Linux: QT_STYLE_OVERRIDE is used as the Qt Quick Controls style → Main.qml fails to load → SIGSEGV

[JSkier21]

Summary

On a GNOME/Wayland desktop that sets QT_STYLE_OVERRIDE (a QtWidgets QStyle name, here Adwaita-Dark from adwaita-qt6), LibrePods passes that name through as the Qt Quick Controls style. There is no QML module by that name, so qrc:/linux/Main.qml never loads, the QQmlApplicationEngine ends up with no root object, and the app then dereferences it and dies with SIGSEGV.

The crash looks intermittent from the outside: a tray instance started with --hide runs fine for hours and dies the moment the tray icon is clicked, while launching from the app grid dies within a few seconds.

Environment

• LibrePods 1.0.0rc1 (Arch/CachyOS package librepods 1.0.0rc1-1)
• Qt 6.11.2 (qt6-base 6.11.2-2, qt6-declarative 6.11.2-1.1), adwaita-qt6 1.4.2-2
• GNOME Shell 50.4, Wayland
• QT_QPA_PLATFORMTHEME=xdgdesktopportal, QT_STYLE_OVERRIDE=Adwaita-Dark

Steps to reproduce

1. export QT_STYLE_OVERRIDE=Adwaita-Dark (any QStyle name that is not also a Qt Quick Controls module)
2. Launch librepods
3. Watch the journal, and click the tray icon

Actual behaviour

Every launch logs (note: LibrePods logs to the journal, not stderr, so a plain 2>&1 capture shows nothing):

librepods[1109873]:  AirPodsTrayApp initialized
librepods[1109873]: QQmlApplicationEngine failed to load component
librepods[1109873]: qrc:/linux/Main.qml: module "Adwaita-Dark" is not installed

and then, on the next tray / D-Bus event, SIGSEGV:

Signal: 11 (SEGV) si_code: SEGV_MAPERR
Command Line: /usr/bin/librepods --hide

#0  0x000055f961ff820c n/a (librepods + 0x2820c)
#1  0x00007fe31cdeeff3 n/a (libQt6Core.so.6 + 0x1eeff3)
#2  0x00007fe31cdeeff3 n/a (libQt6Core.so.6 + 0x1eeff3)
#3  0x00007fe31efc8861 n/a (libQt6Widgets.so.6 + 0x1c8861)
#4  0x00007fe31cdef236 n/a (libQt6Core.so.6 + 0x1ef236)
#5  0x00007fe31ddbf84e n/a (libQt6Gui.so.6 + 0x7bf84e)
#6  0x00007fe31ddc0537 n/a (libQt6Gui.so.6 + 0x7c0537)
#7  0x00007fe31ed78ae9 n/a (libQt6DBus.so.6 + 0x45ae9)
...
#11 0x00007fe31cde6b96 QObject::event(QEvent*) (libQt6Core.so.6 + 0x1e6b96)
#13 0x00007fe31cd822b8 QCoreApplication::notifyInternal2(QObject*, QEvent*) (libQt6Core.so.6 + 0x1822b8)
#21 0x00007fe31cd8685a QCoreApplication::exec() (libQt6Core.so.6 + 0x18685a)
#22 0x000055f961fec0d1 main (librepods + 0x1c0d1)

(The distro binary is stripped, hence the numeric frames; the crash is in LibrePods' own code, reached from a D-Bus-delivered tray event.)

Expected behaviour

An unknown/invalid style should fall back to a valid Qt Quick Controls style, and a failed QML load should not be followed by a null dereference.

Diagnosis

QT_STYLE_OVERRIDE is a QtWidgets variable — its value is a QStyle plugin name (Adwaita-Dark, Breeze, kvantum, …), not a Qt Quick Controls module. The only Qt Quick Controls styles installed here are the ones shipped by Qt: Basic, Fusion, FluentWinUI3, Imagine, Material, Universal. Anything else fails the QML import.

Verified by varying only the environment:

Environment	Result
QT_STYLE_OVERRIDE=Adwaita-Dark (session default)	module "Adwaita-Dark" is not installed, later SIGSEGV
QT_STYLE_OVERRIDE unset	Main.qml loads, no crash
QT_STYLE_OVERRIDE=Fusion	Main.qml loads, no crash (Fusion happens to be both a QStyle and a QML module)
QT_STYLE_OVERRIDE=Adwaita-Dark + QT_QUICK_CONTROLS_STYLE=Basic	still fails — the explicit Qt Quick style is not honoured

That last row is the interesting one: because the style is applied via QQuickStyle::setStyle() at runtime, it takes precedence over QT_QUICK_CONTROLS_STYLE, so users cannot work around it with the documented variable.

Suggested fix

1. Don't derive the Qt Quick Controls style from QT_STYLE_OVERRIDE / the QStyle name. If a style is chosen programmatically, validate it first, e.g. only call QQuickStyle::setStyle(name) when QQuickStyle::availableStyles().contains(name), otherwise leave Qt's own resolution alone.
2. Harden the failure path regardless: connect to QQmlApplicationEngine::objectCreationFailed (or check engine.rootObjects().isEmpty()) and bail out with an error instead of using a null root object — that is what turns a cosmetic style problem into a segfault.

Workaround for other users

Launch with the variable cleared, e.g. in ~/.local/share/applications/me.kavishdevar.librepods.desktop and ~/.config/autostart/librepods.desktop:

Exec=env -u QT_STYLE_OVERRIDE /usr/bin/librepods