'Retrieve all the clients for the AbstractClientStatsCollectors.'

@flow.StateHandler()
def Start(self):

    try:
        self.stats = {}
        self.BeginProcessing()
        root = aff4.FACTORY.Open(aff4.ROOT_URN, token=self.token)
        children_urns = list(root.ListChildren())
        logging.debug('Found    %d    children.', len(children_urns))
        processed_count = 0
        for child in aff4.FACTORY.MultiOpen(children_urns, mode='r', token=self.token, age=aff4.NEWEST_TIME):
            if isinstance(child, aff4_grr.VFSGRRClient):
                self.ProcessClient(child)
                processed_count += 1
            self.HeartBeat()
        self.FinishProcessing()
        for fd in self.stats.values():
            fd.Close()
        logging.info('%s:    processed    %d    clients.', self.__class__.__name__, processed_count)
    except Exception as e:
        logging.exception('Error    while    calculating    stats:    %s', e)
        raise
