audio: Handle min=max when scaling volumes (fixes: #525)

Also add gobject.threads_init() so we can run the audio actor test on its own.

mopidy/audio/actor.py

@@ -544,6 +544,8 @@ class Audio(pykka.ThreadingActor):
         """Convert value between scales."""
         new_min, new_max = new
         old_min, old_max = old
+        if old_min == old_max:
+            return old_max
         scaling = float(new_max - new_min) / (old_max - old_min)
         return int(round(scaling * (value - old_min) + new_min))
 

tests/audio/actor_test.py

@@ -6,6 +6,9 @@ import pygst
 pygst.require('0.10')
 import gst
 
+import gobject
+gobject.threads_init()
+
 import pykka
 
 from mopidy import audio
@@ -80,6 +83,18 @@ class AudioTest(unittest.TestCase):
             self.assertTrue(self.audio.set_volume(value).get())
             self.assertEqual(value, self.audio.get_volume().get())
 
+    def test_set_volume_with_mixer_min_equal_max(self):
+        config = {
+            'audio': {
+                'mixer': 'fakemixer track_max_volume=0',
+                'mixer_track': None,
+                'output': 'fakesink',
+                'visualizer': None,
+            }
+        }
+        self.audio = audio.Audio.start(config=config).proxy()
+        self.assertEqual(0, self.audio.get_volume().get())
+
     @unittest.SkipTest
     def test_set_state_encapsulation(self):
         pass  # TODO