bpd: add dummy command for volume

MPD supports a deprecated command 'volume' which was used to change the
volume by a relative amount unlike its replacement 'setvol' which uses
an absolute amount. As far as I can tell 'volume' always responds with a system
error message "No mixer".

beetsplug/bpd/__init__.py

@@ -356,6 +356,10 @@ class BaseServer(object):
             raise BPDError(ERROR_ARG, u'volume out of range')
         self.volume = vol
 
+    def cmd_volume(self, conn, vol_delta):
+        """Deprecated command to change the volume by a relative amount."""
+        raise BPDError(ERROR_SYSTEM, u'No mixer')
+
     def cmd_crossfade(self, conn, crossfade):
         """Set the number of seconds of crossfading."""
         crossfade = cast_arg(int, crossfade)

test/test_player.py

@@ -46,7 +46,7 @@ gstplayer._GstPlayer = mock.MagicMock(
         "seek"
     ], **{
         'playing': False,
-        'volume': 0.0,
+        'volume': 0,
         'time.return_value': (0, 0),
         'play_file.side_effect': _gstplayer_play,
     })
@@ -362,8 +362,8 @@ class BPDQueryTest(BPDTestHelper):
 class BPDPlaybackTest(BPDTestHelper):
     test_implements_playback = implements({
             'consume', 'random',
-            'repeat', 'setvol', 'single', 'replay_gain_mode',
-            'replay_gain_status', 'volume',
+            'repeat', 'single', 'replay_gain_mode',
+            'replay_gain_status',
             }, expectedFailure=True)
 
     def test_cmd_crossfade(self):
@@ -399,6 +399,23 @@ class BPDPlaybackTest(BPDTestHelper):
         self.assertAlmostEqual(2, float(responses[1].data['mixrampdelay']))
         self.assertNotIn('mixrampdelay', responses[3].data)
 
+    def test_cmd_setvol(self):
+        with self.run_bpd() as client:
+            responses = client.send_commands(
+                    ('setvol', '67'),
+                    ('status',),
+                    ('setvol', '32'),
+                    ('status',),
+                    ('setvol', '101'))
+        self._assert_failed(responses, bpd.ERROR_ARG, pos=4)
+        self.assertEqual('67', responses[1].data['volume'])
+        self.assertEqual('32', responses[3].data['volume'])
+
+    def test_cmd_volume(self):
+        with self.run_bpd() as client:
+            response = client.send_command('volume', '10')
+        self._assert_failed(response, bpd.ERROR_SYSTEM)
+
 
 class BPDControlTest(BPDTestHelper):
     test_implements_control = implements({