audio: Postpone set_metadata until track is playing

docs/changelog.rst

@@ -35,6 +35,9 @@ Bug fix release.
 
 - Audio: Ensure audio tags are never ``None``. (Fixes: :issue:`1449`)
 
+- Audio: Update :meth:`mopidy.audio.Audio.set_metadata` to postpone sending
+  tags if there is a pending track change. (Fixes: :issue:`1357`)
+
 - Core: Avoid endless loop if all tracks in the tracklist are unplayable and
   consume mode is off. (Fixes: :issue:`1221`, :issue:`1454`, PR: :issue:`1455`)
 

mopidy/audio/actor.py

@@ -374,6 +374,10 @@ class _Handler(object):
             logger.debug('Audio event: tags_changed(tags=%r)', tags.keys())
             AudioListener.send('tags_changed', tags=tags.keys())
 
+        if self._audio._pending_metadata:
+            self._audio._playbin.send_event(self._audio._pending_metadata)
+            self._audio._pending_metadata = None
+
     def on_segment(self, segment):
         gst_logger.debug(
             'Got SEGMENT pad event: '
@@ -412,6 +416,7 @@ class Audio(pykka.ThreadingActor):
         self._tags = {}
         self._pending_uri = None
         self._pending_tags = None
+        self._pending_metadata = None
 
         self._playbin = None
         self._outputs = None
@@ -800,12 +805,11 @@ class Audio(pykka.ThreadingActor):
         if track.album and track.album.name:
             set_value(Gst.TAG_ALBUM, track.album.name)
 
-        gst_logger.debug(
-            'Sending TAG event for track %r: %r',
-            track.uri, taglist.to_string())
         event = Gst.Event.new_tag(taglist)
-        # TODO: check if we get this back on our own bus?
-        self._playbin.send_event(event)
+        if not self._pending_uri:
+            self._playbin.send_event(event)
+        else:
+            self._pending_metadata = event
 
     def get_current_tags(self):
         """