[PyQt] Segfault with QRawFont

Sun Oct 28 17:42:35 GMT 2012

Blanket not calling dtors would be problematic, there's no way to be
sure what side effects that would have. One solution would be to
specialize the dealloc function for QRawFont objects to check if the
application is exiting and abort. 

Another might be to register a cleanup function using the atexit module
that calls deletes all QRawFont objects (at least their underlying C++
objects), though since atexit is
called from Py_Finalize() that would only work if Qt hasn't released
FreeType at that time.

Kovid.

On Sun, Oct 28, 2012 at 01:17:05PM +0000, Phil Thompson wrote:
> On Sun, 28 Oct 2012 15:38:59 +0530, Kovid Goyal <kovid at kovidgoyal.net>
> wrote:
> > The backtrace is below. Looks to me like the QRawFont object is being
> > released after Qt has already released the FreeType library, leading to
> > the segfault. The workaround is to ensure that all QRawFont objects are
> > deleted before Py_Finalize() is called. I dont know if there is more
> > elegant way to fix this in PyQt.
> 
> The best solution at the moment is to explicitly del problematic objects
> when the application terminates.
> 
> PyQt does have a mechanism for doing this automatically but only for
> QObjects and it has to be explicitly coded.
> 
> At the moment SIP disables the calling of any Python reimplementations of
> virtuals when an application exits. I could extend this to disable any
> calls to C++ dtors when Python objects get garbage collected. This would be
> optional and enabled by function implemented in the sip module. It would be
> disabled by default (to reflect the current behaviour) as I'm not sure if
> it might break current applications.
> 
> Thoughts?
> 
> Phil
> 
> > #0  0x00007ffff4061e3a in freeServerGlyphSet (id=39845946,
> this=<optimized
> > out>) at text/qfontengine_x11.cpp:1146
> > #1  QFontEngineX11FT::freeServerGlyphSet (this=0xa922f0, id=39845946) at
> > text/qfontengine_x11.cpp:1141
> > #2  0x00007ffff4068e89 in QFontEngineFT::freeGlyphSets (this=0xa922f0)
> at
> > text/qfontengine_ft.cpp:658
> > #3  0x00007ffff4061ccb in QFontEngineX11FT::~QFontEngineX11FT
> > (this=0xa922f0, __in_chrg=<optimized out>) at
> text/qfontengine_x11.cpp:1127
> > #4  0x00007ffff4061d09 in QFontEngineX11FT::~QFontEngineX11FT
> > (this=0xa922f0, __in_chrg=<optimized out>) at
> text/qfontengine_x11.cpp:1128
> > #5  0x00007ffff405f0ef in QRawFontPrivate::cleanUp (this=0xa5c290) at
> > text/qrawfont.cpp:699
> > #6  0x00007ffff405f194 in ~QRawFontPrivate (this=0xa5c290,
> > __in_chrg=<optimized out>) at text/qrawfont_p.h:94
> > #7  ~QExplicitlySharedDataPointer (this=<optimized out>,
> > __in_chrg=<optimized out>) at
> > ../../include/QtCore/../../src/corelib/tools/qshareddata.h:166
> > #8  ~QExplicitlySharedDataPointer (this=<optimized out>,
> > __in_chrg=<optimized out>) at text/qrawfont.cpp:174
> > #9  QRawFont::~QRawFont (this=<optimized out>, __in_chrg=<optimized
> out>)
> > at text/qrawfont.cpp:174
> > #10 0x00007ffff4b55f46 in release_QRawFont (sipCppV=0xa087f0) at
> > sipQtGuiQRawFont.cpp:936
> > #11 0x00007ffff5208e74 in forgetObject (sw=0x7fffec9bf980) at
> > siplib.c:10199
> > #12 0x00007ffff5208ea9 in sipSimpleWrapper_dealloc (self=0x7fffec9bf980)
> > at siplib.c:9449
> > #13 0x00007ffff7ab7b66 in subtype_dealloc (self=0x7fffec9bf980) at
> > Objects/typeobject.c:1014
> > #14 0x00007ffff7a986f7 in insertdict (mp=0x641000, key=0x7ffff7f7d788,
> > hash=13056039271, value=0x7ffff7da4b30 <_Py_NoneStruct>) at
> > Objects/dictobject.c:530
> > #15 0x00007ffff7a9abb4 in PyDict_SetItem (op=0x641000,
> key=0x7ffff7f7d788,
> > value=0x7ffff7da4b30 <_Py_NoneStruct>) at Objects/dictobject.c:775
> > #16 0x00007ffff7a9e5dc in _PyModule_Clear (m=<optimized out>) at
> > Objects/moduleobject.c:138
> > #17 0x00007ffff7b0d7bf in PyImport_Cleanup () at Python/import.c:445
> > #18 0x00007ffff7b1a035 in Py_Finalize () at Python/pythonrun.c:454
> > #19 0x00007ffff7b2aacc in Py_Main (argc=<optimized out>, argv=<optimized
> > out>) at Modules/main.c:668
> > #20 0x00007ffff747760d in __libc_start_main (main=0x400970 <main>,
> argc=3,
> > ubp_av=0x7fffffffda68, init=<optimized out>, fini=<optimized out>,
> > rtld_fini=<optimized out>, stack_end=0x7fffffffda58) at libc-start.c:226
> > #21 0x00000000004008a9 in _start ()
> > 
> > Kovid.
> > 
> > On Fri, Oct 19, 2012 at 07:38:18PM +0530, Kovid Goyal wrote:
> >> Hi Phil,
> >> 
> >> The following snippet causes a segfault:
> >> 
> >> python -c "from PyQt4.Qt import *; app = QApplication([]); f =
> >> QRawFont.fromFont(QFont('Arial')); print f.familyName();  print 1"
> >> 
> >> However, if I explicitly delete the QRawFont first, the segfault goes
> >> away:
> >> 
> >> python -c "from PyQt4.Qt import *; app = QApplication([]); f =
> >> QRawFont.fromFont(QFont('Arial')); print f.familyName(); del f;  print
> 1"
> >> 
> >> Seems to be something wrong in the object lifetime/ownership
> >> semantics for QRawFont.
> >> 
> >> This is on linux PyQt4 4.9.4, SIP 4.14, Qt 4.8.3
> >> 
> >> I am unable to generate a backtrace at the moment as I am travelling,
> >> let me know if you need one, and I'll try to generate it ASAP.
> >> 
> >> Kovid.
> >> 
> >> 
> >> -- 
> >> _____________________________________
> >> 
> >> Dr. Kovid Goyal 
> >> http://www.kovidgoyal.net
> >> http://calibre-ebook.com
> >> _____________________________________
> > 
> > 
> > 
> >> _______________________________________________
> >> PyQt mailing list    PyQt at riverbankcomputing.com
> >> http://www.riverbankcomputing.com/mailman/listinfo/pyqt
> >> 
> >> 
> 
> !DSPAM:3,508d305447621662420518!
> 
> 

-- 
_____________________________________

Dr. Kovid Goyal 
http://www.kovidgoyal.net
http://calibre-ebook.com
_____________________________________
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