Segmentation fault when exiting when linked against Qt 6.9.1

[Christian Ehrlicher]

@Axel-Spoerl : dockwidgets... 😛

[l3u_]

The problem is that the only custom thing I do is the closeEvent override – everything else happens inside the default Qt code … and even if I remove the custom closeEvent, the segfault is the same (which is not really surprising, when I stepped through everything that happens, the function returns, and the crash happens way later, while we're long deep inside Qt …).

@Christian-Ehrlicher : May I suppose from what you wrote that this is not the first time strange things happen when dock widgets are messed with?! ;-)

The only thing I can definitely say is that some change from 6.9.0 to 6.9.1 causes this, because it never happens with 6.9.0, and it always happens with 6.9.1 …

[l3u_]

Luckily, my desktop is still quite decent, so I could git bisect out the commit that introduced this.

If I checkout

commit 19c4db4201e8933fbbbf951809ceff30e99b4458
QDockWidget: don't access a QMainWindow that's under destruction

everything is fine, no crash. One commit later, at

commit ab6f1ad77852a427ae73172ca11dacf876a0cbf7
QMainWindowLayout: Fix leaking of unused tab bars

I get the segfault.

I would call it a regression …

[Axel Spoerl]

Ooops, I am guilty for the guilty commit!

What's obviously crashing is a QDockWidget. Can you tell us more about those?
How are they added? how is QDockWidget::setWidget() used? In which state are they, when the crash occurs (docked on the main window / Floating / Tabbed)?

[l3u_]

@Axel-Spoerl Hi, nice to meet the right guy at once ;-)

I meanwhile filed a bug report about this: https://bugreports.qt.io/browse/QTBUG-137755

For all dock widgets I first instatiate a widget, like this (all are simple QWidgets – with quite complex stuff inside, but still):

m_registrationPage = new RegistrationPage(this, m_sharedObjects);
...

then, I create the dock, format it, set the widget and hide it, like this:

m_playersDock = new QDockWidget(tr("Anmeldung"), this);
m_playersDock->setWidget(m_registrationPage);
formatDockWidget(m_playersDock, QStringLiteral("playersDock"));
addDockWidget(Qt::TopDockWidgetArea, m_playersDock);
connect(m_registrationPage, &RegistrationPage::raiseMe, m_playersDock, &QDockWidget::raise);
m_playersDock->hide();

Where the formatting is:

void MainWindow::formatDockWidget(QDockWidget *dock, const QString &id)
{
    dock->setObjectName(id);
    dock->setContextMenuPolicy(Qt::PreventContextMenu);
    dock->setFeatures(QDockWidget::DockWidgetMovable | QDockWidget::DockWidgetFloatable);
}

When I start the program, all docks are hidden. If I close it again in this state, I get no segfault.

Opening a database makes them show up, by default attached to the main window and tabbed. When closing the program then, the segfault happens.

Interestingly, if I close my database again, the docks are hidden again – but still, I get a segfault on closing.

[l3u_]

Btw. I didn't change this code since Qt 5.6 back in 2017 …

[Axel Spoerl]

Hm. Looks like I went too far and we have to revert or follow up on this patch.
Can you do me a favor: Report it as a bug, assign it to me? I'll fix it this week.

[l3u_]

I already filed one here: https://bugreports.qt.io/browse/QTBUG-137755 – I can't assign it to you though (or better said, I don't know how to do that …)

Somebody already commented on it, and mentioned these two PRs:
https://codereview.qt-project.org/c/qt/qtbase/+/637198
https://codereview.qt-project.org/c/qt/qtbase/+/636652

[SGaist]

@l3u_ done for you :-)

[l3u_]

Thanks :-)

[l3u_]

Just to also leave this here: I meanwhile could strip it down to a minimal example producing the crash, cf. the sources attached to https://bugreports.qt.io/browse/QTBUG-137755

Produces the crash reliably when linked against Qt 6.9.1. You have to run the program twice: The first time, it exits normally. The second time, when the window geometry and state are restored, it segfaults on exiting.

No delete, no deleteLater, nothing special at all …

[Axel Spoerl]

Great reproducer!
Someone decades ago felt like storing unused tab bars for later re-usage in QMainWindowLayout.
Those were soft-leaked and taken care off, when QApplication got destroyed.
Reading the state back from settings causes the original tab bar (created in the C++ part) to become unused, but not removed from its QMainWindow parent. That ultimately lead to a double delete and the crash.
Let's see, if my thorough reviewers let me get away without writing an autotest ;-)

[l3u_]

Wow, that was fast :-) Thanks for the immediate fix! Nice to see my stuff helped here!

That was quite deep inside Qt apparently … where will this land? Qt 6.10.0? Or will it be backported?

I tried to cherry-pick it to a 6.9.1 checkout to test it, but it seems that's no trivial task …

[jsulm]

@l3u_ You can see in the bug report:
Fix Version/s: 6.10.0 Beta2, 6.11.0 FF

[l3u_]

Ah okay. So no 6.9 backport? I just wondered, because in the respective "Change ID" https://codereview.qt-project.org/c/qt/qtbase/+/653727, it's listed: "Fixes: QTBUG-137755 Pick-to: 6.10 6.9"

[jsulm]

@l3u_ Yes, it looks like it was cherry picked to 6.9 branch, so may end up in some 6.9.x version :-)

[Axel Spoerl]

The fix will land in 6.9.2, which we plan to release on August 14th.

If you want to cherry pick and compile locally, please make sure to pick the following 5 commits in the right order:

1. https://codereview.qt-project.org/c/qt/qtbase/+/653890
2. https://codereview.qt-project.org/c/qt/qtbase/+/653891
3. https://codereview.qt-project.org/c/qt/qtbase/+/653892
4. https://codereview.qt-project.org/c/qt/qtbase/+/653893
5. https://codereview.qt-project.org/c/qt/qtbase/+/653894

[l3u_]

Nice :-)

I just tried it locally, cherry-picking the exact comments you linked. Worked without a collision on a v6.9.1 checkout.

And – you may already have thought that – I can confirm the segfault is gone now, also for my quite complex use-case :-)

[Axel Spoerl]

Great that you are using dock widgets! As you may have suspected: QDockWidget is slightly closer to my heart than other creatures in the widget zoo ;-)

[l3u_]

First, it was just a QTabBar interface. But over the years, it got more and more complex, and at some point, I learned about QDockWidgets – and I loved them at first sight ;-) The same as the tab bar if you want – but way more if you need it. The possibility to arrange them as you like and need is really nice. This is actually a powerful tool, with a great user experience. QDockWidgets really rock! They do, from time to time, cause some headache (this is actually not the first bug I filed about them ;-) – but they are really cool.