Fix a race between started/stopped stream

synapse/handlers/events.py

@@ -69,7 +69,12 @@ def started_stream(self, user):
             A deferred that completes once their presence has been updated.
         """
         if user not in self._streams_per_user:
-            self._streams_per_user[user] = 0
+            # Make sure we set the streams per user to 1 here rather than
+            # setting it to zero and incrementing the value below.
+            # Otherwise this may race with stopped_stream causing the
+            # user to be erased from the map before we have a chance
+            # to increment it.
+            self._streams_per_user[user] = 1
             if user in self._stop_timer_per_user:
                 try:
                     self.clock.cancel_call_later(
@@ -79,8 +84,8 @@ def started_stream(self, user):
                     logger.exception("Failed to cancel event timer")
             else:
                 yield started_user_eventstream(self.distributor, user)
-
-        self._streams_per_user[user] += 1
+        else:
+            self._streams_per_user[user] += 1
 
     def stopped_stream(self, user):
         """If there are no streams for a user this starts a timer that will