Crash in the qt_viewer quiting

[ogoffart]

Reproduced on Linux C++ build, opening the fancy demo

==355214== Invalid read of size 8
==355214==    at 0x5EBE919: std::default_delete<QWidget>::operator()(QWidget*) const (unique_ptr.h:85)
==355214==    by 0x5EBDCB9: std::unique_ptr<QWidget, std::default_delete<QWidget> >::~unique_ptr() (unique_ptr.h:361)
==355214==    by 0x5EBC983: __cpp_destructor_12665555240224438248 (qt_window.rs:1235)
==355214==    by 0x5E64DEA: <i_slint_backend_qt::qt_window::QWidgetPtr as core::ops::drop::Drop>::drop (lib.rs:366)
==355214==    by 0x5E63ABA: core::ptr::drop_in_place<i_slint_backend_qt::qt_window::QWidgetPtr> (mod.rs:188)
==355214==    by 0x5E63A73: core::ptr::drop_in_place<i_slint_backend_qt::qt_window::QtWindow> (mod.rs:188)
==355214==    by 0x5F2EB3F: core::ptr::drop_in_place<dyn i_slint_core::window::PlatformWindow> (mod.rs:188)
==355214==    by 0x5F33E25: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop (rc.rs:1511)
==355214==    by 0x5F2F30A: core::ptr::drop_in_place<alloc::rc::Rc<dyn i_slint_core::window::PlatformWindow>> (mod.rs:188)
==355214==    by 0x5F2CBC2: core::ptr::drop_in_place<core::option::Option<alloc::rc::Rc<dyn i_slint_core::window::PlatformWindow>>> (mod.rs:188)
==355214==    by 0x5F2D12A: core::ptr::drop_in_place<core::cell::UnsafeCell<core::option::Option<alloc::rc::Rc<dyn i_slint_core::window::PlatformWindow>>>> (mod.rs:188)
==355214==    by 0x5F2CD7A: core::ptr::drop_in_place<once_cell::unsync::OnceCell<alloc::rc::Rc<dyn i_slint_core::window::PlatformWindow>>> (mod.rs:188)
==355214==  Address 0xb2508e0 is 0 bytes inside a block of size 48 free'd
==355214==    at 0x4848B6F: operator delete(void*, unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==355214==    by 0x5EBE822: SlintWidget::~SlintWidget() (qt_window.rs:74)
==355214==    by 0x76B7589: QObjectPrivate::deleteChildren() (in /usr/lib/libQt6Core.so.6.3.0)
==355214==    by 0x6807790: QWidget::~QWidget() (in /usr/lib/libQt6Widgets.so.6.3.0)
==355214==    by 0x68A09FD: QFrame::~QFrame() (in /usr/lib/libQt6Widgets.so.6.3.0)
==355214==    by 0x76B7589: QObjectPrivate::deleteChildren() (in /usr/lib/libQt6Core.so.6.3.0)
==355214==    by 0x6807790: QWidget::~QWidget() (in /usr/lib/libQt6Widgets.so.6.3.0)
==355214==    by 0x10CDD6: main (in /home/rust/sixtyfps/cppbuild_2/examples/qt_viewer/qt_viewer)
==355214==  Block was alloc'd at
==355214==    at 0x4846013: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==355214==    by 0x5EBDB3B: std::_MakeUniq<SlintWidget>::__single_object std::make_unique<SlintWidget>() (unique_ptr.h:962)
==355214==    by 0x5EBB4D5: __cpp_closure_18363416283276074912_impl() (qt_window.rs:1250)
==355214==    by 0x5EBB54B: __cpp_closure_18363416283276074912 (qt_window.rs:1255)
==355214==    by 0x5E8DD42: i_slint_backend_qt::qt_window::QtWindow::new (qt_window.rs:1248)
==355214==    by 0x5E7C92D: <i_slint_backend_qt::Backend as i_slint_core::backend::Backend>::create_window::{{closure}} (lib.rs:147)
==355214==    by 0x5E79033: i_slint_core::window::Window::new (window.rs:210)
==355214==    by 0x5E944DE: <i_slint_backend_qt::Backend as i_slint_core::backend::Backend>::create_window (lib.rs:147)
==355214==    by 0x4C1C3AA: slint_interpreter::dynamic_component::ComponentDescription::create (dynamic_component.rs:377)
==355214==    by 0x4C9AB2E: slint_interpreter::api::ComponentDefinition::create (api.rs:566)
==355214==    by 0x4B959C7: slint_interpreter_component_instance_create (ffi.rs:573)
==355214==    by 0x10EB26: slint::interpreter::ComponentDefinition::create() const (in /home/rust/sixtyfps/cppbuild_2/examples/qt_viewer/qt_viewer)

[ogoffart]

The qt_viewer reparent the QWidget to anothe QWidget window which is on the stack. and closing the qt_viewer dostroy that widget (and hence the slint QWidget (SlintWidget) before the window is closed.) We probbly should do some cleanup in the ~SlintWidget destructor to avoid that.

[ogoffart]

So the crash is worked around in the qt_viewer by commit 590ef33

The question is: is that something we should support? (destroying deleting QWidget* before destroying the Slint handle?)

[hunger]

If this causes problems in the little apps we have right now, this will surely bite us or our users later when they build bigger things.

[ogoffart]

Let's just say it's undefined behavior