Linearize state resolution to help caches

synapse/state.py

@@ -23,6 +23,7 @@ from synapse.api.constants import EventTypes
 from synapse.api.errors import AuthError
 from synapse.api.auth import AuthEventTypes
 from synapse.events.snapshot import EventContext
+from synapse.util.async import Linearizer
 
 from collections import namedtuple
 
@@ -84,6 +85,7 @@ class StateHandler(object):
 
         # dict of set of event_ids -> _StateCacheEntry.
         self._state_cache = None
+        self.resolve_linearizer = Linearizer()
 
     def start_caching(self):
         logger.debug("start_caching")
@@ -283,6 +285,7 @@ class StateHandler(object):
                 state_group=name,
             ))
 
+        with (yield self.resolve_linearizer.queue(group_names)):
             if self._state_cache is not None:
                 cache = self._state_cache.get(group_names, None)
                 if cache: