src/rw_mutex.h

/*
 *  Copyright (C) 2004-2021 Savoir-faire Linux Inc.
 *
 *  Author: Adrien Béraud <adrien.beraud@savoirfairelinux.com>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA.
 */

#ifndef RW_MUTEX_H_
#define RW_MUTEX_H_

#include "noncopyable.h"

#include <mutex>
#include <atomic>
#include <condition_variable>
#include <string>
#include <sstream>

namespace jami {

/**
 * rw_mutex is a shared mutex meant to protect
 * rarely-modified, often-read data structures.
 *
 * Its goal is to optimize read throughput and latency.
 * Multiple threads can concurrently read data while
 * a writer thread gets exclusive access when needed.
 */
class rw_mutex
{
public:
    rw_mutex()
        : mutex()
        , canRead()
        , canWrite()
        , readers(0)
        , writing(false)
    {}
    void read_enter()
    {
        std::unique_lock<std::mutex> lck(mutex);
        canRead.wait(lck, [this]() { return !writing; });
        readers++;
    }
    void read_exit()
    {
        // std::lock_guard<std::mutex> lck(mutex);
        readers--;
        canWrite.notify_one();
    }
    void write_enter()
    {
        std::unique_lock<std::mutex> lck(mutex);
        canWrite.wait(lck, [this]() { return !writing && readers == 0; });
        writing = true;
    }
    void write_exit()
    {
        std::lock_guard<std::mutex> lck(mutex);
        writing = false;
        canWrite.notify_one();
        canRead.notify_all();
    }

    struct read_lock
    {
    public:
        read_lock(rw_mutex& m)
            : sem(m)
        {
            sem.read_enter();
        }
        ~read_lock() { sem.read_exit(); }

    private:
        rw_mutex& sem;
    };

    struct write_lock
    {
    public:
        write_lock(rw_mutex& m)
            : sem(m)
        {
            sem.write_enter();
        }
        ~write_lock() { sem.write_exit(); }

    private:
        rw_mutex& sem;
    };

    read_lock read() { return read_lock(*this); }

    write_lock write() { return write_lock(*this); }

    std::string toString()
    {
        std::stringstream ss;
        ss << "[rw_mutex write:" << (writing ? "LOCKED" : "unlocked") << " read:" << readers << "]";
        return ss.str();
    }

private:
    NON_COPYABLE(rw_mutex);
    std::mutex mutex;
    std::condition_variable canRead, canWrite;
    std::atomic<unsigned> readers;
    bool writing;
};

} // namespace jami

#endif