Raise error if prev_ids is not a list

Signed-off-by: Carl Bordum Hansen <carl@bordum.dk>

Author: carlbordum

changelog.d/12829.bugfix

@@ -0,0 +1,2 @@
+Fixed a bug where device list updates over federation would fail if they
+were not a list. Contributed by Carl Bordum Hansen.

synapse/handlers/device.py

@@ -763,6 +763,8 @@ async def incoming_device_list_update(
         device_id = edu_content.pop("device_id")
         stream_id = str(edu_content.pop("stream_id"))  # They may come as ints
         prev_ids = edu_content.pop("prev_id", [])
+        if not isinstance(prev_ids, list):
+            raise SynapseError(400, "Device list update had an invalid 'prev_ids' field")
         prev_ids = [str(p) for p in prev_ids]  # They may come as ints
 
         if get_domain_from_id(user_id) != origin: