common/include/QtCore/5.5.0/QtCore/private/qorderedmutexlocker_p.h

/****************************************************************************
**
** Copyright (C) 2015 The Qt Company Ltd.
** Contact: http://www.qt.io/licensing/
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL21$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see http://www.qt.io/terms-conditions. For further
** information use the contact form at http://www.qt.io/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 or version 3 as published by the Free
** Software Foundation and appearing in the file LICENSE.LGPLv21 and
** LICENSE.LGPLv3 included in the packaging of this file. Please review the
** following information to ensure the GNU Lesser General Public License
** requirements will be met: https://www.gnu.org/licenses/lgpl.html and
** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** As a special exception, The Qt Company gives you certain additional
** rights. These rights are described in The Qt Company LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** $QT_END_LICENSE$
**
****************************************************************************/

#ifndef QORDEREDMUTEXLOCKER_P_H
#define QORDEREDMUTEXLOCKER_P_H

//
//  W A R N I N G
//  -------------
//
// This file is not part of the Qt API.  It exists purely as an
// implementation detail.  This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.
//

#include <QtCore/qmutex.h>

QT_BEGIN_NAMESPACE

/*
  Locks 2 mutexes in a defined order, avoiding a recursive lock if
  we're trying to lock the same mutex twice.
*/
class QOrderedMutexLocker
{
public:
    QOrderedMutexLocker(QMutex *m1, QMutex *m2)
        : mtx1((m1 == m2) ? m1 : (m1 < m2 ? m1 : m2)),
          mtx2((m1 == m2) ?  0 : (m1 < m2 ? m2 : m1)),
          locked(false)
    {
        relock();
    }
    ~QOrderedMutexLocker()
    {
        unlock();
    }

    void relock()
    {
        if (!locked) {
            if (mtx1) mtx1->lock();
            if (mtx2) mtx2->lock();
            locked = true;
        }
    }

    void unlock()
    {
        if (locked) {
            if (mtx1) mtx1->unlock();
            if (mtx2) mtx2->unlock();
            locked = false;
        }
    }

    static bool relock(QMutex *mtx1, QMutex *mtx2)
    {
        // mtx1 is already locked, mtx2 not... do we need to unlock and relock?
        if (mtx1 == mtx2)
            return false;
        if (mtx1 < mtx2) {
            mtx2->lock();
            return true;
        }
        if (!mtx2->tryLock()) {
            mtx1->unlock();
            mtx2->lock();
            mtx1->lock();
        }
        return true;
    }

private:
    QMutex *mtx1, *mtx2;
    bool locked;
};

QT_END_NAMESPACE

#endif