Application Hangs when Main Window Closes

Brian_H

Hi,

I have a very simple dimension converting application I developed with Desktop Qt 6.8.1: Three text boxes and three labels; typing a number in one of the boxes displays that number converted to other units (e.g. inches to millimeters).

On a Debian 12 system, when I close the window, the process doesn't exit. Debugging shows that the QApplication destructor gets stuck waiting for a QXcbEventQueue to be destroyed, itself stuck in a call to QThread::wait. This problem doesn't seem to occur on Windows.

The QXcbEventQueue thread itself is stuck inside a call to xcb_wait_for_event.

The QDBusConnection thread also seems to be stuck inside a call to g_main_context_iteration.

My main.cpp:

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char * argv[]) {
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

and my MainWindow constructor and destructor:

MainWindow::MainWindow(QWidget * parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->txtIn, SIGNAL(textEdited(QString)), this, SLOT(txtInChanged(QString)));
    connect(ui->txtMils, SIGNAL(textEdited(QString)), this, SLOT(txtMilsChanged(QString)));
    connect(ui->txtMm, SIGNAL(textEdited(QString)), this, SLOT(txtMmChanged(QString)));
}

MainWindow::~MainWindow() 
{
    delete ui;
}

The slots are simple methods that convert the text to a number, do the math, and put the result in the other two fields.

This smells like a deadlock, but I don't know what might be causing it.

I happen to be running this through an X2Go remote X session, which could be part of the problem; at application startup, the message qt.qpa.xcb: X server does not support XInput 2 is emitted. I haven't had a chance to check for this issue at the physical console (display :0).

Are there any known issues with Qt 6.8 and remote X sessions? What else might be causing this?

JonB

@Brian_H
I don't know anything about your contexts, but why do you need any widgets and connections at all? Remove them and just test with QWidget w;, works right or same issue? Then you know if it's just on exiting any Qt app.

SGaist

Hi,

Beside the suggestion of @JonB, are you using the Qt version provided with your distribution ?

Kent-Dorfman

Now, for the 20,000$ question...Are you using pure X11, or running the app under xwayland?