Add the tiniest of tiny one-element caches to get_room_events_max_id() as it's...

Add the tiniest of tiny one-element caches to get_room_events_max_id() as it's read every time someone hits eventstream

synapse/storage/__init__.py

@@ -111,6 +111,7 @@ class DataStore(RoomMemberStore, RoomStore,
                 is_new_state=is_new_state,
                 current_state=current_state,
             )
+            self.get_room_events_max_id.invalidate()
         except _RollbackButIsFineException:
             pass
 

synapse/storage/stream.py

@@ -35,7 +35,7 @@ what sort order was used:
 
 from twisted.internet import defer
 
-from ._base import SQLBaseStore
+from ._base import SQLBaseStore, cached
 from synapse.api.constants import EventTypes
 from synapse.api.errors import SynapseError
 from synapse.util.logutils import log_function
@@ -413,6 +413,7 @@ class StreamStore(SQLBaseStore):
             "get_recent_events_for_room", get_recent_events_for_room_txn
         )
 
+    @cached(num_args=0)
     def get_room_events_max_id(self):
         return self.runInteraction(
             "get_room_events_max_id",