Hard crash when updating QTreeWidget items while handling changed event

VirtualFloat

I'm writing an application in python that used PySide6/Qt for a gui. Sometimes the application hard-crashes as result of user actions. This is hard to debug as the error messages do not contain any info as on where the error occured.

See for example the following:

The following example causes a exit code -1073740791 (0xC0000409) when one of the boxes is checked the second time:

from PySide6.QtCore import Qt
from PySide6.QtWidgets import QApplication, QWidget, QVBoxLayout, QTreeWidget, \
    QTreeWidgetItem

app = QApplication([])
widget = QWidget()
layout = QVBoxLayout(widget)
treewidget = QTreeWidget()
layout.addWidget(treewidget)

def add_items():

    for i in range(5):
        item1 = QTreeWidgetItem()
        item1.setText(0, f'Check to crash{i}')
        item1.setCheckState(0, Qt.CheckState.Unchecked)
        treewidget.invisibleRootItem().addChild(item1)
        treewidget.itemChanged.connect(on_item_changed)

def on_item_changed(item):
    print('clearing')
    treewidget.clear()
    add_items()

add_items()
widget.show()
app.exec()

Uinsg windows-11 with PySide6-Essentials 6.6.1 (pip)
also crashes with 6.6.2

How can I debug errors like this?

JonB

@VirtualFloat
You could try running under Python debugger to see if that tells you anything. You could try running the actual python process under gdb (or whatever your system/C++ debugger) and examining the stack trace when the crash occurs: it is true that will only give information about the python process itself, but sometimes there are clues there.

You are calling treewidget.itemChanged.connect(on_item_changed) multiple times, for each QTreeWidgetItem you add. This is not right. You have only one treewidget, you should only connect the signal once. Try changing that. Your slot clears the tree widget out of its items and adds new ones, but you call this in the middle of an existing item being changed. That may not be good, especially since you are calling it multiple times, so by the time it fires a second time the original item being changed has already been cleared (and new ones added) from the tree.

VirtualFloat

@JonB , thanks for the reply.
the script is buggy on purpose, but you are right about connecting the signal multiple times, that was not needed so I moved that out of the loop.

The add_items part is now as follows:

    for i in range(5):
        item1 = QTreeWidgetItem()
        item1.setText(0, f'Check to crash{i}')
        item1.setCheckState(0, Qt.CheckState.Unchecked)
        treewidget.invisibleRootItem().addChild(item1)
    treewidget.itemChanged.connect(on_item_changed)

which gives me the following stack trace:

