[PyQt] Segfault: QGraphicsView, cyclic references, and threads

[Erik Janssens]

I can confirm the issue in pyqt 4.8.3 / qt 4.7.2

it segfaults, but with the following assert statement :

.QObject::killTimers: timers cannot be stopped from another thread
ASSERT failure in QCoreApplication::sendEvent: "Cannot send events to
objects owned by a different thread. Current thread b6b08490. Receiver
'qt_scrollarea_hcontainer' (of type 'QWidget') was created in thread
94eeee8", file kernel/qcoreapplication.cpp, line 349


Phil,

does this mean that a QGraphicsScene is only garbage collected
when the QApplication has been garbage collected, thus creating
a memory leak ?

The PySide equivalent however crashes as well for me, but only on
exit of the app :

#0  0x00273c81 in Shiboken::Object::destroy(SbkObject*, void*) ()
from /home/tw55413/workspaces/cpd/trunk/linux2-debug/install/lib/libshiboken-python2.7.so.1.0
#1  0x03ebbac7 in QGraphicsViewWrapper::~QGraphicsViewWrapper() ()
from /home/tw55413/workspaces/cpd/trunk/linux2-debug/install/lib/python2.7/site-packages/PySide/QtGui.so
#2  0x005e98dc in PySide::destructionVisitor(SbkObject*, void*) ()
from /home/tw55413/workspaces/cpd/trunk/linux2-debug/install/lib/libpyside-python2.7.so.1.0
#3  0x002788f8 in Shiboken::BindingManager::visitAllPyObjects(void
(*)(SbkObject*, void*), void*) ()
from /home/tw55413/workspaces/cpd/trunk/linux2-debug/install/lib/libshiboken-python2.7.so.1.0
#4  0x005e9833 in PySide::destroyQCoreApplication() ()
from /home/tw55413/workspaces/cpd/trunk/linux2-debug/install/lib/libpyside-python2.7.so.1.0
#5  0x005e9c77 in PySide::runCleanupFunctions() ()
from /home/tw55413/workspaces/cpd/trunk/linux2-debug/install/lib/libpyside-python2.7.so.1.0
#6  0x00ac4cb5 in SbkQtCoreModule___moduleShutdown(_object*) ()
from /home/tw55413/workspaces/cpd/trunk/linux2-debug/install/lib/python2.7/site-packages/PySide/QtCore.so
#7  0x00708bf1 in PyCFunction_Call (func=0xb775ccac, arg=0xb780402c,
kw=0x9bca0b4) at Objects/methodobject.c:90
#8  0x0076d504 in ext_do_call (f=0x9b4f744, throwflag=0) at
Python/ceval.c:4322
#9  PyEval_EvalFrameEx (f=0x9b4f744, throwflag=0) at Python/ceval.c:2704
#10 0x0076f270 in PyEval_EvalCodeEx (co=0xb7751728, globals=0xb7752f0c,
locals=0x0, args=0xb7804038, argcount=0, kws=0x0, kwcount=0, defs=0x0,
defcount=0, closure=0x0) at Python/ceval.c:3252
#11 0x006eef17 in function_call (func=0xb7755d84, arg=0xb780402c,
kw=0x0) at Objects/funcobject.c:526
#12 0x006bf59c in PyObject_Call (func=0xb7755d84, arg=0xb780402c,
kw=0x0) at Objects/abstract.c:2529
#13 0x00767b94 in PyEval_CallObjectWithKeywords (func=0xb7755d84,
arg=0xb780402c, kw=0x0) at Python/ceval.c:3881
#14 0x0078dc5e in call_sys_exitfunc () at Python/pythonrun.c:1726
#15 Py_Finalize () at Python/pythonrun.c:412
#16 0x007a30e1 in Py_Main (argc=2, argv=0xbfbf6e84) at
Modules/main.c:624
#17 0x08048657 in main (argc=2, argv=0xbfbf6e84)
at ./Modules/python.c:23


On Mon, 2011-03-28 at 12:57 +0100, Phil Thompson wrote:
> On Sun, 27 Mar 2011 22:29:38 -0400, Luke Campagnola
> <lcampagn at email.unc.edu> wrote:
> > Greetings All-Knowing PyQters,
> > I have a strange bug that's causing segmentation faults in my program. I
> > have identified some of the key contributors to the bug and a
> workaround,
> > but I would love for someone to explain what causes the crash so I can
> > avoid
> > similar problems in the future.
> > 
> > Here are the things required to cause the crash:
> > 1) Repeatedly creating and deleting a GraphicsView and GraphicsItem
> > 2) The each GraphicsItem has a reference to the GraphicsView it is shown
> in
> > 3) Having a second thread running and actively working
> > 
> > Usually a segmentation fault will occur after roughly 10-1000
> create/delete
> > cycles. Taking away any of these components prevents the crash. The
> second
> > thread does not have to be doing anything specific and does not even
> have
> > to
> > be a QThread; it just has to be doing _something_. If the Item has a
> > weakref
> > to its View, there is no crash (and this is a perfectly acceptable
> > workaround).
> > 
> > I have attached a GDB backtrace and a short script demonstrating the
> bug.
> > When I run the script on my system (Ubuntu 10.04, PyQt 4.7.2), about
> half
> > of
> > the time it causes a segfault, and the rest of the time it just freezes.
> > I've also tested the script on WinXp with PyQt 4.8.3 and 4.5.4, with
> > similar
> > results. PySide does not have the bug, so I suspect it is not a Qt
> issue.
> > 
> > Any advice would be greatly appreciated!
> > 
> > Luke
> 
> PyQt keeps track of certain types of object that are known to cause
> problems if they are garbage collected before the QApplication -
> QGraphicsScene is one of these. It seems that QObjectCleanupHandler has
> problems in a threaded application.
> 
> Fixed (or at least worked around) in tonight's PyQt snapshot.
> 
> Phil
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