Ensure state DAG rooms do not federate

- /createRoom requests forcibly set m.federate=false
- /send_join requests forcibly bail out for state DAG rooms.

synapse/federation/federation_client.py

@@ -1108,6 +1108,11 @@ class FederationClient(FederationBase):
             SynapseError: if the chosen remote server returns a 300/400 code, or
                 no servers successfully handle the request.
         """
+        # See related restriction in /createRoom requests in handlers/room.py
+        if room_version.msc4242_state_dags:
+            raise UnsupportedRoomVersionError(
+                "Homeserver does not support this room version over federation"
+            )
 
         async def send_request(destination: str) -> SendJoinResult:
             response = await self._do_send_join(

synapse/handlers/room.py

@@ -1237,6 +1237,10 @@ class RoomCreationHandler:
         creation_content = config.get("creation_content", {})
         # override any attempt to set room versions via the creation_content
         creation_content["room_version"] = room_version.identifier
+        # We do not currently support federating state DAG rooms.
+        # See related restriction in /send_join requests in federation_client.py.
+        if room_version.msc4242_state_dags:
+            creation_content[EventContentFields.FEDERATE] = False
 
         # trusted private chats have the invited users marked as additional creators
         if (