Crash (SIGSEGV) via raw exit() during waveform-type change, inside fcitx5-qt QWindow teardown (Wayland)

Bug Description

Mixxx segfaults a few seconds after changing the waveform display type in Preferences (e.g. to "RGB (GLSL)"), while running under Wayland with the fcitx5 Qt input-context plugin active (QT_IM_MODULE=fcitx).

dmesg confirms a NULL-pointer dereference:

mixxx[544960]: segfault at 0 ip 00007f1895d71721 sp 00007ffdc6443380 error 4 in libQt6Core.so.6.11.2[371721,7f1895a94000+430000]

The crashing thread's backtrace (from coredumpctl info, symbol names resolved for Qt/libc; Mixxx's own frames are n/a because the Arch package ships stripped and debuginfod.archlinux.org was unreachable when I tried to resolve them):

#0  QThreadStorageData::get()                                  (libQt6Core.so.6)
#1  QOpenGLContext::currentContext()                            (libQt6Gui.so.6)
#2  QWindow::~QWindow()                                         (libQt6Gui.so.6)
#3  ?? (libfcitx5platforminputcontextplugin.so + 0x28815)
#4  QObject::event(QEvent*)                                     (libQt6Core.so.6)
#5  QApplicationPrivate::notify_helper(QObject*, QEvent*)       (libQt6Widgets.so.6)
#6  mixxx + 0x43a436   (custom QApplication::notify override)
#7  QCoreApplication::notifyInternal2(QObject*, QEvent*)        (libQt6Core.so.6)
#8  QCoreApplicationPrivate::sendPostedEvents(...)               (libQt6Core.so.6)
#9  libQt6Core.so.6 + 0x371e1a
#10 libQt6Core.so.6 + 0x371f74
#11 libc.so.6 + 0x40fbf   (__run_exit_handlers-ish)
#12 exit()                                                       (libc.so.6)
#13 mixxx + 0x4448c8      (Mixxx code calling ::exit())
#14 libQt6Core.so.6 + 0x1eeff3
#15 QOpenGLWindow::resizeEvent(QResizeEvent*)                    (libQt6OpenGL.so.6)
#16 QWindow::event(QEvent*)                                      (libQt6Gui.so.6)
#17 mixxx + 0x9f65cc
#18 QApplicationPrivate::notify_helper(...)
#19 mixxx + 0x43a436
#20 QCoreApplication::notifyInternal2(...)
#21 QGuiApplicationPrivate::processGeometryChangeEvent(...)
#22 QWindowSystemInterface::handleGeometryChange<SynchronousDelivery>(...)
#23 QtWaylandClient::QWaylandWindow::setGeometry(QRect const&)
#24 QtWaylandClient::QWaylandWindow::setVisible(bool)
#25 QWindowContainer::event(QEvent*)
...
#29 QWidgetPrivate::show_helper()
#30 QWidgetPrivate::setVisible(bool)
#31 mixxx + 0x5f99bf
...
#38 mixxx + 0x285579
#39 mixxx + 0x2050dc
#40 __libc_start_main
#42 mixxx + 0x2793a5

What I think is happening

Reading the stack bottom-up: while a widget is being shown/resized (rebuilding the OpenGL waveform windows after the display-type change), some Mixxx code path (mixxx+0x4448c8) calls raw exit() instead of going through a normal QApplication::quit() / return-from-main() shutdown. exit()'s atexit/static-destruction machinery then flushes one more round of posted Qt events (frames #8–#12) before the process actually terminates. One of those leftover events gets delivered into the fcitx5 Qt platform input-context plugin, which destroys a QWindow. That destructor calls QOpenGLContext::currentContext(), which crashes inside QThreadStorageData::get() with a null this/pointer — consistent with the underlying QThreadStorage already having been torn down by that point in the exit sequence.

This looks like an ordering hazard specific to calling exit() directly from inside GUI code rather than returning normally from the Qt event loop — the fcitx5 plugin's window teardown wasn't expecting to run in a partially-destructed Qt runtime. I'm filing a parallel report against fcitx5-qt for the plugin side of this (linked below), since the actual faulting instruction is in their teardown path, but the trigger is Mixxx calling exit() mid-event-handling.

Steps to Reproduce

1. Run Mixxx on Wayland (Hyprland) with fcitx5 set as the Qt input method (QT_IM_MODULE=fcitx, fcitx5 running).
2. Open Preferences → Waveforms, change the waveform display type (e.g. to "RGB (GLSL)").
3. Mixxx segfaults a few seconds later.

I have only reproduced this once so far; will update if I get a more reliable repro.

Expected Behavior

Mixxx applies the preference change (or prompts for/performs a clean restart) without crashing.

Actual Behavior

SIGSEGV, QThreadStorageData::get(), address 0.

Version

2.5.6-7 (Arch Linux extra package)

OS

Arch Linux (Omarchy), Hyprland 0.56.2, Wayland session, kernel 7.2.3-arch1-3

Additional environment

• Qt: qt6-base 6.11.2-3, qt6-wayland 6.11.2-1
• fcitx5 5.1.22-1, fcitx5-qt 5.1.15-1
• Mesa 26.2.2 (Intel UHD Graphics 620 / WHL GT2)
• QT_IM_MODULE=fcitx, XMODIFIERS=@im=fcitx

Related: fcitx/fcitx5-qt#TBD (companion report, plugin-side teardown crash)

Filed by Claude Sonnet 5 via Claude Code.