Patch to temporarily drop cross-user m.key_share_requests (#8675)

Cross-user `m.key_share_requests` are a relatively new `to_device` message that allows user to re-request session keys for a message from another user if they were otherwise unable to retrieve them.

Unfortunately, these have had performance concerns on matrix.org. This is a temporary patch to disable them while we investigate a better solution.

changelog.d/8675.misc

@@ -0,0 +1 @@
+Temporarily drop cross-user m.room_key_request to_device messages over performance concerns.

synapse/federation/federation_server.py

@@ -915,6 +915,10 @@ async def on_edu(self, edu_type: str, origin: str, content: dict):
         if not self.config.use_presence and edu_type == "m.presence":
             return
 
+        # Temporary patch to drop cross-user key share requests
+        if edu_type == "m.room_key_request":
+            return
+
         # Check if we have a handler on this instance
         handler = self.edu_handlers.get(edu_type)
         if handler:

synapse/handlers/devicemessage.py

@@ -153,6 +153,10 @@ async def send_device_message(
         local_messages = {}
         remote_messages = {}  # type: Dict[str, Dict[str, Dict[str, JsonDict]]]
         for user_id, by_device in messages.items():
+            # Temporary patch to disable sending local cross-user key requests.
+            if message_type == "m.room_key_request" and user_id != sender_user_id:
+                continue
+
             # we use UserID.from_string to catch invalid user ids
             if self.is_mine(UserID.from_string(user_id)):
                 messages_by_device = {