Baton.h

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/*
 * Copyright 2018-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#pragma once

#include <atomic>
#include <experimental/coroutine>

namespace folly {
namespace coro {

class Baton {
 public:
  class WaitOperation;

  explicit Baton(bool initiallySignalled = false) noexcept;

  ~Baton();

  bool ready() const noexcept;

  [[nodiscard]] WaitOperation operator co_await() const noexcept;

  void post() noexcept;

  void reset() noexcept;

  class WaitOperation {
   public:
    explicit WaitOperation(const Baton& baton) noexcept : baton_(baton) {}

    bool await_ready() const noexcept {
      return baton_.ready();
    }

    bool await_suspend(
        std::experimental::coroutine_handle<> awaitingCoroutine) noexcept {
      awaitingCoroutine_ = awaitingCoroutine;
      return baton_.waitImpl(this);
    }

    void await_resume() noexcept {}

   protected:
    friend class Baton;

    const Baton& baton_;
    std::experimental::coroutine_handle<> awaitingCoroutine_;
    WaitOperation* next_;
  };

 private:
  // Try to register the awaiter as
  bool waitImpl(WaitOperation* awaiter) const noexcept;

  // this  - Baton is in the signalled/posted state.
  // other - Baton is not signalled/posted and this is a pointer to the head
  //         of a potentially empty linked-list of Awaiter nodes that were
  //         waiting for the baton to become signalled.
  mutable std::atomic<void*> state_;
};

inline Baton::Baton(bool initiallySignalled) noexcept
    : state_(initiallySignalled ? static_cast<void*>(this) : nullptr) {}

inline bool Baton::ready() const noexcept {
  return state_.load(std::memory_order_acquire) ==
      static_cast<const void*>(this);
}

inline Baton::WaitOperation Baton::operator co_await() const noexcept {
  return Baton::WaitOperation{*this};
}

inline void Baton::reset() noexcept {
  state_.store(nullptr, std::memory_order_relaxed);
}

} // namespace coro
} // namespace folly