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  1. Qt
  2. QTBUG-119406

QFuture::then() with context object deadlocks

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Details

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

    Description

      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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          duplicates

          Bug - A problem which impairs or prevents the functions of the product. QTBUG-119579 [Reg 6.5 -> 6.6] Calling QPromise::finish() from the same thread deadlocks i

          • P1: Critical - Urgent and Important, will STOP the release if matched with set FixVersion field. This includes regressions from the last version that are not edge cases; Data loss; Build issues; All but the most unlikely crashes/lockups/hanging; Serious usability issues and embarrassing bugs & Bugs critical to a deliverable. This is the highest possible priority for requirements.
          • Closed
          is duplicated by

          Bug - A problem which impairs or prevents the functions of the product. QTBUG-117918 Debug builds with QFuture::waitForFinished and same-thread continuations may deadlock

          • P2: Important - Urgent, should be fixed, but will not stop the release.
          • Closed

          Bug - A problem which impairs or prevents the functions of the product. QTBUG-119103 QFuture hangs when continuation uses a context object and is resolved from the same thread

          • P2: Important - Urgent, should be fixed, but will not stop the release.
          • Closed
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              ivan.solovev Ivan Solovev
              msarehn Arno Rehn
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                Created:
                Updated:
                Resolved:

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