kdevplatform/util/foregroundlock.cpp

/*
    SPDX-FileCopyrightText: 2010 David Nolden <david.nolden.kdevelop@art-master.de>

    SPDX-License-Identifier: GPL-2.0-or-later
*/

#include "foregroundlock.h"

#include <QCoreApplication>
#include <QMutex>
#include <QThread>

using namespace KDevelop;

namespace {

QMutex internalMutex;
QMutex tryLockMutex;
QMutex waitMutex;
QMutex finishMutex;
QWaitCondition condition;

volatile QThread* holderThread = nullptr;
volatile int recursion = 0;

void lockForegroundMutexInternal()
{
    if (holderThread == QThread::currentThread()) {
        // We already have the mutex
        ++recursion;
    } else {
        internalMutex.lock();
        Q_ASSERT(recursion == 0 && holderThread == nullptr);
        holderThread = QThread::currentThread();
        recursion = 1;
    }
}

bool tryLockForegroundMutexInternal(int interval = 0)
{
    if (holderThread == QThread::currentThread()) {
        // We already have the mutex
        ++recursion;
        return true;
    } else {
        if (internalMutex.tryLock(interval)) {
            Q_ASSERT(recursion == 0 && holderThread == nullptr);
            holderThread = QThread::currentThread();
            recursion = 1;
            return true;
        } else {
            return false;
        }
    }
}

void unlockForegroundMutexInternal(bool duringDestruction = false)
{
    /// Note: QThread::currentThread() might already be invalid during destruction.
    if (!duringDestruction) {
        Q_ASSERT(holderThread == QThread::currentThread());
    }

    Q_ASSERT(recursion > 0);
    recursion -= 1;
    if (recursion == 0) {
        holderThread = nullptr;
        internalMutex.unlock();
    }
}
}

ForegroundLock::ForegroundLock(bool lock)
{
    if (lock)
        relock();
}

void KDevelop::ForegroundLock::relock()
{
    Q_ASSERT(!m_locked);

    if (!QCoreApplication::instance() || // Initialization isn't complete yet
        QThread::currentThread() == QCoreApplication::instance()->thread()
        || // We're the main thread (deadlock might happen if we'd enter the trylock loop)
        holderThread == QThread::currentThread()) { // We already have the foreground lock (deadlock might happen if
                                                    // we'd enter the trylock loop)
        lockForegroundMutexInternal();
    } else {
        QMutexLocker lock(&tryLockMutex);

        while (!tryLockForegroundMutexInternal(10)) {
            // In case an additional event-loop was started from within the foreground, we send
            // events to the foreground to temporarily release the lock.

            class ForegroundReleaser : public DoInForeground
            {
public:
                void doInternal() override
                {
                    // By locking the mutex, we make sure that the requester is actually waiting for the condition
                    waitMutex.lock();
                    // Now we release the foreground lock
                    TemporarilyReleaseForegroundLock release;
                    // And signalize to the requester that we've released it
                    condition.wakeAll();
                    // Allow the requester to actually wake up, by unlocking m_waitMutex
                    waitMutex.unlock();
                    // Now wait until the requester is ready
                    QMutexLocker lock(&finishMutex);
                }
            };

            static ForegroundReleaser releaser;

            QMutexLocker lockWait(&waitMutex);
            QMutexLocker lockFinish(&finishMutex);

            QMetaObject::invokeMethod(&releaser, "doInternalSlot", Qt::QueuedConnection);
            // We limit the waiting time here, because sometimes it may happen that the foreground-lock is released,
            // and the foreground is waiting without an event-loop running. (For example through TemporarilyReleaseForegroundLock)
            condition.wait(&waitMutex, 30);

            if (tryLockForegroundMutexInternal()) {
                //success
                break;
            } else {
                //Probably a third thread has creeped in and
                //got the foreground lock before us. Just try again.
            }
        }
    }
    m_locked = true;
    Q_ASSERT(holderThread == QThread::currentThread());
    Q_ASSERT(recursion > 0);
}

bool KDevelop::ForegroundLock::isLockedForThread()
{
    return QThread::currentThread() == holderThread
        || QThread::currentThread() == QCoreApplication::instance()->thread();
}

bool KDevelop::ForegroundLock::tryLock()
{
    if (tryLockForegroundMutexInternal()) {
        m_locked = true;
        return true;
    }
    return false;
}

void KDevelop::ForegroundLock::unlock()
{
    Q_ASSERT(m_locked);
    unlockForegroundMutexInternal();
    m_locked = false;
}

TemporarilyReleaseForegroundLock::TemporarilyReleaseForegroundLock()
{
    Q_ASSERT(holderThread == QThread::currentThread());

    m_recursion = 0;

    while (holderThread == QThread::currentThread()) {
        unlockForegroundMutexInternal();
        ++m_recursion;
    }
}

TemporarilyReleaseForegroundLock::~TemporarilyReleaseForegroundLock()
{
    for (int a = 0; a < m_recursion; ++a)
        lockForegroundMutexInternal();

    Q_ASSERT(recursion == m_recursion && holderThread == QThread::currentThread());
}

KDevelop::ForegroundLock::~ForegroundLock()
{
    if (m_locked)
        unlock();
}

bool KDevelop::ForegroundLock::isLocked() const
{
    return m_locked;
}

namespace KDevelop {
void DoInForeground::doIt()
{
    if (QThread::currentThread() == QCoreApplication::instance()->thread()) {
        // We're already in the foreground, just call the handler code
        doInternal();
    } else {
        QMutexLocker lock(&m_mutex);
        QMetaObject::invokeMethod(this, "doInternalSlot", Qt::QueuedConnection);
        m_wait.wait(&m_mutex);
    }
}

DoInForeground::~DoInForeground()
{
}

DoInForeground::DoInForeground()
{
    moveToThread(QCoreApplication::instance()->thread());
}

void DoInForeground::doInternalSlot()
{
    VERIFY_FOREGROUND_LOCKED
        doInternal();
    QMutexLocker lock(&m_mutex);
    m_wait.wakeAll();
}
}

// Important: The foreground lock has to be held by default, so lock it during static initialization
static struct StaticLock
{
    StaticLock()
    {
        lockForegroundMutexInternal();
    }
    ~StaticLock()
    {
        unlockForegroundMutexInternal(true);
    }
private:
    Q_DISABLE_COPY(StaticLock)
} staticLock;