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  1. Qt
  2. QTBUG-119406

QFuture::then() with context object deadlocks

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    • 59e21a536 (dev), f89d80c90 (6.7), d5528cdec (6.6)

      QFuture::then(context, ...) (and probably the other continuation attachers) can deadlock in Qt 6.6.0. Example:

      #include <QCoreApplication>
#include <QFuture>
#include <QPromise>
#include <QtDebug>

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

    QPromise<int> p;
    auto f = p.future();
    f.then(&app, [](int i) {
        qDebug() << i;
    });

    QMetaObject::invokeMethod(&app, [&] { app.quit(); }, Qt::QueuedConnection);

    p.start();
    p.addResult(42);
    p.finish();  // Deadlocks. Remove to have the app quit.

    return app.exec();
}

      I've tracked the cause to QFutureInterfaceBase::reportFinished(), which sends a QFutureCallOut without unlocking the mutex. The responders to this will want to access the future's result, which again tries to lock the mutex -> deadlock.
      This happens only if the context object lives in the same thread that calls QPromise::finish().

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            ivan.solovev Ivan Solovev
            msarehn Arno Rehn
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              Created:
              Updated:
              Resolved:

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