Reject history insertion during partial joins (#14291)

Author: David Robertson

changelog.d/14291.bugfix

@@ -0,0 +1 @@
+Prevent history insertion ([MSC2716](https://github.com/matrix-org/matrix-spec-proposals/pull/2716)) during an partial join ([MSC3706](https://github.com/matrix-org/matrix-spec-proposals/pull/3706)).

synapse/rest/client/room_batch.py

@@ -108,6 +108,13 @@ async def on_POST(
                 errcode=Codes.MISSING_PARAM,
             )
 
+        if await self.store.is_partial_state_room(room_id):
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST,
+                "Cannot insert history batches until we have fully joined the room",
+                errcode=Codes.UNABLE_DUE_TO_PARTIAL_STATE,
+            )
+
         # Verify the batch_id_from_query corresponds to an actual insertion event
         # and have the batch connected.
         if batch_id_from_query: