text: /*
   * 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 {
  
  /// A baton is a synchronisation primitive for coroutines that allows a
  /// coroutine to co_await the baton and suspend until the baton is posted by
  /// some thread via a call to .post().
  ///
  /// The Baton supports being awaited by multiple coroutines at a time. If the
  /// baton is not ready at the time it is awaited then an awaiting coroutine
  /// suspends. All suspended coroutines waiting for the baton to be posted will
  /// be resumed when some thread next calls .post().
  ///
  /// Example usage:
  ///
  ///   folly::coro::Baton baton;
  ///   std::string sharedValue;
  ///
  ///   folly::coro::Task<void> consumer()
  ///   {
  ///     // Wait until the baton is posted.
  ///     co_await baton;
  ///
  ///     // Now safe to read shared state.
  ///     std::cout << sharedValue << std::cout;
  ///   }
  ///
  ///   void producer()
  ///   {
  ///     // Write to shared state
  ///     sharedValue = "some result";
  ///
  ///     // Publish the value by 'posting' the baton.
  ///     // This will resume the consumer if it was currently suspended.
  ///     baton.post();
  ///   }
  class Baton {
   public:
    class WaitOperation;
  
    /// Initialise the Baton to either the signalled or non-signalled state.
    explicit Baton(bool initiallySignalled = false) noexcept;
  
    ~Baton();
  
    /// Query whether the Baton is currently in the signalled state.
    bool ready() const noexcept;
  
    /// Asynchronously wait for the Baton to enter the signalled state.
    ///
    /// The returned object must be co_awaited from a coroutine. If the Baton
    /// is already signalled then the awaiting coroutine will continue without
    /// suspending. Otherwise, if the Baton is not yet signalled then the
    /// awaiting coroutine will suspend execution and will be resumed when some
    /// thread later calls post().
    [[nodiscard]] WaitOperation operator co_await() const noexcept;
  
    /// Set the Baton to the signalled state if it is not already signalled.
    ///
    /// This will resume any coroutines that are currently suspended waiting
    /// for the Baton inside 'co_await baton'.
    void post() noexcept;
  
    /// Atomically reset the baton back to the non-signalled state.
    ///
    /// This is a no-op if the baton was already in the non-signalled state.
    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 {
    // Transition from 'signalled' (ie. 'this') to not-signalled (ie. nullptr).
    void* oldState = this;
    (void)state_.compare_exchange_strong(
        oldState, nullptr, std::memory_order_acq_rel, std::memory_order_relaxed);
  }
  
  } // namespace coro
  } // namespace folly
  