cmd/remote-compile/rwmutex.h

// Copyright 2020 The Tint Authors
//
// 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
//
//     https://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.

#ifndef TOOLS_SRC_CMD_REMOTE_COMPILE_RWMUTEX_H_
#define TOOLS_SRC_CMD_REMOTE_COMPILE_RWMUTEX_H_

#include <condition_variable>  // NOLINT
#include <mutex>               // NOLINT

////////////////////////////////////////////////////////////////////////////////
// RWMutex
////////////////////////////////////////////////////////////////////////////////

/// A RWMutex is a reader/writer mutual exclusion lock.
/// The lock can be held by an arbitrary number of readers or a single writer.
/// Also known as a shared mutex.
class RWMutex {
 public:
  inline RWMutex() = default;

  /// lockReader() locks the mutex for reading.
  /// Multiple read locks can be held while there are no writer locks.
  inline void lockReader();

  /// unlockReader() unlocks the mutex for reading.
  inline void unlockReader();

  /// lockWriter() locks the mutex for writing.
  /// If the lock is already locked for reading or writing, lockWriter blocks
  /// until the lock is available.
  inline void lockWriter();

  /// unlockWriter() unlocks the mutex for writing.
  inline void unlockWriter();

 private:
  RWMutex(const RWMutex&) = delete;
  RWMutex& operator=(const RWMutex&) = delete;

  int readLocks = 0;
  int pendingWriteLocks = 0;
  std::mutex mutex;
  std::condition_variable cv;
};

void RWMutex::lockReader() {
  std::unique_lock<std::mutex> lock(mutex);
  readLocks++;
}

void RWMutex::unlockReader() {
  std::unique_lock<std::mutex> lock(mutex);
  readLocks--;
  if (readLocks == 0 && pendingWriteLocks > 0) {
    cv.notify_one();
  }
}

void RWMutex::lockWriter() {
  std::unique_lock<std::mutex> lock(mutex);
  if (readLocks > 0) {
    pendingWriteLocks++;
    cv.wait(lock, [&] { return readLocks == 0; });
    pendingWriteLocks--;
  }
  lock.release();  // Keep lock held
}

void RWMutex::unlockWriter() {
  if (pendingWriteLocks > 0) {
    cv.notify_one();
  }
  mutex.unlock();
}

////////////////////////////////////////////////////////////////////////////////
// RLock
////////////////////////////////////////////////////////////////////////////////

/// RLock is a RAII read lock helper for a RWMutex.
class RLock {
 public:
  /// Constructor.
  /// Locks `mutex` with a read-lock for the lifetime of the WLock.
  /// @param mutex the mutex
  explicit inline RLock(RWMutex& mutex);
  /// Destructor.
  /// Unlocks the RWMutex.
  inline ~RLock();

  /// Move constructor
  /// @param other the other RLock to move into this RLock.
  inline RLock(RLock&& other);
  /// Move assignment operator
  /// @param other the other RLock to move into this RLock.
  /// @returns this RLock so calls can be chained
  inline RLock& operator=(RLock&& other);

 private:
  RLock(const RLock&) = delete;
  RLock& operator=(const RLock&) = delete;

  RWMutex* m;
};

RLock::RLock(RWMutex& mutex) : m(&mutex) {
  m->lockReader();
}

RLock::~RLock() {
  if (m != nullptr) {
    m->unlockReader();
  }
}

RLock::RLock(RLock&& other) {
  m = other.m;
  other.m = nullptr;
}

RLock& RLock::operator=(RLock&& other) {
  m = other.m;
  other.m = nullptr;
  return *this;
}

////////////////////////////////////////////////////////////////////////////////
// WLock
////////////////////////////////////////////////////////////////////////////////

/// WLock is a RAII write lock helper for a RWMutex.
class WLock {
 public:
  /// Constructor.
  /// Locks `mutex` with a write-lock for the lifetime of the WLock.
  /// @param mutex the mutex
  explicit inline WLock(RWMutex& mutex);

  /// Destructor.
  /// Unlocks the RWMutex.
  inline ~WLock();

  /// Move constructor
  /// @param other the other WLock to move into this WLock.
  inline WLock(WLock&& other);
  /// Move assignment operator
  /// @param other the other WLock to move into this WLock.
  /// @returns this WLock so calls can be chained
  inline WLock& operator=(WLock&& other);

 private:
  WLock(const WLock&) = delete;
  WLock& operator=(const WLock&) = delete;

  RWMutex* m;
};

WLock::WLock(RWMutex& mutex) : m(&mutex) {
  m->lockWriter();
}

WLock::~WLock() {
  if (m != nullptr) {
    m->unlockWriter();
  }
}

WLock::WLock(WLock&& other) {
  m = other.m;
  other.m = nullptr;
}

WLock& WLock::operator=(WLock&& other) {
  m = other.m;
  other.m = nullptr;
  return *this;
}

#endif  // TOOLS_SRC_CMD_REMOTE_COMPILE_RWMUTEX_H_