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// Copyright 2020 Google LLC
//
// 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 dap_rwmutex_h
#define dap_rwmutex_h
#include <condition_variable>
#include <mutex>
namespace dap {
////////////////////////////////////////////////////////////////////////////////
// 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:
inline RLock(RWMutex& mutex);
inline ~RLock();
inline RLock(RLock&&);
inline RLock& operator=(RLock&&);
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:
inline WLock(RWMutex& mutex);
inline ~WLock();
inline WLock(WLock&&);
inline WLock& operator=(WLock&&);
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;
}
} // namespace dap
#endif
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