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Details

Type: Bug
Resolution: Done
Priority: P2: Important
Fix Version/s: 5.15.4, 6.0.4, 6.1.0 RC, 6.2.0
Affects Version/s: 5.15.2, 6.2
Component/s: Core: Event loop, Core: Threads
Labels:
None

Platform/s:

Windows
Commits:
f274f91cebb0a4fd2ebe37bb3a605c47d6acd404 ba0a105c617ad40f2a3eb2bb2ac2993ed6b11133 (qt/qtbase/6.1) 864b17ec1bbea826a19dfad113e560c58cc65e61 (qt/tqtc-qtbase/tqtc/lts-5.15) 955ce882f7c94f6085e26ef3f987fbfa72a1a8d7 (qt/qtbase/6.0)

Description

QThread::quit() has a race condition on Windows that means the quit() call is ignored. The following example fails for me within 30 seconds. With failure I mean the event loop doesn't quit correctly and the program prints the "Failed to quit" message and then triggers the qFatal for deleting a running thread. In my real world code it fails with a much higher frequency - in 1 of ~50 runs, but this is the best I could do with a minimal example. When I try to reduce the example further, the race condition isn't triggered and the for-loop continues to run. The problem does not reproduce with Qt 5.15.2 on Fedora 32 x86_64 Linux. I've only seen it with 5.15.2 on Windows x64. I have not tried reproducing with older versions of Qt 5 or with Qt 6.

#include <future>
#include <iostream>
#include <QCoreApplication>
#include <QThread>
#include <QTimer>

class TestThread : public QThread
{
public:
    ~TestThread()
    {
        quit();
        if (!wait(10000)) {
            std::cerr << "Failed to quit" << std::endl;
        }
    }

    void run() override
    {
        QObject object;
        m_promise.set_value(&object);
        QThread::run();
    }

    QObject *objectInThread() { return m_future.get(); }

private:
    std::promise<QObject*> m_promise;
    std::shared_future<QObject*> m_future = m_promise.get_future();
};

int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);

    for (int i = 0; i < 10000000; ++i) {
        std::cout << "iteration " << i << std::endl;
        TestThread t;
        t.start();
        std::promise<bool> prom;
        std::shared_future<bool> fut = prom.get_future();
        QMetaObject::invokeMethod(t.objectInThread(),
                                  [&] {
            // When we call prom.set_value we signal to the main thread to call QThread::quit()
            // on "our" thread.
            prom.set_value(true);
            // To trigger the race condition we then immediately after schedule some work
            // for the event loop.
            QTimer::singleShot(0, [] {});
        },
        Qt::QueuedConnection);
        fut.get();
    }
}