GStreamer callbacks keeping mopidy.gstreamer.GStreamer instance from being GC-ed?
Commit e57a71729a8bf937232e36ee70ac96ccb99cf124 fixes the rogue traceback being printed while running tests.

Though, it doesn't fix the root problem, which I believe is that some `GStreamer` instance still alive somewhere is notified by GStreamer about the EOS happening in the `ScannerTest.test_data_is_set` test.

The only place the tests create an `mopidy.gstreamer.GStreamer` instance directly is in `gstreamer_test.py`, and there it doesn't even start as an Pykka actor. Thus, it should normally die when the test referencing it dies.

Could GStreamer still keep the instance alive because of registered callbacks? If so, should we make our `GStreamer` class able to tear down itself more completely?

diff --git a/mopidy/gstreamer.py b/mopidy/gstreamer.py
index d9157a02..6dc7b0aa 100644
--- a/mopidy/gstreamer.py
+++ b/mopidy/gstreamer.py
@@ -1,6 +1,7 @@
 import pygst
 pygst.require('0.10')
 import gst
+import gobject
 
 import logging
 
@@ -9,7 +10,10 @@ from pykka.registry import ActorRegistry
 
 from mopidy import settings, utils
 from mopidy.backends.base import Backend
-from mopidy import mixers # Trigger install of gst mixer plugins.
+from mopidy.utils import process
+
+# Trigger install of gst mixer plugins
+from mopidy import mixers
 
 logger = logging.getLogger('mopidy.gstreamer')
 
@@ -28,6 +32,7 @@ class GStreamer(ThreadingActor):
 
     def __init__(self):
         super(GStreamer, self).__init__()
+
         self._default_caps = gst.Caps("""
             audio/x-raw-int,
             endianness=(int)1234,
@@ -36,16 +41,29 @@ class GStreamer(ThreadingActor):
             depth=(int)16,
             signed=(boolean)true,
             rate=(int)44100""")
+
         self._pipeline = None
         self._source = None
         self._uridecodebin = None
         self._output = None
         self._mixer = None
 
-        self._setup_pipeline()
-        self._setup_output()
-        self._setup_mixer()
-        self._setup_message_processor()
+        self._message_processor_set_up = False
+
+    def on_start(self):
+        try:
+            self._setup_pipeline()
+            self._setup_output()
+            self._setup_mixer()
+            self._setup_message_processor()
+        except gobject.GError as ex:
+            logger.exception(ex)
+            process.exit_process()
+
+    def on_stop(self):
+        self._teardown_message_processor()
+        self._teardown_mixer()
+        self._teardown_pipeline()
 
     def _setup_pipeline(self):
         # TODO: replace with and input bin so we simply have an input bin we
@@ -65,10 +83,18 @@ class GStreamer(ThreadingActor):
         self._uridecodebin.connect('pad-added', self._on_new_pad,
             self._pipeline.get_by_name('queue').get_pad('sink'))
 
+    def _teardown_pipeline(self):
+        self._pipeline.set_state(gst.STATE_NULL)
+
     def _setup_output(self):
-        # This will raise a gobject.GError if the description is bad.
-        self._output = gst.parse_bin_from_description(
-            settings.OUTPUT, ghost_unconnected_pads=True)
+        try:
+            self._output = gst.parse_bin_from_description(
+                settings.OUTPUT, ghost_unconnected_pads=True)
+        except gobject.GError as ex:
+            logger.error('Failed to create output "%s": %s',
+                settings.OUTPUT, ex)
+            process.exit_process()
+            return
 
         self._pipeline.add(self._output)
         gst.element_link_many(self._pipeline.get_by_name('queue'),
@@ -80,8 +106,13 @@ class GStreamer(ThreadingActor):
             logger.info('Not setting up mixer.')
             return
 
-        # This will raise a gobject.GError if the description is bad.
-        mixerbin = gst.parse_bin_from_description(settings.MIXER, False)
+        try:
+            mixerbin = gst.parse_bin_from_description(settings.MIXER,
+                ghost_unconnected_pads=False)
+        except gobject.GError as ex:
+            logger.warning('Failed to create mixer "%s": %s',
+                settings.MIXER, ex)
+            return
 
         # We assume that the bin will contain a single mixer.
         mixer = mixerbin.get_by_interface('GstMixer')
@@ -113,10 +144,21 @@ class GStreamer(ThreadingActor):
                                 gst.interfaces.MIXER_TRACK_OUTPUT):
                 return track
 
+    def _teardown_mixer(self):
+        if self._mixer is not None:
+            (mixer, track) = self._mixer
+            mixer.set_state(gst.STATE_NULL)
+
     def _setup_message_processor(self):
         bus = self._pipeline.get_bus()
         bus.add_signal_watch()
         bus.connect('message', self._on_message)
+        self._message_processor_set_up = True
+
+    def _teardown_message_processor(self):
+        if self._message_processor_set_up:
+            bus = self._pipeline.get_bus()
+            bus.remove_signal_watch()
 
     def _on_new_source(self, element, pad):
         self._source = element.get_property('source')
diff --git a/tests/gstreamer_test.py b/tests/gstreamer_test.py
index 790394f5..ce20d2b4 100644
--- a/tests/gstreamer_test.py
+++ b/tests/gstreamer_test.py
@@ -14,9 +14,10 @@ class GStreamerTest(unittest.TestCase):
         settings.MIXER = 'fakemixer track_max_volume=65536'
         settings.OUTPUT = 'fakesink'
         self.song_uri = path_to_uri(path_to_data_dir('song1.wav'))
-        self.gstreamer = GStreamer()
+        self.gstreamer = GStreamer.start().proxy()
 
     def tearDown(self):
+        self.gstreamer.stop()
         settings.runtime.clear()
 
     def prepare_uri(self, uri):
@@ -25,21 +26,21 @@ class GStreamerTest(unittest.TestCase):
 
     def test_start_playback_existing_file(self):
         self.prepare_uri(self.song_uri)
-        self.assertTrue(self.gstreamer.start_playback())
+        self.assertTrue(self.gstreamer.start_playback().get())
 
     def test_start_playback_non_existing_file(self):
         self.prepare_uri(self.song_uri + 'bogus')
-        self.assertFalse(self.gstreamer.start_playback())
+        self.assertFalse(self.gstreamer.start_playback().get())
 
     def test_pause_playback_while_playing(self):
         self.prepare_uri(self.song_uri)
         self.gstreamer.start_playback()
-        self.assertTrue(self.gstreamer.pause_playback())
+        self.assertTrue(self.gstreamer.pause_playback().get())
 
     def test_stop_playback_while_playing(self):
         self.prepare_uri(self.song_uri)
         self.gstreamer.start_playback()
-        self.assertTrue(self.gstreamer.stop_playback())
+        self.assertTrue(self.gstreamer.stop_playback().get())
 
     @unittest.SkipTest
     def test_deliver_data(self):
@@ -51,8 +52,8 @@ class GStreamerTest(unittest.TestCase):
 
     def test_set_volume(self):
         for value in range(0, 101):
-            self.assertTrue(self.gstreamer.set_volume(value))
-            self.assertEqual(value, self.gstreamer.get_volume())
+            self.assertTrue(self.gstreamer.set_volume(value).get())
+            self.assertEqual(value, self.gstreamer.get_volume().get())
 
     @unittest.SkipTest
     def test_set_state_encapsulation(self):
['mopidy/gstreamer.py', 'tests/gstreamer_test.py']