:return: List of handles, or empty list. Should never return None.
        :raise: SolrSwitchedOff
        :raise SolrError: If both strategies to find file handles failed.

def retrieve_file_handles_of_same_dataset(self, **args):
        '''
        :return: List of handles, or empty list. Should never return None.
        :raise: SolrSwitchedOff
        :raise SolrError: If both strategies to find file handles failed.
        '''

        mandatory_args = ['drs_id', 'version_number', 'data_node']
        esgfpid.utils.check_presence_of_mandatory_args(args, mandatory_args)
        LOGGER.debug('Looking for files of dataset "%s", version "%s".',
                     args['drs_id'], str(args['version_number']))

        if self.__switched_on:
            return self.__retrieve_file_handles_of_same_dataset(**args)
        else:
            msg = 'Cannot retrieve handles of files of the same dataset.'
            raise esgfpid.exceptions.SolrSwitchedOff(msg)