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Details

Type: Bug
Resolution: Fixed
Priority: P1: Critical
Fix Version/s: 6.8.1, 6.8.2, 6.8, 6.9.0 FF
Affects Version/s: 6.8.0
Component/s: QML: Declarative and Javascript Engine
Labels:
None

Commits:
d148d8d78 (dev), fbdbed919 (6.8), b34fca349 (6.8.1)

Description

https://bugs.kde.org/show_bug.cgi?id=494804 appears to be a regression in Qt 6.8.0:
SDDM creates a QQuickView for each screen (with the default constructor, i.e. every instance has its own QQmlEngine) and adds some context properties. One of them is a simple QObject derived proxy object to call back into SDDM:

void GreeterApp::addViewForScreen(QScreen *screen) {
    QQuickView *view = new QQuickView()
    ...
    // m_proxy was created earlier on startup, it's QObject derived without parent.
    view->rootContext()->setContextProperty(QStringLiteral("sddm"), m_proxy);
    ...
    // show
    qDebug() << "Adding view for" << screen->name() << screen->geometry();
    view->showFullScreen();
}

However, that context property is suddenly not accessible anymore on all screens except one, resulting in error messages like

Okt 19 17:49:30  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Main.qml:221:25: Unable to assign [undefined] to bool
Okt 19 17:49:30  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Main.qml:215:25: Unable to assign [undefined] to bool
Okt 19 17:49:30  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Main.qml:209:25: Unable to assign [undefined] to bool
...
Okt 19 17:49:32  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Login.qml:134:17: Unable to assign QString to QObject*
Okt 19 17:49:32  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Main.qml:348:25: Unable to assign [undefined] to bool
Okt 19 17:49:32  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Main.qml:354:25: Unable to assign [undefined] to bool
Okt 19 17:49:32  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Main.qml:360:25: Unable to assign [undefined] to bool
...
Okt 19 17:49:50  sddm-greeter-qt6[850]: file:///usr/share/sddm/themes/breeze/Main.qml:339: TypeError: Property 'login' of object Loader is not a function

Apparently sddm is suddenly a QString or Loader on some views?

The issue disappears when using QV4_MM_AGGRESSIVE_GC=1 QV4_GC_TIMELIMIT=0, which points to the incremental GC being the issue.

I added some logging to qv4mm.cpp and see that callDestroyObject does indeed try to destroy the context property:

        // we need to call destroyObject on qobjectwrappers now, so that they can emit the destroyed
        // signal before we start sweeping the heap
        if (QObjectWrapper *qobjectWrapper = m->as<QObjectWrapper>()) {
            if(qobjectWrapper->object())
                DEBUG << "Destroying weak value object" << qobjectWrapper->object() << QQmlEngine::objectOwnership(qobjectWrapper->object());
            qobjectWrapper->destroyObject(/*lastSweep =*/false);
        }

Okt 31 16:27:51 fvogt-thinkpad sddm-greeter-qt6[110315]: MM: Destroying weak value object SDDM::GreeterProxy(0x5578658fa320) 0
Okt 31 16:27:51 fvogt-thinkpad sddm-greeter-qt6[110315]: MM: Destroying weak value object SDDM::GreeterProxy(0x5578658fa320) 0
Okt 31 16:27:51 fvogt-thinkpad sddm-greeter-qt6[110315]: MM: Destroying weak value object QQuickMouseEvent(0x557866873408) 0

It's destroyed twice, which matches that two of the three QQuickView s are broken.

Using a single QQmlEngine shared between all QQuickView s also avoids the issue.

My guess is that either the incremental GC is confused by QObjects being used in multiple engines. Maybe the incremental runs affect each other (one engine clearing the marks set by another? That should break more violently though) or the incremental GC has issues with tainted objects?

Unfortunately I'm unable to reproduce the issue outside of SDDM, so I can't provide some minimal reproducer
I can try out patches easily though.