docs: def GetInput(self):
      """Yield client urns."""
      client_list = GetAllClients(token=self.token)
      logging.debug("Got %d clients", len(client_list))
      for client_group in collection.Batch(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):
            # Skip if older than max_age
            oldest_time = (time.time() - self.max_age) * 1e6
          if fd.Get(aff4_grr.VFSGRRClient.SchemaCls.PING) >= oldest_time:
            yield fd