[Qt] Qt deffered deletion (QObject_deleteLater())

Tue Jan 13 17:27:56 CET 2009

Hi all,

! PLEASE READ THIS UNTIL THE END (not debugging info but my text after that) !

I've found strange crashes inside qt libs, and of course found why it happens.
It rarely happens on small apps, but with big apps and heavy pressure (many 
forms opened & closed), our lcl app segfaults like this :

[Switching to Thread 1073923872 (LWP 26845)]
0x0636dc11 in QHash<QObject*, QHashDummyValue>::remove () 
from /usr/lib/libQtCore.so.4
(gdb) bt
#0  0x0636dc11 in QHash<QObject*, QHashDummyValue>::remove () 
from /usr/lib/libQtCore.so.4
#1  0x06369f61 in qt_removeObject () from /usr/lib/libQtCore.so.4
#2  0x0636ba9d in QObject::~QObject () from /usr/lib/libQtCore.so.4
#3  0x05b7d2a4 in QAction::~QAction () from /usr/lib/libQtGui.so.4
#4  0x06366753 in QObjectPrivate::deleteChildren () 
from /usr/lib/libQtCore.so.4
#5  0x05bd4a4e in QWidget::~QWidget () from /usr/lib/libQtGui.so.4
#6  0x05f490ec in QMenu::~QMenu () from /usr/lib/libQtGui.so.4
#7  0x063653a6 in qDeleteInEventHandler () from /usr/lib/libQtCore.so.4
#8  0x063679d6 in QObject::event () from /usr/lib/libQtCore.so.4
#9  0x05bcd2c8 in QWidget::event () from /usr/lib/libQtGui.so.4
#10 0x05f4440a in QMenu::event () from /usr/lib/libQtGui.so.4
#11 0x05b80f41 in QApplicationPrivate::notify_helper () 
from /usr/lib/libQtGui.so.4
#12 0x05b810ae in QApplication::notify () from /usr/lib/libQtGui.so.4
#13 0x063578f0 in QCoreApplication::notifyInternal () 
from /usr/lib/libQtCore.so.4
#14 0x0635ad4b in QCoreApplicationPrivate::sendPostedEvents () 
from /usr/lib/libQtCore.so.4
#15 0x0635ade5 in QCoreApplication::sendPostedEvents () 
from /usr/lib/libQtCore.so.4
#16 0x0635b036 in QCoreApplication::processEvents () 
from /usr/lib/libQtCore.so.4
#17 0x4b7393ee in QCoreApplication_processEvents () 
from /usr/lib/libqt4intf.so

OR LIKE THIS

[Thread debugging using libthread_db enabled]
[New Thread 1073923872 (LWP 3553)]
*** glibc detected *** corrupted double-linked list: 0x09bc4f00 ***

Program received signal SIGABRT, Aborted.
[Switching to Thread 1073923872 (LWP 3553)]
0x40000424 in __kernel_vsyscall ()
(gdb) bt
#0  0x40000424 in __kernel_vsyscall ()
#1  0x00a32955 in raise () from /lib/tls/libc.so.6
#2  0x00a34319 in abort () from /lib/tls/libc.so.6
#3  0x00a65f9a in __libc_message () from /lib/tls/libc.so.6
#4  0x00a6bf00 in malloc_consolidate () from /lib/tls/libc.so.6
#5  0x00a6ce03 in _int_malloc () from /lib/tls/libc.so.6
#6  0x00a6e135 in _int_realloc () from /lib/tls/libc.so.6
#7  0x00a6f2a6 in realloc () from /lib/tls/libc.so.6
#8  0x0627a874 in qRealloc () from /usr/lib/libQtCore.so.4
#9  0x062a27e1 in QListData::realloc () from /usr/lib/libQtCore.so.4
#10 0x062a28d9 in QListData::append () from /usr/lib/libQtCore.so.4
#11 0x05b8a4b3 in QList<QWidget*>::append () from /usr/lib/libQtGui.so.4
#12 0x05b8ad39 in QSet<QWidget*>::toList () from /usr/lib/libQtGui.so.4
#13 0x05b84546 in QApplication::allWidgets () from /usr/lib/libQtGui.so.4
#14 0x05b8459d in QApplication::topLevelWidgets () from /usr/lib/libQtGui.so.4
#15 0x05b88269 in QApplicationPrivate::leaveModal () 
from /usr/lib/libQtGui.so.4
#16 0x05c017c6 in QWidget::destroy () from /usr/lib/libQtGui.so.4
#17 0x05bd434e in QWidget::~QWidget () from /usr/lib/libQtGui.so.4
#18 0x063653a6 in qDeleteInEventHandler () from /usr/lib/libQtCore.so.4
#19 0x063679d6 in QObject::event () from /usr/lib/libQtCore.so.4
#20 0x05bcd2c8 in QWidget::event () from /usr/lib/libQtGui.so.4
#21 0x05b80f41 in QApplicationPrivate::notify_helper () 
from /usr/lib/libQtGui.so.4
#22 0x05b810ae in QApplication::notify () from /usr/lib/libQtGui.so.4
#23 0x063578f0 in QCoreApplication::notifyInternal () 
from /usr/lib/libQtCore.so.4
#24 0x0635ad4b in QCoreApplicationPrivate::sendPostedEvents () 
from /usr/lib/libQtCore.so.4
#25 0x0635ade5 in QCoreApplication::sendPostedEvents () 
from /usr/lib/libQtCore.so.4
#26 0x0635b036 in QCoreApplication::processEvents () 
from /usr/lib/libQtCore.so.4
#27 0x4b7393ee in QCoreApplication_processEvents () 
from /usr/lib/libqt4intf.so

