Re: Qt 5.13: Segfault on exit with Windows + QtWebEngine

Florian Bruhin <[email protected]> Wed, 17 Jul 2019 18:10:24 +0200
Newsgroups	gmane.comp.python.pyqt-pykde
Message-ID	<[email protected]>
On Wed, Jul 17, 2019 at 04:58:22PM +0100, Phil Thompson wrote:
> On 17/07/2019 12:48, Florian Bruhin wrote:
> > On Wed, Jul 17, 2019 at 11:54:10AM +0100, Phil Thompson wrote:
> > > On 13/07/2019 16:43, Florian Bruhin wrote:
> > > > With a quite minimal QtWebEngine example like this:
> > > >
> > > >     from PyQt5.QtWebEngineWidgets import QWebEngineProfile
> > > >     from PyQt5.QtWidgets import QApplication
> > > >     from PyQt5.QtCore import QTimer
> > > >
> > > >     qapp = QApplication([])
> > > >     p = QWebEngineProfile()
> > > >     QTimer.singleShot(500, qapp.quit)
> > > >     qapp.exec_()
> > > >
> > > > I get a segfault on exit with Python 3.7.4 and PyQt5/PyQtWebEngine
> > > > 5.13.0
> > > > on Windows 10.
> > > >
> > > > When I downgrade to PyQt5 5.12.3 and PyQtWebEngine 5.12.1 things work
> > > > fine.
> > > >
> > > > Unfortunately I haven't been able to get a reliable stacktrace. Here are
> > > > some
> > > > examples of the stacks I got:
> > > > https://paste.the-compiler.org/view/5774da86
> > > > I don't think they really make sense though.
> > > 
> > > I can't reproduce this - although I am using Python v3.7.0.
> > 
> > I can reproduce it with Python 3.7.0 as well (both 32 and 64-bit).
> > 
> > Note that I don't get a "Python.exe has stopped" window, but "echo $?"
> > shows
> > False in Powershell.
> > 
> > Similarly, running it from Python:
> > 
> >   subprocess.run([sys.executable, 'segv.py'], check=True)
> > 
> > Gives me:
> > 
> >   subprocess.CalledProcessError: Command '['...\\python.exe',
> > 'segv.py']' returned non-zero exit status 3221225477.
> > 
> > Where 3221225477 is 0xC0000005, i.e. an access violation.
> 
> I can reproduce that. However if I build the latest PyQt against Qt v5.12.4
> the problem goes away, which suggests a Qt problem. I can only suggest
> playing with things to try and get things destroyed in a different order,
> then I might be able to add a workaround.

Using qapp as a context manager seems to fix it. 
Putting everything into a run() function (without "global qapp") and calling
that also does.

In other words, if the QApplication is destroyed *before* whatever QtWebEngine
initializes internally it seems to not segfault.

That seems the exact opposite of what's recommended here though:
https://www.riverbankcomputing.com/static/Docs/PyQt5/gotchas.html#crashes-on-exit

Florian

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