Fix race condition in dispatcher start/stop (#887)

fixes #881

telegram/ext/dispatcher.py

@@ -183,14 +183,20 @@ def run_async(self, func, *args, **kwargs):
         self.__async_queue.put(promise)
         return promise
 
-    def start(self):
+    def start(self, ready=None):
         """Thread target of thread 'dispatcher'.
 
         Runs in background and processes the update queue.
 
+        Args:
+            ready (:obj:`threading.Event`, optional): If specified, the event will be set once the
+                dispatcher is ready.
+
         """
         if self.running:
             self.logger.warning('already running')
+            if ready is not None:
+                ready.set()
             return
 
         if self.__exception_event.is_set():
@@ -202,6 +208,9 @@ def start(self):
         self.running = True
         self.logger.debug('Dispatcher started')
 
+        if ready is not None:
+            ready.set()
+
         while 1:
             try:
                 # Pop update from update queue.

telegram/ext/updater.py

@@ -199,10 +199,13 @@ def start_polling(self,
 
                 # Create & start threads
                 self.job_queue.start()
-                self._init_thread(self.dispatcher.start, "dispatcher")
+                dispatcher_ready = Event()
+                self._init_thread(self.dispatcher.start, "dispatcher", ready=dispatcher_ready)
                 self._init_thread(self._start_polling, "updater", poll_interval, timeout,
                                   read_latency, bootstrap_retries, clean, allowed_updates)
 
+                dispatcher_ready.wait()
+
                 # Return the update queue so the main thread can insert updates
                 return self.update_queue