Enforce compatibility when processing make_join requests

Reject make_join requests from servers which do not support the room version.

Also include the room version in the response.

synapse/federation/federation_server.py

@@ -27,7 +27,13 @@ from twisted.internet.abstract import isIPAddress
 from twisted.python import failure
 
 from synapse.api.constants import EventTypes
-from synapse.api.errors import AuthError, FederationError, NotFoundError, SynapseError
+from synapse.api.errors import (
+    AuthError,
+    FederationError,
+    IncompatibleRoomVersionError,
+    NotFoundError,
+    SynapseError,
+)
 from synapse.crypto.event_signing import compute_event_signature
 from synapse.federation.federation_base import FederationBase, event_from_pdu_json
 from synapse.federation.persistence import TransactionActions
@@ -323,12 +329,21 @@ class FederationServer(FederationBase):
         defer.returnValue((200, resp))
 
     @defer.inlineCallbacks
-    def on_make_join_request(self, origin, room_id, user_id):
+    def on_make_join_request(self, origin, room_id, user_id, supported_versions):
         origin_host, _ = parse_server_name(origin)
         yield self.check_server_matches_acl(origin_host, room_id)
+
+        room_version = yield self.store.get_room_version(room_id)
+        if room_version not in supported_versions:
+            logger.warn("Room version %s not in %s", room_version, supported_versions)
+            raise IncompatibleRoomVersionError(room_version=room_version)
+
         pdu = yield self.handler.on_make_join_request(room_id, user_id)
         time_now = self._clock.time_msec()
-        defer.returnValue({"event": pdu.get_pdu_json(time_now)})
+        defer.returnValue({
+            "event": pdu.get_pdu_json(time_now),
+            "room_version": room_version,
+        })
 
     @defer.inlineCallbacks
     def on_invite_request(self, origin, content):