Why is QGraphicsObject getting deleted in QCoreApplication event loop?

patrickkidd

I have a QGraphicsObject that is getting deleted when I let the QApplication's event loop run for a little bit. I can reproduce this in a unit test. This is a PyQt5 app. The QGraphicsObject is not being removed from the QGraphicsScene, but a delete event is obviously getting posted for deferred deletion. There is about a ~500ms delay between calling QMessageBox::information (to let the event loop run a little) and the actual deletion. Here is C++ backtrace on macOS:

I am emitting a signal from my PathItemBase (my QGraphicsObject subclass) so that I can set a python breakpoint on the deletion. The QObject::destroyed has not been emitted yet.

C++ dtor:

PathItemBase::~PathItemBase() {
    print_stack_trace();
    qDebug() << "~PathItemBase() objectName: " << this->objectName() << " className: " << this->metaObject()->className() << " parentItem: " << this->parentItem() << " parent: " << this->parent();
    emit deleting(this);
}

Python deleting slot:

        def _deleting(o):
            nonlocal self

            _log.info(
                f">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> ABOUT TO BE DESTROYED "
                f"objectName: {o.objectName()}, className: {o.metaObject().className()}"
            )
            x = 1

        self.deleting.connect(_deleting)

Which prints:

C++ Stack trace:
0   _pkdiagram.cpython-310-darwin.so    0x00000001090b6516 _Z17print_stack_tracev + 54
1   _pkdiagram.cpython-310-darwin.so    0x00000001090bb0ef _ZN12PathItemBaseD2Ev + 47
2   _pkdiagram.cpython-310-darwin.so    0x00000001090af23d _ZN15sipPathItemBaseD0Ev + 61
3   QtCore                              0x000000028a93088e _ZN7QObject5eventEP6QEvent + 158
4   _pkdiagram.cpython-310-darwin.so    0x00000001090af7d1 _ZN15sipPathItemBase5eventEP6QEvent + 113
5   QtWidgets                           0x000000029671e9ea _ZN19QApplicationPrivate13notify_helperEP7QObjectP6QEvent + 266
6   QtWidgets                           0x000000029671fe11 _ZN12QApplication6notifyEP7QObjectP6QEvent + 497
7   QtWidgets.abi3.so                   0x0000000296e89e06 _ZN15sipQApplication6notifyEP7QObjectP6QEvent + 230
8   QtCore                              0x000000028a905a34 _ZN16QCoreApplication15notifyInternal2EP7QObjectP6QEvent + 212
9   QtCore                              0x000000028a906d79 _ZN23QCoreApplicationPrivate16sendPostedEventsEP7QObjectiP11QThreadData + 809
10  QtCore                              0x000000028a969719 _ZN20QEventDispatcherUNIX13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE + 73
11  libqoffscreen.dylib                 0x000000029983f74e _ZN23QUnixEventDispatcherQPA13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE + 14
12  QtCore                              0x000000028a901acf _ZN10QEventLoop4execE6QFlagsINS_17ProcessEventsFlagEE + 431
13  QtWidgets                           0x000000029691ee9e _ZN7QDialog4execEv + 526
14  QtWidgets                           0x000000029695ca72 _ZL17showNewMessageBoxP7QWidgetN11QMessageBox4IconERK7QStringS5_6QFlagsINS1_14StandardButtonEES7_ + 354
15  QtWidgets.abi3.so                   0x0000000296ef12f7 _ZL28meth_QMessageBox_informationP7_objectS0_S0_ + 231
16  python3.10                          0x0000000106ca6045 cfunction_call + 69
17  python3.10                          0x0000000106c5f218 _PyObject_MakeTpCall + 376
18  python3.10                          0x0000000106d5228e call_function + 974
19  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
20  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
21  python3.10                          0x0000000106c5f5eb PyVectorcall_Call + 155
22  python3.10                          0x0000000106d4ee65 _PyEval_EvalFrameDefault + 30021
23  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
24  python3.10                          0x0000000106d4ee65 _PyEval_EvalFrameDefault + 30021
25  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
26  python3.10                          0x0000000106d5218a call_function + 714
27  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
28  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
29  python3.10                          0x0000000106c61aed method_vectorcall + 205
30  python3.10                          0x0000000106d5218a call_function + 714
31  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
32  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
33  python3.10                          0x0000000106c5efee _PyObject_FastCallDictTstate + 174
34  python3.10                          0x0000000106c5fa9b _PyObject_Call_Prepend + 139
35  python3.10                          0x0000000106cc5e5c slot_tp_call + 204
36  python3.10                          0x0000000106c5f218 _PyObject_MakeTpCall + 376
37  python3.10                          0x0000000106d5228e call_function + 974
38  python3.10                          0x0000000106d4ea29 _PyEval_EvalFrameDefault + 28937
39  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
40  python3.10                          0x0000000106d5218a call_function + 714
41  python3.10                          0x0000000106d4e94c _PyEval_EvalFrameDefault + 28716
42  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
43  python3.10                          0x0000000106d4ee65 _PyEval_EvalFrameDefault + 30021
44  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
45  python3.10                          0x0000000106d5218a call_function + 714
46  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
47  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
48  python3.10                          0x0000000106c61aed method_vectorcall + 205
49  python3.10                          0x0000000106d5218a call_function + 714
50  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
51  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
52  python3.10                          0x0000000106c5efee _PyObject_FastCallDictTstate + 174
53  python3.10                          0x0000000106c5fa9b _PyObject_Call_Prepend + 139
54  python3.10                          0x0000000106cc5e5c slot_tp_call + 204
55  python3.10                          0x0000000106c5f73d _PyObject_Call + 141
56  python3.10                          0x0000000106d4ee65 _PyEval_EvalFrameDefault + 30021
57  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
58  python3.10                          0x0000000106d5218a call_function + 714
59  python3.10                          0x0000000106d47f5b _PyEval_EvalFrameDefault + 1595
60  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
61  python3.10                          0x0000000106c61aed method_vectorcall + 205
62  python3.10                          0x0000000106d5218a call_function + 714
63  python3.10                          0x0000000106d4ea29 _PyEval_EvalFrameDefault + 28937
64  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
65  python3.10                          0x0000000106d4ee65 _PyEval_EvalFrameDefault + 30021
66  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
67  python3.10                          0x0000000106d5218a call_function + 714
68  python3.10                          0x0000000106d47f5b _PyEval_EvalFrameDefault + 1595
69  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
70  python3.10                          0x0000000106d5218a call_function + 714
71  python3.10                          0x0000000106d4ea29 _PyEval_EvalFrameDefault + 28937
72  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
73  python3.10                          0x0000000106d4ee65 _PyEval_EvalFrameDefault + 30021
74  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
75  python3.10                          0x0000000106d5218a call_function + 714
76  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
77  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
78  python3.10                          0x0000000106c61aed method_vectorcall + 205
79  python3.10                          0x0000000106d5218a call_function + 714
80  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
81  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
82  python3.10                          0x0000000106c5efee _PyObject_FastCallDictTstate + 174
83  python3.10                          0x0000000106c5fa9b _PyObject_Call_Prepend + 139
84  python3.10                          0x0000000106cc5e5c slot_tp_call + 204
85  python3.10                          0x0000000106c5f218 _PyObject_MakeTpCall + 376
86  python3.10                          0x0000000106d5228e call_function + 974
87  python3.10                          0x0000000106d4ea29 _PyEval_EvalFrameDefault + 28937
88  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
89  python3.10                          0x0000000106d4ee65 _PyEval_EvalFrameDefault + 30021
90  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
91  python3.10                          0x0000000106d5218a call_function + 714
92  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
93  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
94  python3.10                          0x0000000106c61aed method_vectorcall + 205
95  python3.10                          0x0000000106d5218a call_function + 714
96  python3.10                          0x0000000106d4e978 _PyEval_EvalFrameDefault + 28760
97  python3.10                          0x0000000106d477f2 _PyEval_Vector + 146
98  python3.10                          0x0000000106c5efee _PyObject_FastCallDictTstate + 174
99  python3.10                          0x0000000106c5fa9b _PyObject_Call_Prepend + 139
2024-08-11 11:11:52,145 INFO :0                         ~PathItemBase() objectName:  "Marriage"  className:  PathItemBase  parentItem:  QGraphicsItem(0)  parent:  QObject(0x0)
2024-08-11 11:11:52,146 INFO pathitem.py:104            >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> ABOUT TO BE DESTROYED objectName: Marriage, className: PathItemBase

