Faster Room Joins: fix /make_knock blocking indefinitely when the room in question is a partial-stated room.

changelog.d/13583.bugfix

@@ -0,0 +1 @@
+Faster Room Joins: fix `/make_knock` blocking indefinitely when the room in question is a partial-stated room.

synapse/federation/federation_server.py

@@ -763,6 +763,17 @@ async def on_make_knock_request(
             The partial knock event.
         """
         origin_host, _ = parse_server_name(origin)
+
+        if await self.store.is_partial_state_room(room_id):
+            # Before we do anything: check if the room is partial-stated.
+            # Note that at the time this check was added, `on_make_knock_request` would
+            # block due to https://github.com/matrix-org/synapse/issues/12997.
+            raise SynapseError(
+                404,
+                "Unable to handle /make_knock right now; this server is not fully joined.",
+                errcode=Codes.NOT_FOUND,
+            )
+
         await self.check_server_matches_acl(origin_host, room_id)
 
         room_version = await self.store.get_room_version(room_id)