remote join processing: get create event from state, not auth_chain (#12039)

A follow-up to #12005, in which I apparently missed that there are a bunch of other places that assume the create event is in the auth chain.

changelog.d/12039.misc

@@ -0,0 +1 @@
+Preparation for faster-room-join work: when parsing the `send_join` response, get the `m.room.create` event from `state`, not `auth_chain`.

synapse/federation/federation_client.py

@@ -870,13 +870,15 @@ class FederationClient(FederationBase):
             for s in signed_state:
                 s.internal_metadata = copy.deepcopy(s.internal_metadata)
 
-            # double-check that the same create event has ended up in the auth chain
+            # double-check that the auth chain doesn't include a different create event
             auth_chain_create_events = [
                 e.event_id
                 for e in signed_auth
                 if (e.type, e.state_key) == (EventTypes.Create, "")
             ]
-            if auth_chain_create_events != [create_event.event_id]:
+            if auth_chain_create_events and auth_chain_create_events != [
+                create_event.event_id
+            ]:
                 raise InvalidResponseError(
                     "Unexpected create event(s) in auth chain: %s"
                     % (auth_chain_create_events,)

synapse/handlers/federation.py

@@ -516,7 +516,7 @@ class FederationHandler:
             await self.store.upsert_room_on_join(
                 room_id=room_id,
                 room_version=room_version_obj,
-                auth_events=auth_chain,
+                state_events=state,
             )
 
             max_stream_id = await self._federation_event_handler.process_remote_join(

synapse/storage/databases/main/room.py

@@ -1498,7 +1498,7 @@ class RoomStore(RoomBackgroundUpdateStore, RoomWorkerStore):
         self._event_reports_id_gen = IdGenerator(db_conn, "event_reports", "id")
 
     async def upsert_room_on_join(
-        self, room_id: str, room_version: RoomVersion, auth_events: List[EventBase]
+        self, room_id: str, room_version: RoomVersion, state_events: List[EventBase]
     ) -> None:
         """Ensure that the room is stored in the table
 
@@ -1511,7 +1511,7 @@ class RoomStore(RoomBackgroundUpdateStore, RoomWorkerStore):
         has_auth_chain_index = await self.has_auth_chain_index(room_id)
 
         create_event = None
-        for e in auth_events:
+        for e in state_events:
             if (e.type, e.state_key) == (EventTypes.Create, ""):
                 create_event = e
                 break