bpd: support short form of list command for albums

Some clients list the albums belonging to an artist by issuing the
command `list album <ARTIST NAME>`. This change inserts the tag `artist`
before the artist name so that this succeeds. Fixes #3007

beetsplug/bpd/__init__.py

@@ -1404,16 +1404,31 @@ class Server(BaseServer):
         filtered by matching match_tag to match_term.
         """
         show_tag_canon, show_key = self._tagtype_lookup(show_tag)
+        if len(kv) == 1:
+            if show_tag_canon == 'Album':
+                # If no tag was given, assume artist. This is because MPD
+                # supports a short version of this command for fetching the
+                # albums belonging to a particular artist, and some clients
+                # rely on this behaviour (e.g. MPDroid, M.A.L.P.).
+                kv = ('Artist', kv[0])
+            else:
+                raise BPDError(ERROR_ARG, u'should be "Album" for 3 arguments')
+        elif len(kv) % 2 != 0:
+            raise BPDError(ERROR_ARG, u'Incorrect number of filter arguments')
         query = self._metadata_query(dbcore.query.MatchQuery, None, kv)
 
         clause, subvals = query.clause()
         statement = 'SELECT DISTINCT ' + show_key + \
                     ' FROM items WHERE ' + clause + \
                     ' ORDER BY ' + show_key
+        self._log.debug(statement)
         with self.lib.transaction() as tx:
             rows = tx.query(statement, subvals)
 
         for row in rows:
+            if not row[0]:
+                # Skip any empty values of the field.
+                continue
             yield show_tag_canon + u': ' + six.text_type(row[0])
 
     def cmd_count(self, conn, tag, value):

test/test_player.py

@@ -774,11 +774,21 @@ class BPDDatabaseTest(BPDTestHelper):
         with self.run_bpd() as client:
             responses = client.send_commands(
                     ('list', 'album'),
-                    ('list', 'track'))
+                    ('list', 'track'),
-        self._assert_ok(*responses)
+                    ('list', 'album', 'artist', 'Artist Name', 'track'))
+        self._assert_failed(responses, bpd.ERROR_ARG, pos=2)
         self.assertEqual('Album Title', responses[0].data['Album'])
         self.assertEqual(['1', '2'], responses[1].data['Track'])
 
+    def test_cmd_list_three_arg_form(self):
+        with self.run_bpd() as client:
+            responses = client.send_commands(
+                    ('list', 'album', 'artist', 'Artist Name'),
+                    ('list', 'album', 'Artist Name'),
+                    ('list', 'track', 'Artist Name'))
+        self._assert_failed(responses, bpd.ERROR_ARG, pos=2)
+        self.assertEqual(responses[0].data, responses[1].data)
+
     def test_cmd_lsinfo(self):
         with self.run_bpd() as client:
             response1 = client.send_command('lsinfo')