Both QObject::parent() and QGraphicsItem::parentItem() return 0x0, so it shouldn't be a QObject parent-child hierarchy deletion. What else could post this deletion event?

ChrisW67

@patrickkidd said in Why is QGraphicsObject getting deleted in QCoreApplication event loop?:

I can reproduce this in a unit test.

Then you should post the unit test so we can see exactly what you are doing.

patrickkidd

OK, but it's a fairly mature and complex app and there is a lot going on here. I bet this problem is best taken hypothetically, like when could a QGraphicsItem possibly get a deferred delete call if it isn't getting explicitly removed from the scene or anything.

But here is the unit/integration test:

https://github.com/patrickkidd/familydiagram/blob/cf161d739c01747c52756dba9f2b64ed68f80c51/tests/test_addanythingdialog_scripts.py#L292

ChrisW67

You are right, the test is not much use. It is a not a small, self-contained demonstration of the behaviour.

The QGraphicsObject will be destroyed when the QGraphicsScene containing it is destroyed. This is using the QGraphicsItem hierarchy, not the QObject hierarchy.

In a Python environment, using QGraphicsScene::removeItem() will probably result in removed items being garbage collected at a later point in time if you are not holding other references. QGraphicsScene::clear() will delete all items in the scene.

patrickkidd

Yeah, I still have references in python, and in fact I first discovered this problem because PyQt was complaining about the underlying C++ object being deleted from a still valid python reference.

The QGraphicsScene isn't being destroyed so far as I can tell. The QGraphicsScene.destroyed signal never gets emitted and removeItem is never called. I've been using Qt for 25 years and this is such a strange bug. How in the world could the QGraphicsItem be getting deleted, and not only that but in a deferred way??

I can debug the test with lldb and then set a breakpoint in QObject::deleteLater() (I am using the pip-installed PyQt5 bins on macOS and they accurately reflect the C++ object attrs) but I can't figure out how to set a lldb breakpoint condition for when the classname is "PathItemBase", or at least not QQml*.

patrickkidd

I wonder, could this possibly have something to do with the QGraphicsObject getting deleted being garbage collected from a reference in the Qml JS engine?

patrickkidd

I need more time testing, but it looks like setting object ownership to C++ when a Object is returned from a pyqt slot like this:

    @pyqtSlot(int, result=QObject)
    def item(self, id):
        ret = self.scene.findById(id)
        QQmlEngine.setObjectOwnership(ret, QQmlEngine.CppOwnership)
        return ret

fixes it. I can't figure out where to find this in the docs, but a fresh ChatGPT 4 thread says that default ownership by objects returned to calls from QMl/javascript is to javascript and not python/C++.

Man, that was a tough and strange one. I would really love to find a place in the Qt JS engine where the deleteLater is called.