OR LIKE THIS

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 1073923872 (LWP 18964)]
0x0636b7db in QObject::~QObject () from /usr/lib/libQtCore.so.4
(gdb) bt
#0  0x0636b7db in QObject::~QObject () from /usr/lib/libQtCore.so.4
#1  0x05b7d2a4 in QAction::~QAction () from /usr/lib/libQtGui.so.4
#2  0x06366753 in QObjectPrivate::deleteChildren () 
from /usr/lib/libQtCore.so.4
#3  0x05bd4a4e in QWidget::~QWidget () from /usr/lib/libQtGui.so.4
#4  0x05f490ec in QMenu::~QMenu () from /usr/lib/libQtGui.so.4
#5  0x063653a6 in qDeleteInEventHandler () from /usr/lib/libQtCore.so.4
#6  0x063679d6 in QObject::event () from /usr/lib/libQtCore.so.4
#7  0x05bcd2c8 in QWidget::event () from /usr/lib/libQtGui.so.4
#8  0x05f4440a in QMenu::event () from /usr/lib/libQtGui.so.4
#9  0x05b80f41 in QApplicationPrivate::notify_helper () 
from /usr/lib/libQtGui.so.4
#10 0x05b810ae in QApplication::notify () from /usr/lib/libQtGui.so.4
#11 0x063578f0 in QCoreApplication::notifyInternal () 
from /usr/lib/libQtCore.so.4
#12 0x0635ad4b in QCoreApplicationPrivate::sendPostedEvents () 
from /usr/lib/libQtCore.so.4
#13 0x0635ade5 in QCoreApplication::sendPostedEvents () 
from /usr/lib/libQtCore.so.4
#14 0x0635b036 in QCoreApplication::processEvents () 
from /usr/lib/libQtCore.so.4
#15 0x4b7393ee in QCoreApplication_processEvents () 
from /usr/lib/libqt4intf.so

END DEBUGGING INFO

So I decided to find out what's up.
1.Problem exists on Linux, Win32 and Mac.
2.Tested with our ERP app , ported from K3/D7
3.There's no mem leaks in our app (according to heaptrc).
4.Problem raises after few hours of work, when many forms opened & closed.
   (see debug logs above).
5.I'm talking about qt-4.4 and qt-4.5 !

So after googling (about similar error gdb traces in qt4) and some small tests 
I decide to remove QEvent::DefferedDelete from our 
QApplication_processMessages(), and of course changed some code inside 
qtwidgets & qtobjects (there's no more QObject_deleteLater() per default 
inside our qtlcl !)
BINGO ! Seem that our app is working faster (seem like Laz IDE work faster 
too - no more wait for deffered deletion) AND NO MORE STRANGE SEGFAULTS ! :).
QEvent::DefferedDelete produced mess & unnecessary breaks inside 
processMessages() (trolltech guys said that DefferedDelete should not be used 
anymore as of Qt-4.4 (currently, we are using workaround inside 
TQtWidgetSet.AppProcessMessages() )!.

Patch is ready - less or more (still some problem pending with destroying of 
popup menus)...working on that one at the moment.

So my question would be (since I don't want to break qt-4.3 users), should I 
ifdef it like USE_QT_44 OR QT_NO_DEFFERED_DELETION ?

Question for Den: Why we don't have QObject_destroyed_event() inside bindings, 
there's public signal QObject::destroyed(QObject* obj = 0) according to qt 
docs ?

thanks,

  zeljko