template <typename Val>
template <typename C, typename D>
int KVWorker<Val>::AddPullCB(
    const SArray<Key>& keys, C* vals, D* lens, int cmd,
    const Callback& cb) {
  int ts = obj_->NewRequest(kServerGroup);
  AddCallback(ts, [this, ts, keys, vals, lens, cb]() mutable {
      ...
  
      if (vals->empty()) {
        vals->resize(total_val);
      } else {
        CHECK_EQ(vals->size(), total_val);    // here
      }
     ...
    });

  return ts;
}