mpd: Add volume command

docs/changelog.rst

@@ -39,6 +39,9 @@ Bug fix release.
 - MPD: Don't return tracks with empty URIs. (Partly fixes: :issue:`1340`, PR:
   :issue:`1343`)
 
+- MPD: Add ``volume`` command that was reintroduced, though still as a
+  deprecated command, in MPD 0.18 and is in use by some clients like mpc.
+
 - Proxy: Handle case where :confval:`proxy/port` is either missing from config
   or set to an empty string. (PR: :issue:`1371`)
 

mopidy/mpd/protocol/playback.py

@@ -426,3 +426,27 @@ def stop(context):
         Stops playing.
     """
     context.core.playback.stop()
+
+
+@protocol.commands.add('volume', change=protocol.INT)
+def volume(context, change):
+    """
+    *musicpd.org, playback section:*
+
+        ``volume {CHANGE}``
+
+        Changes volume by amount ``CHANGE``.
+
+        Note: ``volume`` is deprecated, use ``setvol`` instead.
+    """
+    if change < -100 or change > 100:
+        raise exceptions.MpdArgError('Invalid volume value', command='volume')
+
+    old_volume = context.core.mixer.get_volume().get()
+    if old_volume is None:
+        raise exceptions.MpdSystemError('problems setting volume')
+
+    new_volume = min(max(0, old_volume + change), 100)
+    success = context.core.mixer.set_volume(new_volume).get()
+    if not success:
+        raise exceptions.MpdSystemError('problems setting volume')

tests/mpd/protocol/test_playback.py

@@ -453,6 +453,38 @@ class VolumeTest(protocol.BaseTestCase):
         self.assertEqual(50, self.core.mixer.get_volume().get())
         self.assertInResponse('OK')
 
+    def test_volume_plus(self):
+        self.core.mixer.set_volume(50)
+
+        self.send_request('volume +20')
+
+        self.assertEqual(70, self.core.mixer.get_volume().get())
+        self.assertInResponse('OK')
+
+    def test_volume_minus(self):
+        self.core.mixer.set_volume(50)
+
+        self.send_request('volume -20')
+
+        self.assertEqual(30, self.core.mixer.get_volume().get())
+        self.assertInResponse('OK')
+
+    def test_volume_less_than_minus_100(self):
+        self.core.mixer.set_volume(50)
+
+        self.send_request('volume -110')
+
+        self.assertEqual(50, self.core.mixer.get_volume().get())
+        self.assertInResponse('ACK [2@0] {volume} Invalid volume value')
+
+    def test_volume_more_than_plus_100(self):
+        self.core.mixer.set_volume(50)
+
+        self.send_request('volume +110')
+
+        self.assertEqual(50, self.core.mixer.get_volume().get())
+        self.assertInResponse('ACK [2@0] {volume} Invalid volume value')
+
 
 class VolumeWithNoMixerTest(protocol.BaseTestCase):
     enable_mixer = False
@@ -460,3 +492,7 @@ class VolumeWithNoMixerTest(protocol.BaseTestCase):
     def test_setvol_without_mixer_fails(self):
         self.send_request('setvol "100"')
         self.assertInResponse('ACK [52@0] {setvol} problems setting volume')
+
+    def test_volume_without_mixer_failes(self):
+        self.send_request('volume +100')
+        self.assertInResponse('ACK [52@0] {volume} problems setting volume')