Fix race when persisting create event (#4404)

* Fix race when persisting create event

When persisting a chunk of DAG it is sometimes requried to do a state
resolution, which requires knowledge of the room version. If this
happens while we're persisting the create event then we need to use that
event rather than attempting to look it up in the database.

changelog.d/4404.bugfix

@@ -0,0 +1 @@
+Fix potential bug where creating or joining a room could fail

synapse/storage/events.py

@@ -739,6 +739,17 @@ class EventsStore(StateGroupWorkerStore, EventFederationStore, EventsWorkerStore
         }
 
         events_map = {ev.event_id: ev for ev, _ in events_context}
+
+        # We need to get the room version, which is in the create event.
+        # Normally that'd be in the database, but its also possible that we're
+        # currently trying to persist it.
+        room_version = None
+        for ev, _ in events_context:
+            if ev.type == EventTypes.Create and ev.state_key == "":
+                room_version = ev.content.get("room_version", "1")
+                break
+
+        if not room_version:
             room_version = yield self.get_room_version(room_id)
 
         logger.debug("calling resolve_state_groups from preserve_events")