Fix-up 771c573 to properly stop background update workers

The "special" StopIteration object didn't actually make any sense.
Instead looping forever, workers now loop while there are workers,
so that they stop looping once the count is cleared.

Dummy values are still inserted so that they don't need to timeout
on the queue before exiting (these values are None) so in essence,
this keeps the best of both of worlds.

telethon/update_state.py

@@ -39,19 +39,15 @@ class UpdateState:
         return not self._updates.empty()
 
     def poll(self, timeout=None):
-        """Polls an update or blocks until an update object is available.
-           If 'timeout is not None', it should be a floating point value,
-           and the method will 'return None' if waiting times out.
+        """
+        Polls an update or blocks until an update object is available.
+        If 'timeout is not None', it should be a floating point value,
+        and the method will 'return None' if waiting times out.
         """
         try:
-            update = self._updates.get(timeout=timeout)
+            return self._updates.get(timeout=timeout)
         except Empty:
-            return
-
-        if isinstance(update, Exception):
-            raise update  # Some error was set through (surely StopIteration)
-
-        return update
+            return None
 
     def get_workers(self):
         return self._workers
@@ -60,27 +56,24 @@ class UpdateState:
         """Changes the number of workers running.
            If 'n is None', clears all pending updates from memory.
         """
-        self.stop_workers()
-        self._workers = n
-        if n is not None:
+        if n is None:
+            self.stop_workers()
+        else:
+            self._workers = n
             self.setup_workers()
 
     workers = property(fget=get_workers, fset=set_workers)
 
     def stop_workers(self):
         """
-        Raises "StopIterationException" on the worker threads to stop
-        them, and also clears all the workers/updates from the lists.
+        Waits for all the worker threads to stop.
         """
-        if self._workers:
-            with self._updates_lock:
-                # Insert at the beginning so the very next poll causes an error
-                # on all the worker threads
-                # TODO Should this reset the pts and such?
-                while self._updates:
-                    self._updates.get()
-                for _ in range(self._workers):
-                    self._updates.put(StopIteration())
+        # Put dummy ``None`` objects so that they don't need to timeout.
+        n = self._workers
+        self._workers = None
+        with self._updates_lock:
+            for _ in range(n):
+                self._updates.put(None)
 
         for t in self._worker_threads:
             t.join()
@@ -103,7 +96,7 @@ class UpdateState:
             thread.start()
 
     def _worker_loop(self, wid):
-        while True:
+        while self._workers is not None:
             try:
                 update = self.poll(timeout=UpdateState.WORKER_POLL_TIMEOUT)
                 if update and self.handler: