async_request.h

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/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
 
#pragma once
 
#if __cplusplus >= 201703L
#include <optional>
#else
#include <dispenso/detail/op_result.h>
#endif // C++17
 
#include <dispenso/platform.h>
 
namespace dispenso {
 
template <typename T>
class AsyncRequest {
 public:
  // A lightweight std::optional-like type with a subset of functionality.
#if __cplusplus >= 201703L
  using OpResult = std::optional<T>;
#else
  using OpResult = detail::OpResult<T>;
#endif // C++17
 
  void requestUpdate() {
    RequestState state = kNone;
    state_.compare_exchange_strong(state, kNeedsUpdate, std::memory_order_acq_rel);
  }
 
  bool updateRequested() const {
    return state_.load(std::memory_order_acquire) == kNeedsUpdate;
  }
 
  template <typename... Args>
  bool tryEmplaceUpdate(Args&&... args) {
    RequestState state = kNeedsUpdate;
    if (!state_.compare_exchange_strong(state, kUpdating, std::memory_order_acq_rel)) {
      return false;
    }
    obj_.emplace(std::forward<Args>(args)...);
    state_.store(kReady, std::memory_order_release);
    return true;
  }
 
  OpResult getUpdate() {
    if (state_.load(std::memory_order_acquire) == kReady) {
      auto obj = std::move(obj_);
      state_.store(kNone, std::memory_order_release);
      return obj;
    }
    return {};
  }
 
 private:
  enum RequestState { kNone, kNeedsUpdate, kUpdating, kReady };
  alignas(kCacheLineSize) std::atomic<RequestState> state_ = {kNone};
  OpResult obj_;
};
 
} // namespace dispenso