[PyKDE] Double free() problem in PyQt 3.15.1??

[Simon Edwards]

On Saturday 22 April 2006 21:17, Phil Thompson wrote:
> On Saturday 22 April 2006 4:04 pm, Simon Edwards wrote:
> > I'm confused as to what the newer exec_loop() code in QDialog was meant to
> > do.
> > ---------------------------------
> > %If (Qt_3_0_0 -)
> > 	int exec() /PyName=exec_loop, ReleaseGIL,
> > 		    PreHook=__pyQtPreEventLoopHook__,
> > 		    PostHook=__pyQtPostEventLoopHook__/;

> > It looks like it is meant to take ownership away from the parent dialog 
and
> > let Python handle destruction of the popup. But the dumpObjectTree() lines
> > in my test code say that the parent dialog always keeps the popup on it's
> > list of children objects.
> Correct.
> > When the parent dialog is destroyed so are the children objects. This is a
> > problem if Python thinks that it should destroy the popup too.
> 
> It's not a problem because QObject's dtor does the right thing in taking 
> itself out of any parent's list of children. In other words, it doesn't 
> matter (in the case of QObject and its subclasses) if both Python and C++ 
> think they should call the dtor.

I think I follow what you are saying. But that is only going to work correctly 
in the case of Python and Qt wanting to destroy the child dialog if Python 
calls the d'tor first. If the child dialog is destroyed by Qt when the parent 
dialog is destroyed, and THEN Python comes along later wanting to destroy the 
child dialog again, then it goes wrong.

valgrind output suggests that this is what is happening. The parent dialog is 
destroyed. The child dialog is destroyed at the same time. And then Python 
tries the child again. valgrind says that the illegal free/delete is coming 
from Python itself. (the first stack trace below) And is refering to memory 
that was free'd via Qt (the second stack trace).

---------------------------------------------------
Double QObject deletion detected.
==7997==
==7997== Invalid free() / delete / delete[]
==7997==    at 0x401D268: operator delete(void*) (vg_replace_malloc.c:246)
==7997==    by 0x4E22609: QObject::~QObject() (in /usr/lib/libqt-mt.so.3.3.6)
==7997==    by 0x608452A: (within /usr/lib/python2.4/site-packages/kdeui.so)
==7997==    by 0x402A927: (within /usr/lib/python2.4/site-packages/sip.so)
==7997==    by 0x8089DDF: (within /usr/bin/python2.4)
==7997==    by 0x80FA46D: (within /usr/bin/python2.4)
==7997==    by 0x80B6FF5: PyEval_EvalFrame (in /usr/bin/python2.4)
==7997==    by 0x80B76BE: PyEval_EvalCodeEx (in /usr/bin/python2.4)
==7997==    by 0x80B7904: PyEval_EvalCode (in /usr/bin/python2.4)
==7997==    by 0x80D946B: PyRun_FileExFlags (in /usr/bin/python2.4)
==7997==    by 0x80D970B: PyRun_SimpleFileExFlags (in /usr/bin/python2.4)
==7997==    by 0x8055B02: Py_Main (in /usr/bin/python2.4)
==7997==    by 0x4080EA1: __libc_start_main 
(in /lib/tls/i686/cmov/libc-2.3.6.so)


==7997==  Address 0x56CFFE0 is 0 bytes inside a block of size 2,044 free'd
==7997==    at 0x401D268: operator delete(void*) (vg_replace_malloc.c:246)
==7997==    by 0x6073D6B: sipKDialogBase::~sipKDialogBase() 
(in /usr/lib/python2.4/site-packages/kdeui.so)
==7997==    by 0x608452A: (within /usr/lib/python2.4/site-packages/kdeui.so)
==7997==    by 0x402A927: (within /usr/lib/python2.4/site-packages/sip.so)
==7997==    by 0x8089DDF: (within /usr/bin/python2.4)
==7997==    by 0x80FC987: (within /usr/bin/python2.4)
==7997==    by 0x4029030: (within /usr/lib/python2.4/site-packages/sip.so)
==7997==    by 0x607840C: sipKDialogBase::removeChild(QObject*) 
(in /usr/lib/python2.4/site-packages/kdeui.so)
==7997==    by 0x4E22AA8: QObject::~QObject() (in /usr/lib/libqt-mt.so.3.3.6)
==7997==    by 0x4E5D05C: QWidget::~QWidget() (in /usr/lib/libqt-mt.so.3.3.6)
==7997==    by 0x4FD7A07: QDialog::~QDialog() (in /usr/lib/libqt-mt.so.3.3.6)
==7997==    by 0x4916B05: sipQDialog::~sipQDialog() 
(in /usr/lib/python2.4/site-packages/qt.so)
==7997==    by 0x491DF56: (within /usr/lib/python2.4/site-packages/qt.so)
==7997==    by 0x402A927: (within /usr/lib/python2.4/site-packages/sip.so)
==7997==    by 0x8089DDF: (within /usr/bin/python2.4)
==7997==    by 0x807A2D4: (within /usr/bin/python2.4)
==7997==    by 0x402A8A4: (within /usr/lib/python2.4/site-packages/sip.so)
==7997==    by 0x402A8E0: (within /usr/lib/python2.4/site-packages/sip.so)
==7997==    by 0x8089DDF: (within /usr/bin/python2.4)
==7997==    by 0x80FA46D: (within /usr/bin/python2.4)
==7997==    by 0x80B6FF5: PyEval_EvalFrame (in /usr/bin/python2.4)
==7997==    by 0x80B76BE: PyEval_EvalCodeEx (in /usr/bin/python2.4)
==7997==    by 0x80B7904: PyEval_EvalCode (in /usr/bin/python2.4)
==7997==    by 0x80D946B: PyRun_FileExFlags (in /usr/bin/python2.4)
==7997==    by 0x80D970B: PyRun_SimpleFileExFlags (in /usr/bin/python2.4)
==7997==    by 0x8055B02: Py_Main (in /usr/bin/python2.4)
==7997==    by 0x4080EA1: __libc_start_main 
(in /lib/tls/i686/cmov/libc-2.3.6.so)
---------------------------------------------------


-- 
Simon Edwards             | KDE-NL, Guidance tools, Guarddog Firewall
simon at simonzone.com       | http://www.simonzone.com/software/
Nijmegen, The Netherlands | "ZooTV? You made the right choice."