'Yield client urns.'

def GetInput(self):

    client_list = GetAllClients(token=self.token)
    logging.debug('Got    %d    clients', len(client_list))
    for client_group in utils.Grouper(client_list, self.client_chunksize):
        for fd in aff4.FACTORY.MultiOpen(client_group, mode='r', aff4_type=aff4_grr.VFSGRRClient, token=self.token):
            if isinstance(fd, aff4_grr.VFSGRRClient):
                oldest_time = ((time.time() - self.max_age) * 1000000.0)
            if (fd.Get(aff4_grr.VFSGRRClient.SchemaCls.PING) >= oldest_time):
                (yield fd)