# Child-SP          RetAddr               Call Site
00 (Inline Function) --------`--------     VCRUNTIME140!GetImageBaseFromCompleteObjectLocator+0x3 [D:\a\_work\1\s\src\vctools\crt\vcruntime\src\eh\rtti.cpp @ 44] 
01 00000039`b45e9560 00007ffb`bf6f6230     VCRUNTIME140!__RTtypeid+0x15 [D:\a\_work\1\s\src\vctools\crt\vcruntime\src\eh\rtti.cpp @ 152] 
02 00000039`b45e95b0 00007ffc`2c983202     QtWidgets+0x46230
03 00000039`b45e95f0 00007ffc`2c97c9f9     pyside6_abi3!PySide::PySideName::parameters+0xe2
04 00000039`b45e9650 00007ffc`2c985135     pyside6_abi3!PySide::SignalManager::callPythonMetaMethod+0x59
05 00000039`b45e96d0 00007ffb`db248fc1     pyside6_abi3!PySide::PySideName::qtEmit+0x995
06 00000039`b45e97d0 00007ffb`db27bed2     Qt6Core!QMetaObject::metacall+0x41
07 00000039`b45e9810 00007ffb`db27eae4     Qt6Core!QObject::qt_static_metacall+0x1832
08 00000039`b45e9950 00007ffb`b714e093     Qt6Core!QMetaObject::activate+0x84
09 00000039`b45e9980 00007ffb`db27beb6     Qt6Widgets!QTreeWidget::qt_static_metacall+0x123
0a 00000039`b45e9a20 00007ffb`db27eae4     Qt6Core!QObject::qt_static_metacall+0x1816
0b 00000039`b45e9b60 00007ffb`db457e6f     Qt6Core!QMetaObject::activate+0x84
0c 00000039`b45e9b90 00007ffb`b7154548     Qt6Core!QAbstractItemModel::dataChanged+0x3f
0d 00000039`b45e9bf0 00007ffb`b715bc14     Qt6Widgets!QTreeWidget::editItem+0x1d8
0e 00000039`b45e9ca0 00007ffb`bf72cd55     Qt6Widgets!QTreeWidgetItem::setData+0x744
0f 00000039`b45e9e50 00007ffb`b715b4bb     QtWidgets+0x7cd55
10 00000039`b45e9eb0 00007ffb`b70f60db     Qt6Widgets!QTreeWidget::setCurrentItem+0xeb
11 00000039`b45e9ef0 00007ffb`b70d4fcc     Qt6Widgets!QStyledItemDelegate::editorEvent+0x2cb
12 00000039`b45ea0c0 00007ffb`b70cb557     Qt6Widgets!QAbstractItemView::selectionCommand+0x51c
13 00000039`b45ea250 00007ffb`bf70708d     Qt6Widgets!QAbstractItemView::edit+0x107
14 00000039`b45ea300 00007ffb`b70d1208     QtWidgets+0x5708d
15 00000039`b45ea380 00007ffb`b7145822     Qt6Widgets!QAbstractItemView::mouseReleaseEvent+0x2b8
16 00000039`b45ea560 00007ffb`bf7202a9     Qt6Widgets!QTreeView::mouseReleaseEvent+0x92
17 00000039`b45ea5d0 00007ffb`b6e66bc4     QtWidgets+0x702a9
18 00000039`b45ea620 00007ffb`b6f02540     Qt6Widgets!QWidget::event+0x164
19 00000039`b45ea700 00007ffb`b70d8efd     Qt6Widgets!QFrame::event+0x30
1a 00000039`b45ea730 00007ffb`bf735f6a     Qt6Widgets!QAbstractItemView::viewportEvent+0x45d
1b 00000039`b45ea8c0 00007ffb`db23eb87     QtWidgets+0x85f6a
1c 00000039`b45ea930 00007ffb`b6e21f23     Qt6Core!QCoreApplicationPrivate::sendThroughObjectEventFilters+0xd7
1d 00000039`b45ea990 00007ffb`b6e1fef0     Qt6Widgets!QApplicationPrivate::notify_helper+0xf3
1e 00000039`b45ea9c0 00007ffb`bfac880c     Qt6Widgets!QApplication::notify+0x750
1f 00000039`b45eae70 00007ffb`db23b6ff     QtWidgets!PyInit_QtWidgets+0x390624
20 00000039`b45eaef0 00007ffb`b6e25eaf     Qt6Core!QCoreApplication::notifyInternal2+0x11f
21 00000039`b45eaf60 00007ffb`b6e89529     Qt6Widgets!QApplicationPrivate::sendMouseEvent+0x3ef
22 00000039`b45eb080 00007ffb`b6e86c02     Qt6Widgets!QWidgetRepaintManager::updateStaticContentsSize+0x34f9
23 00000039`b45eb590 00007ffb`b6e21f3e     Qt6Widgets!QWidgetRepaintManager::updateStaticContentsSize+0xbd2
24 00000039`b45eb6b0 00007ffb`b6e21061     Qt6Widgets!QApplicationPrivate::notify_helper+0x10e
25 00000039`b45eb6e0 00007ffb`bfac880c     Qt6Widgets!QApplication::notify+0x18c1
26 00000039`b45ebb90 00007ffb`db23b6ff     QtWidgets!PyInit_QtWidgets+0x390624
27 00000039`b45ebc10 00007ffb`a9ba3f67     Qt6Core!QCoreApplication::notifyInternal2+0x11f
28 00000039`b45ebc80 00007ffb`a9c00dea     Qt6Gui!QGuiApplicationPrivate::processMouseEvent+0x777
29 00000039`b45ec1b0 00007ffb`db3c2d70     Qt6Gui!QWindowSystemInterface::sendWindowSystemEvents+0xea
2a 00000039`b45ec1e0 00007ffb`a9e91989     Qt6Core!QEventDispatcherWin32::processEvents+0x90
2b 00000039`b45ef370 00007ffb`db242f8f     Qt6Gui!QWindowsGuiEventDispatcher::processEvents+0x19
2c 00000039`b45ef3a0 00007ffb`db238c8d     Qt6Core!QEventLoop::exec+0x19f
2d 00000039`b45ef440 00007ffb`bfa71bf0     Qt6Core!QCoreApplication::exec+0x15d
2e 00000039`b45ef4a0 00007ffb`dec59126     QtWidgets!PyInit_QtWidgets+0x339a08
2f 00000039`b45ef4d0 00007ffb`dec5a544     python311!PyObject_Vectorcall+0x606
30 00000039`b45ef5e0 00007ffb`dec6fa77     python311!PyEval_EvalFrameDefault+0x784
31 00000039`b45ef7f0 00007ffb`dec6f137     python311!PyMapping_Check+0x1eb
32 00000039`b45ef830 00007ffb`dec6d80a     python311!PyEval_EvalCode+0x97
33 00000039`b45ef8b0 00007ffb`dec6d786     python311!PyMapping_Items+0x11e
34 00000039`b45ef8e0 00007ffb`dedba17e     python311!PyMapping_Items+0x9a
35 00000039`b45ef920 00007ffb`dec233a5     python311!PyThread_tss_is_created+0x53ce
36 00000039`b45ef990 00007ffb`ded1a620     python311!PyRun_SimpleFileObject+0x11d
37 00000039`b45efa00 00007ffb`ded1aaef     python311!PyRun_AnyFileObject+0x54
38 00000039`b45efa30 00007ffb`ded1ab5f     python311!Py_MakePendingCalls+0x38f
39 00000039`b45efb00 00007ffb`ded1b964     python311!Py_MakePendingCalls+0x3ff
3a 00000039`b45efb30 00007ffb`ded1b7f5     python311!Py_RunMain+0x184
3b 00000039`b45efba0 00007ffb`dec660d9     python311!Py_RunMain+0x15
3c 00000039`b45efbd0 00007ff6`08131230     python311!Py_Main+0x25
3d 00000039`b45efc20 00007ffc`6532257d     python+0x1230
3e 00000039`b45efc60 00007ffc`6628aa48     KERNEL32!BaseThreadInitThunk+0x1d
3f 00000039`b45efc90 00000000`00000000     ntdll!RtlUserThreadStart+0x28

