futriix/src/readwritelock.h

#pragma once
#include <condition_variable>

class readWriteLock {
    std::mutex m_readLock;
    std::recursive_mutex m_writeLock;
    std::condition_variable m_cv;
    int m_readCount = 0;
    int m_writeCount = 0;
    bool m_writeWaiting = false;
public:
    void acquireRead() {
        std::unique_lock<std::mutex> rm(m_readLock);
        while (m_writeCount > 0 || m_writeWaiting)
            m_cv.wait(rm);
        m_readCount++;
    }
    
    bool tryAcquireRead() {
        std::unique_lock<std::mutex> rm(m_readLock, std::defer_lock);
        if (!rm.try_lock())
            return false;
        if (m_writeCount > 0 || m_writeWaiting)
            return false;
        m_readCount++;
        return true;
    }

    void acquireWrite(bool exclusive = true) {
        std::unique_lock<std::mutex> rm(m_readLock);
        m_writeWaiting = true;
        while (m_readCount > 0)
            m_cv.wait(rm);
        if (exclusive) {
            /* Another thread might have the write lock while we have the read lock
               but won't be able to release it until they can acquire the read lock
               so release the read lock and try again instead of waiting to avoid deadlock */
            while(!m_writeLock.try_lock())
                m_cv.wait(rm);
        }
        m_writeCount++;
        m_writeWaiting = false;
    }

    void upgradeWrite(bool exclusive = true) {
        std::unique_lock<std::mutex> rm(m_readLock);
        m_writeWaiting = true;
        while (m_readCount > 1)
            m_cv.wait(rm);
        if (exclusive) {
            /* Another thread might have the write lock while we have the read lock
               but won't be able to release it until they can acquire the read lock
               so release the read lock and try again instead of waiting to avoid deadlock */
            while(!m_writeLock.try_lock())
                m_cv.wait(rm);
        }
        m_writeCount++;
        m_readCount--;
        m_writeWaiting = false;
    }

    bool tryAcquireWrite(bool exclusive = true) {
        std::unique_lock<std::mutex> rm(m_readLock, std::defer_lock);
        if (!rm.try_lock())
            return false;
        if (m_readCount > 0)
            return false;
        if (exclusive)
            if (!m_writeLock.try_lock())
                return false;
        m_writeCount++;
        return true;
    }

    void releaseRead() {
        std::unique_lock<std::mutex> rm(m_readLock);
        serverAssert(m_readCount > 0);
        m_readCount--;
        m_cv.notify_all();
    }

    void releaseWrite(bool exclusive = true) {
        std::unique_lock<std::mutex> rm(m_readLock);
        serverAssert(m_writeCount > 0);
        if (exclusive)
            m_writeLock.unlock();
        m_writeCount--;
        m_cv.notify_all();
    }

    void downgradeWrite(bool exclusive = true) {
        std::unique_lock<std::mutex> rm(m_readLock);
        serverAssert(m_writeCount > 0);
        if (exclusive)
            m_writeLock.unlock();
        m_writeCount--;
        while (m_writeCount > 0 || m_writeWaiting)
            m_cv.wait(rm);
        m_readCount++;
    }

    bool hasReader() {
        return m_readCount > 0;
    }

    bool hasWriter() {
        return m_writeCount > 0;
    }

    bool writeWaiting() {
        return m_writeWaiting;
    }
};