Unable to submit results to certification.canonical.com - sortkey attribute not accepted by server

Result submissions to certification.canonical.com fail with "Inconsistent message" errors on the clientside. Serverside, the message is this:

2012-03-13T19:44:49 ERROR canonical.certify.message Error handling message 'set-distribution' for hardware 201101-6950 (OOPS 2263production38): {'distribution': {'release': '12.04', 'codename': 'precise', 'distributor_id': 'Ubuntu', 'description': 'Ubuntu precise (development branch)', 'sortkey': 0}, 'type': 'set-distribution'}
Traceback (most recent call last):
  File "./src/canonical/certify/message/messageapis.py", line 230, in _run
    self.handle(message["type"], message)
  File "./src/canonical/message/api.py", line 57, in handle
    return handler(type, body)
  File "./src/canonical/checkbox/arguments.py", line 81, in replacement
    kwargs)
  File "./src/canonical/checkbox/arguments.py", line 54, in replace_arguments
    args[name_index] = self.argument_replacer(name, value)
  File "./src/canonical/checkbox/arguments.py", line 74, in replacer
    return schema.coerce(value)
  File "/srv/certification.canonical.com/checkbox/checkbox/properties.py", line 71, in coerce
    return self._variable_class(**self._variable_kwargs).coerce(value)
  File "/srv/certification.canonical.com/checkbox/checkbox/variables.py", line 265, in coerce
    % (attribute, value, e))
ValueError: Value of 'distribution' key of dict {'distribution': {'release': '12.04', 'codename': 'precise', 'distributor_id': 'Ubuntu', 'description': 'Ubuntu precise (development branch)', 'sortkey': 0}, 'type': 'set-distribution'} could not be converted: 'sortkey' is not a valid key as per {'release': <functools.partial object at 0xba72b1c>, 'codename': <functools.partial object at 0xba72b44>, 'distributor_id': <functools.partial object at 0xba72b6c>, 'description': <functools.partial object at 0xba72b94>}

I can see the troublesome part is the "sortkey" element of the distribution dictionary/message. The sortkey attribute is added in code from checkbox revision 1291 to visually sort jobs in the user interface. However, sending these to either c3 or launchpad doesn't make sense.

The possible solution is to strip the sortkey element prior to sending messages, which is done in the launchpad_report and certify_report plugins.