which indeed gives me some clues.

I think I was hoping for some magic debugging switch in PySide that I could trigger from python and would output but this is going to help me as well. Thanks!

JonB

@VirtualFloat
It's not ideal, but maybe it gives us a clue.

03 00000039`b45e95f0 00007ffc`2c97c9f9     pyside6_abi3!PySide::PySideName::parameters+0xe2
04 00000039`b45e9650 00007ffc`2c985135     pyside6_abi3!PySide::SignalManager::callPythonMetaMethod+0x59
05 00000039`b45e96d0 00007ffb`db248fc1     pyside6_abi3!PySide::PySideName::qtEmit+0x995

We are inside some kind of (Python) emit. It is calling some Python "meta method", doubtless for the signal-to-slot connection, and perhaps falling over setting up or passing parameters.

[I have reproduced your "crash" with your code in my PySide2. Examining now and will post whatever correction shortly....]

JonB

@VirtualFloat
Findings:

It does not crash every time. Sometimes I can click several times before it dumps. I suspect it might be to do with the speed at which I click, not sure.
It behaves the same (dumps immediately or after a few clicks) under PyQt5 as under PySide2.
Replacing the signal connection to treewidget.itemChanged.connect(lambda: print("Received!")) never crashes.
Commenting out both the treewidget.clear() & add_items() in the slot, so it only does print('clearing'), works.

You have connected add_items() slot to be called on signal itemChanged(). That means it is inside a call for a particular item being changed. In your slot you do 3 "worrying" things:

treewidget.clear() clears all the existing items. That includes the item in the midst of being changed.
You then add a number of new items in place of the old ones.
And because you put treewidget.itemChanged.connect() inside add_items() you create a connection again, which you do not want to do, and you are doing that while still inside the signal handling. In your code the connect() belongs in the main line, e.g. immediately before add_items() call there, so it can only execute once.

I can only say that you cannot afford to have treewidget.clear() and/or add_items() in the slot being executed for when an item has been changed.

VirtualFloat

@JonB , I really appreciate you help. Just want to stress again that I'm not looking for a solution to this particular situation. A solution is to create a single shot timer to handle the refresh of the items after the updates have been done.

JonB

@VirtualFloat
Yes, if for whatever reason that we do not know it is the fact that you clear/refill the items inside the slot for the itemChanged signal which causes the "crash" then moving it to a 0-delay QTimer::singleShot() would be recommended.