Bug#1057620: doomsday: segfault in _XFlush() when Qt is using native Wayland

[Simon McVittie]

On Sat, 13 Dec 2025 at 20:06:57 +0200, Adrian Bunk wrote:
>On Mon, May 06, 2024 at 10:28:40AM +0100, Simon McVittie wrote:
>> If Doomsday needs to use both SDL and Qt for graphics/windowing, probably
>> the right way to implement this would be to let one of those libraries
>> choose its backend (X11 or Wayland) according to its usual heuristic,
>> and then call configuration functions that force the other library to
>> make the same choice.
>
>Is this the same bug as #1113738?

That depends how you define "same bug", but it's certainly closely-related.

>If yes, does the workaround from Bernhard I NMUed there look correct to
>you?

If doomsday's Qt code doesn't work when using Qt's Wayland backend, then 
the change you applied is necessary, but it might not be sufficient.

My concern about it would be what happens when Qt has been forced to use 
X11 (by the patch you applied or by QT_QPA_PLATFORM=xcb in the 
environment), but SDL is using native Wayland (via 
SDL_VIDEODRIVER=wayland or replacing libsdl2-2.0-0 with 
libsdl2-compat-shim). If doomsday code assumes that Qt windows on Unix 
are always X11 windows, it is probably also assuming that SDL on Unix 
will always be using the X11 backend.

If doomsday has that assumption, one way to make it be true would be to 
call

SDL_SetHintWithPriority(SDL_HINT_VIDEODRIVER, "x11", SDL_HINT_OVERRIDE);

near the beginning of main(), before SDL_Init(). Or it could set 
SDL_VIDEODRIVER=wayland as an environment variable, equivalent to what 
you've done for Qt, which would have a similar effect. 
All of this needs to happen before SDL initialization, and before the 
program has had a chance to create a second thread, because setting 
environment variables is not thread-safe and setting SDL hints is 
probably also not thread-safe.

After discussion with SDL upstream, I'm hoping to make src:sdl2-compat 
take over libsdl2-2.0-0 early in 2026, after which SDL 2 will default to 
native Wayland in many environments, like GTK and Qt already do. Games 
that assumed SDL will use X11 forever will need to adjust for that, 
similar to what happened for GTK and Qt.

Users and developers can get a preview of the future SDL implementation 
by replacing libsdl2-2.0-0 with libsdl2-compat-shim, and replacing 
libsdl2-dev (if installed) with libsdl2-compat-dev.

     smcv