core: Switch back to correct track if seek happens before stream changed

Technically the seek still needs to be postponed for this to work right, but
it's a step closer.

mopidy/core/playback.py

@@ -437,10 +437,10 @@ class PlaybackController(object):
         if self.get_state() == PlaybackState.STOPPED:
             self.play()
 
-        # TODO: uncomment once we have tests for this. Should fix seek after
+        # Make sure we switch back to previous track if we get a seek while we
-        # about to finish doing wrong track.
+        # have a pending track.
-        # if self._current_tl_track and self._pending_tl_track:
+        if self._current_tl_track and self._pending_tl_track:
-        #     self.play(self._current_tl_track)
+            self._change(self._current_tl_track, self.get_state())
 
         # We need to prefer the still playing track, but if nothing is playing
         # we fall back to the pending one.

tests/core/test_playback.py

@@ -6,6 +6,8 @@ import mock
 
 import pykka
 
+import pytest
+
 from mopidy import backend, core
 from mopidy.internal import deprecation
 from mopidy.models import Track
@@ -529,6 +531,25 @@ class EventEmissionTest(BaseTest):
             ],
             listener_mock.send.mock_calls)
 
+    @pytest.mark.xfail
+    def test_seek_race_condition_emits_events(self, listener_mock):
+        tl_tracks = self.core.tracklist.get_tl_tracks()
+
+        self.core.playback.play(tl_tracks[0])
+        self.trigger_about_to_finish(replay_until='stream_changed')
+        listener_mock.reset_mock()
+
+        self.core.playback.seek(1000)
+        self.replay_events()
+
+        # When we trigger seek after an about to finish the other code that
+        # emits track stopped/started and playback state changed events gets
+        # triggered as we have to switch back to the previous track.
+        # The correct behavior would be to only emit seeked.
+        self.assertListEqual(
+            [mock.call('seeked', time_position=1000)],
+            listener_mock.send.mock_calls)
+
     def test_previous_emits_events(self, listener_mock):
         tl_tracks = self.core.tracklist.get_tl_tracks()
 
@@ -632,6 +653,19 @@ class SeekTest(BaseTest):
         self.core.playback.seek(1000)
         self.assertEqual(self.core.playback.state, core.PlaybackState.PAUSED)
 
+    def test_seek_race_condition_after_about_to_finish(self):
+        tl_tracks = self.core.tracklist.get_tl_tracks()
+
+        self.core.playback.play(tl_tracks[0])
+        self.replay_events()
+
+        self.trigger_about_to_finish(replay_until='stream_changed')
+        self.core.playback.seek(1000)
+        self.replay_events()
+
+        current_tl_track = self.core.playback.get_current_tl_track()
+        self.assertEqual(current_tl_track, tl_tracks[0])
+
 
 class TestStream(BaseTest):