Mutex.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>
#include <mutex>

namespace folly {
namespace coro {

class Mutex {
  class ScopedLockOperation;
  class LockOperation;

 public:
  Mutex() noexcept : state_(unlockedState()), waiters_(nullptr) {}

  Mutex(const Mutex&) = delete;
  Mutex(Mutex&&) = delete;
  Mutex& operator=(const Mutex&) = delete;
  Mutex& operator=(Mutex&&) = delete;

  ~Mutex();

  bool try_lock() noexcept {
    void* oldValue = unlockedState();
    return state_.compare_exchange_strong(
        oldValue,
        nullptr,
        std::memory_order_acquire,
        std::memory_order_relaxed);
  }

  [[nodiscard]] ScopedLockOperation co_scoped_lock() noexcept;

  [[nodiscard]] LockOperation co_lock() noexcept;

  void unlock() noexcept;

 private:
  class LockOperation {
   public:
    bool await_ready() noexcept {
      return mutex_.try_lock();
    }

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

    void await_resume() noexcept {}

   protected:
    friend class Mutex;

    explicit LockOperation(Mutex& mutex) noexcept : mutex_(mutex) {}

    Mutex& mutex_;
    std::experimental::coroutine_handle<> awaitingCoroutine_;
    LockOperation* next_;
  };

  class ScopedLockOperation : public LockOperation {
   public:
    std::unique_lock<Mutex> await_resume() noexcept {
      return std::unique_lock<Mutex>{mutex_, std::adopt_lock};
    }

   private:
    friend class Mutex;
    using LockOperation::LockOperation;
  };

  friend class LockOperation;

  // Special value for state_ that indicates the mutex is not locked.
  void* unlockedState() noexcept {
    return this;
  }

  // Try to lock the mutex.
  //
  // Returns true if the lock could not be acquired synchronously and awaiting
  // coroutine should suspend. In this case the coroutine will be resumed later
  // once it acquires the mutex. Returns false if the lock was acquired
  // synchronously and the awaiting coroutine should continue without
  // suspending.
  bool lockAsyncImpl(LockOperation* awaiter);

  // This contains either:
  // - this    => Not locked
  // - nullptr => Locked, no newly queued waiters (ie. empty list of waiters)
  // - other   => Pointer to first LockAwaiterBase* in a linked-list of newly
  //              queued awaiters in LIFO order.
  std::atomic<void*> state_;

  // Linked-list of waiters in FIFO order.
  // Only the current lock holder is allowed to access this member.
  LockOperation* waiters_;
};

inline Mutex::ScopedLockOperation Mutex::co_scoped_lock() noexcept {
  return ScopedLockOperation{*this};
}

inline Mutex::LockOperation Mutex::co_lock() noexcept {
  return LockOperation{*this};
}

} // namespace coro
} // namespace folly