'Get the configuration, doing some sanity checks along the way.
:param service: A ``ConfigurationPersistenceService``.
:return: Result of ``service.configuration_hash()``.'

def get_hash(self, service):

    result1 = service.configuration_hash()
    result2 = service.configuration_hash()
    self.assertEqual(result1, result2)
    self.assertIsInstance(result1, bytes)
    return result1
