Fix `room_version` in `on_invite_request` flow (#6827)

I messed this up a bit in #6805, but fortunately we weren't actually doing
anything with the room_version so it didn't matter that it was a str not a RoomVersion.

changelog.d/6827.misc

+Refactoring work in preparation for changing the event redaction algorithm.

synapse/federation/federation_server.py

@@ -54,7 +54,7 @@ from synapse.replication.http.federation import (
     ReplicationFederationSendEduRestServlet,
     ReplicationGetQueryRestServlet,
 )
-from synapse.types import get_domain_from_id
+from synapse.types import JsonDict, get_domain_from_id
 from synapse.util import glob_to_regex, unwrapFirstError
 from synapse.util.async_helpers import Linearizer, concurrently_execute
 from synapse.util.caches.response_cache import ResponseCache
@@ -396,20 +396,23 @@ class FederationServer(FederationBase):
         time_now = self._clock.time_msec()
         return {"event": pdu.get_pdu_json(time_now), "room_version": room_version}
 
-    async def on_invite_request(self, origin, content, room_version):
-        if room_version not in KNOWN_ROOM_VERSIONS:
+    async def on_invite_request(
+        self, origin: str, content: JsonDict, room_version_id: str
+    ):
+        room_version = KNOWN_ROOM_VERSIONS.get(room_version_id)
+        if not room_version:
             raise SynapseError(
                 400,
                 "Homeserver does not support this room version",
                 Codes.UNSUPPORTED_ROOM_VERSION,
             )
 
-        format_ver = room_version_to_event_format(room_version)
+        format_ver = room_version.event_format
 
         pdu = event_from_pdu_json(content, format_ver)
         origin_host, _ = parse_server_name(origin)
         await self.check_server_matches_acl(origin_host, pdu.room_id)
-        pdu = await self._check_sigs_and_hash(room_version, pdu)
+        pdu = await self._check_sigs_and_hash(room_version.identifier, pdu)
         ret_pdu = await self.handler.on_invite_request(origin, pdu, room_version)
         time_now = self._clock.time_msec()
         return {"event": ret_pdu.get_pdu_json(time_now)}

synapse/federation/transport/server.py

@@ -579,7 +579,7 @@ class FederationV1InviteServlet(BaseFederationServlet):
         # state resolution algorithm, and we don't use that for processing
         # invites
         content = await self.handler.on_invite_request(
-            origin, content, room_version=RoomVersions.V1.identifier
+            origin, content, room_version_id=RoomVersions.V1.identifier
         )
 
         # V1 federation API is defined to return a content of `[200, {...}]`
@@ -606,7 +606,7 @@ class FederationV2InviteServlet(BaseFederationServlet):
         event.setdefault("unsigned", {})["invite_room_state"] = invite_room_state
 
         content = await self.handler.on_invite_request(
-            origin, event, room_version=room_version
+            origin, event, room_version_id=room_version
         )
         return 200, content