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Details

Type: Bug
Resolution: Done
Priority: P1: Critical
Fix Version/s: 5.6.3, 5.9.0 Beta 2
Affects Version/s: 5.7.1, 5.8.0 RC
Component/s: QML: Declarative and Javascript Engine
Labels:
None
Environment:
macOS, windows

Description

Hi,

I am the author of a library called QuickFlux (www.github.com/benlau/quickflux ) . One of the function is a central message dispatcher for component-to-component communication that avoid to propagate a signal across uncertain amount of components .

I just got a bug report that it will crash on Qt 5.7.1/5.8RC in Mac/Windows (but works with Qt 5.7.0) . After traced out the code, I found that it happen when emitting a signal from C++ to QML with a QJSValue (isUndefined() == true) as argument.

For example, if we have such C++ class:

class Messenger : public QQuickItem
{
    Q_OBJECT
public:
    explicit Messenger(QQuickItem *parent = 0);

signals:
    void received(QJSValue message);

public slots:
    void send (QJSValue message = QJSValue());
};

The send() function is actually a proxy to forward the message

void Messenger::send(QJSValue message)
{
    emit received(message);
}

Then call send() in QML, the program will crash immediately

    Messenger {
        id: messenger
        onReceived: {
            console.log("hello");
        }
    }

    Component.onCompleted: {
        messenger.send(); // crash here
    }

Part of backtrace:

QML debugging is enabled. Only use this in a safe environment.
Error: signal 11:
0   qtbug                               0x000000010add87a2 _Z15handleBacktracei + 34
1   libsystem_platform.dylib            0x00007fffe45c2bba _sigtramp + 26
2   ???                                 0x000000011684e7ad 0x0 + 4672776109
3   QtQml                               0x000000010b33f39d _ZN15QQmlBoundSignal10setEnabledEb + 477
4   QtQml                               0x000000010b38106d _ZN12QQmlNotifier10emitNotifyEP20QQmlNotifierEndpointPPv + 781
5   QtCore                              0x000000010b850fc9 _ZN11QMetaObject8activateEP7QObjectiiPPv + 121
6   qtbug                               0x000000010addac4f _ZN9Messenger8receivedE8QJSValue + 63

The program and backtrace has been attached .

Thanks