audio: Add gst.DEBUG_BIN_TO_DOT_FILE pipeline debuging

docs/troubleshooting.rst

@@ -89,4 +89,12 @@ level 3, you can run::
     GST_DEBUG=3 mopidy -v
 
 This will produce a lot of output, but given some GStreamer knowledge this is
-very useful for debugging GStreamer pipeline issues.
+very useful for debugging GStreamer pipeline issues. Additionally
+:envvar:`GST_DEBUG_FILE=gstreamer.log` can be used to redirect the debug
+logging to a file instead of standard out.
+
+Lastly :envvar:`GST_DEBUG_DUMP_DOT_DIR` can be used to get descriptions of the
+current pipeline in dot format. Currently we trigger a dump of the pipeline on
+every completed state change::
+
+    GST_DEBUG_DUMP_DOT_DIR=. mopidy

mopidy/audio/actor.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import logging
+import os
 
 import gobject
 
@@ -351,6 +352,10 @@ class _Handler(object):
             logger.debug('Audio event: stream_changed(uri=None)')
             AudioListener.send('stream_changed', uri=None)
 
+        if 'GST_DEBUG_DUMP_DOT_DIR' in os.environ:
+            gst.DEBUG_BIN_TO_DOT_FILE(
+                self._audio._playbin, gst.DEBUG_GRAPH_SHOW_ALL, 'mopidy')
+
     def on_buffering(self, percent):
         gst_logger.debug('Got buffering message: percent=%d%%', percent)