Limit concurrent event creation for a room to avoid state resolution when sending bursts of events to a local room (#14977)

changelog.d/14977.misc

@@ -0,0 +1 @@
+Limit concurrent event creation for a room to avoid state resolution when sending bursts of events to a local room.

synapse/handlers/message.py

@@ -499,9 +499,9 @@ class EventCreationHandler:
 
         self.request_ratelimiter = hs.get_request_ratelimiter()
 
-        # We arbitrarily limit concurrent event creation for a room to 5.
-        # This is to stop us from diverging history *too* much.
-        self.limiter = Linearizer(max_count=5, name="room_event_creation_limit")
+        # We limit concurrent event creation for a room to 1. This prevents state resolution
+        # from occurring when sending bursts of events to a local room
+        self.limiter = Linearizer(max_count=1, name="room_event_creation_limit")
 
         self._bulk_push_rule_evaluator = hs.get_bulk_push_rule_evaluator()