Merge pull request #2790 from matrix-org/rav/preserve_event_logcontext_leak

Fix a logcontext leak in persist_events

synapse/storage/events.py

@@ -146,6 +146,9 @@ class _EventPeristenceQueue(object):
             try:
                 queue = self._get_drainining_queue(room_id)
                 for item in queue:
+                    # handle_queue_loop runs in the sentinel logcontext, so
+                    # there is no need to preserve_fn when running the
+                    # callbacks on the deferred.
                     try:
                         ret = yield per_item_callback(item)
                         item.deferred.callback(ret)
@@ -157,7 +160,11 @@ class _EventPeristenceQueue(object):
                     self._event_persist_queues[room_id] = queue
                 self._currently_persisting_rooms.discard(room_id)
 
-        preserve_fn(handle_queue_loop)()
+        # set handle_queue_loop off on the background. We don't want to
+        # attribute work done in it to the current request, so we drop the
+        # logcontext altogether.
+        with PreserveLoggingContext():
+            handle_queue_loop()
 
     def _get_drainining_queue(self, room_id):
         queue = self._event_persist_queues.setdefault(room_id, deque())