Fixed stop handler for vlc plugin.

For some reason, vlc event handlers are not re-entrant (https://github.com/oaubert/python-vlc/issues/44#issuecomment-378520074).

This means that the vlc API can't be used from an event handler,
and that an event handler that reacts to stop/end-of-stream by
releasing the player and the vlc instance will likely get stuck
and the app may eventually die with SIGSEGV.

Because of this design limitation on the vlc side, the plugin has
to run another thread in the main app that monitors the stop event
set by the event handler and releases the resources appropriately.

platypush/plugins/media/vlc.py

@@ -1,5 +1,7 @@
 import os
+import threading
 import urllib.parse
+from typing import Optional
 
 from platypush.context import get_bus
 from platypush.plugins.media import PlayerState, MediaPlugin
@@ -47,6 +49,8 @@ class MediaVlcPlugin(MediaPlugin):
         self._on_stop_callbacks = []
         self._title = None
         self._filename = None
+        self._monitor_thread: Optional[threading.Thread] = None
+        self._on_stop_event = threading.Event()
 
     @classmethod
     def _watched_event_types(cls):
@@ -64,11 +68,18 @@ class MediaVlcPlugin(MediaPlugin):
 
     def _init_vlc(self, resource):
         import vlc
+
+        if self._instance:
+            self.logger.info('Another instance is running, waiting for it to terminate')
+            self._on_stop_event.wait()
+
         self._reset_state()
 
         for k, v in self._env.items():
             os.environ[k] = v
 
+        self._monitor_thread = threading.Thread(target=self._player_monitor)
+        self._monitor_thread.start()
         self._instance = vlc.Instance(*self._args)
         self._player = self._instance.media_player_new(resource)
 
@@ -76,16 +87,24 @@ class MediaVlcPlugin(MediaPlugin):
             self._player.event_manager().event_attach(
                 eventtype=evt, callback=self._event_callback())
 
+    def _player_monitor(self):
+        self._on_stop_event.wait()
+        self.logger.info('VLC stream terminated')
+        self._reset_state()
+
     def _reset_state(self):
         self._latest_seek = None
         self._title = None
         self._filename = None
+        self._on_stop_event.clear()
 
         if self._player:
+            self.logger.info('Releasing VLC player resource')
             self._player.release()
             self._player = None
 
         if self._instance:
+            self.logger.info('Releasing VLC instance resource')
             self._instance.release()
             self._instance = None
 
@@ -105,7 +124,7 @@ class MediaVlcPlugin(MediaPlugin):
                 self._post_event(MediaPauseEvent)
             elif event.type == EventType.MediaPlayerStopped or \
                     event.type == EventType.MediaPlayerEndReached:
-                self._reset_state()
+                self._on_stop_event.set()
                 self._post_event(MediaStopEvent)
                 for cbk in self._on_stop_callbacks:
                     cbk()