Method TryAcquire

cpp/src/arrow/util/async_util.cc:48–59

  }

  std::optional<Future<>> TryAcquire(int amt) override {
    std::lock_guard<std::mutex> lk(mutex_);
    if (backoff_.is_valid()) {
      return backoff_;
    }
    if (amt <= available_cost_) {
      available_cost_ -= amt;
      return std::nullopt;
    }
    backoff_ = Future<>::Make();
    return backoff_;
  }

  void Release(int amt) override {
    std::unique_lock lk(mutex_);