bpd: fix crossfade command

Although crossfade is not implemented in bpd, we can store the setting
and repeat is back to clients. Also log a warning that the operation is
not implemented.

The real MPD doesn't show the crossfade in status if it's zero since
that means no crossfade, so now we don't either.

beetsplug/bpd/__init__.py

@@ -307,9 +307,11 @@ class BaseServer(object):
             u'random: ' + six.text_type(int(self.random)),
             u'playlist: ' + six.text_type(self.playlist_version),
             u'playlistlength: ' + six.text_type(len(self.playlist)),
-            u'xfade: ' + six.text_type(self.crossfade),
         )
 
+        if self.crossfade > 0:
+            yield u'xfade: ' + six.text_type(self.crossfade)
+
         if self.current_index == -1:
             state = u'stop'
         elif self.paused:
@@ -353,6 +355,8 @@ class BaseServer(object):
         crossfade = cast_arg(int, crossfade)
         if crossfade < 0:
             raise BPDError(ERROR_ARG, u'crossfade time must be nonnegative')
+        self._log.warning(u'crossfade is not implemented in bpd')
+        self.crossfade = crossfade
 
     def cmd_clear(self, conn):
         """Clear the playlist."""

test/test_player.py

@@ -356,11 +356,24 @@ class BPDQueryTest(BPDTestHelper):
 
 class BPDPlaybackTest(BPDTestHelper):
     test_implements_playback = implements({
-            'consume', 'crossfade', 'mixrampd', 'mixrampdelay', 'random',
+            'consume', 'mixrampd', 'mixrampdelay', 'random',
             'repeat', 'setvol', 'single', 'replay_gain_mode',
             'replay_gain_status', 'volume',
             }, expectedFailure=True)
 
+    def test_cmd_crossfade(self):
+        with self.run_bpd() as client:
+            responses = client.send_commands(
+                    ('status',),
+                    ('crossfade', '123'),
+                    ('status',),
+                    ('crossfade', '-2'))
+            response = client.send_command('crossfade', '0.5')
+        self._assert_failed(responses, bpd.ERROR_ARG, pos=3)
+        self._assert_failed(response, bpd.ERROR_ARG)
+        self.assertNotIn('xfade', responses[0].data)
+        self.assertAlmostEqual(123, int(responses[2].data['xfade']))
+
 
 class BPDControlTest(BPDTestHelper):
     test_implements_control = implements({