_slot && db_name == NULL)
 		pg_fatal("could not establish database-specific replication connection");
 
 	/*

src/bin/pg_basebackup/t/030_pg_recvlogical.pl

@@ -127,4 +127,12 @@
 	],
 	'replayed a two-phase transaction');
 
+$node->command_ok(
+	[
+		'pg_recvlogical',
+		'--slot' => 'test',
+		'--drop-slot'
+	],
+	'drop could work without dbname');
+
 done_testing();