[PyQt] Object lifetime related segfault on linux

Kovid Goyal kovid at kovidgoyal.net
Thu Oct 9 12:28:39 BST 2014

I've recently started getting a seemingly random crash on program
termination on linux x64. It can be reproduced with this script:

from PyQt5.Qt import QApplication, QMainWindow

app = QApplication([])

w = QMainWindow()

Running it with python2 causes a segfault about once in 20 executions.
Unfortunately, there is stack corruption so the backtrace is not very
useful. And the crash does not occur running under gdb.

#0  0x00007f0ab58d5069 in ?? ()
#1  0x00007f0ab3bb0f10 in ?? ()
#2  0x00000000018c8920 in ?? ()
#3  0x00007f0ab3bb0f50 in ?? ()
#4  0x00000000018c8c80 in ?? ()
#5  0x00007f0ab3bb1700 in ?? ()
#6  0x00007f0acd8b03be in QThreadPrivate::start (arg=0x18c8920) at thread/qthread_unix.cpp:345
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

I can however avoid the crash by adding

del w
del app

to the end of the script. In other words explicitly deleting the
QMainWindow before the QApplication prevents the crash. As I understand
it this should not be necessary as qapplication.sip should be
transferring ownership of top level widgets to C++ in the QApplication
destructor, and indeed, sip.ispyowned() indicates that ownership is
transferred in the QApplication destructor, so there is something else
going on here. It might just be a timing issue that the explicit
deletion avoids.

This is with Qt 5.3.2 and PyQt 5.3.2. In the above back trace there is a
reference to the start method of QThreadPrivate. Looking through the Qt
source code I can find no reason for a thread being started during
application shutdown. The xcb QPA plugin does start a thread to talk to
the X server, but that is during startup.

Obviously, for this simple script the fix is to delete the QMainWindow
before QApplication. However doing that in a more complex application is
not always possible (the best I can think of is to save the main window as an
attribute of the application object). That should ensure that main
window is either deleted before the application or not at all because of
a reference cycle. But I would like to have a proper fix for this, since
if it is a timing issue then this fix may not be robust.



Dr. Kovid Goyal 
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