Don't spuriously create new listeners

synapse/notifier.py

@@ -350,14 +350,15 @@ class Notifier(object):
             while not result and not timed_out[0]:
                 new_token = yield deferred
 
+                result = yield callback(current_token, new_token)
+                current_token = new_token
+
+                if not result:
                     deferred = defer.Deferred()
                     listener = _NotificationListener(deferred, timeout)
                     listeners.append(listener)
                     user_stream.listeners.add(listener)
 
-                result = yield callback(current_token, new_token)
-                current_token = new_token
-
         if timer[0] is not None:
             try:
                 self.clock.cancel_call_later(timer[0])