Why do so many (but not all) Qt-based applications SIGABRT when $DISPLAY is unset (instead of invoking, as GTK3-based ones do)?

RokeJulianLockhart

Per bugzilla.redhat.com/show_bug.cgi?id=2378661#c13, I can crash basically all Qt-based applications I've tried (mostly KDE Frameworks-based ones, but certainly not solely those), but not some, like VLC, when I invoke them over SSH. VLC gracefully handles it, and invokes on the host, as all GTK3-based applications I've evaluated do. At the cited thread is a trace, if of use.

The reason that I ask is so that I can advise the KDE Frameworks developers.

Axel Spoerl

Qt applications attempt to detect the backend, which can be anything from XCB, offscreen, eglfs to wayland (and more).
This fails without DISPLAY being correctly set. It triggers an assertion in debug mode, as @SGaist wrote.

Instead of crashing immediately, VLC Player brings up its CLI when DISPLAY is unset.
However, it does crash when you even attempt to add a video to the playlist:

VLC media player 3.0.21 Vetinari
Command Line Interface initialized. Type `help' for help.
> add /home/axel/Videos/IMG_0954.MP4
> [00007fb26c0013c0] gl vout display error: parent window not available
[00007fb26c0013c0] xcb vout display error: window not available
[00007fb26c0013c0] fb vout display error: cannot get terminal mode (Inappropriate ioctl for device)
Segmentation fault (core dumped)
axel@axelsqt:~>

The reason that I ask is so that I can advise the KDE Frameworks developers.

All KDE developers I know are pretty proficient in QtGui. KDE even implements its own platform theme.
On behalf of which "KDE Frameworks developers" are you speaking?
I am confused.

SGaist

Hi,

It might be nitpicking, but it's not a crash per se, you are triggering an assert in a debug build which means some conditions are off.
Did you already check the bug report system ?

wrosecrans

@RokeJulianLockhart Appearing on a potentially unrelated console sounds like a pretty un-desirable behavior in a lot of circumstances. Exiting when DISPLAY isn't set sounds like the right choice to me. I understand that reasonable people could disagree, but it's certainly not the case the displaying on :0 is "obviously correct" behavior even if some people would prefer it. If you are annoyed by the behavior, you can always just set DISPLAY and get exactly he behavior you want.

RokeJulianLockhart

it's not a crash per se, you are triggering an assert in a debug build which means some conditions are off.

@SGaist, can you elaborate on "some conditions are off"? (Do you think that coredumpctl's capturing things it shouldn't, because Fedora's building the packages in a non-release mode? It seems to me that SIGABRTs usually indicate a fatal error, per stackoverflow.com/revisions/3413215/2.) I'm familiar with what "debug build" would mean in an AOSP context (isDebuggable), but not here.

Did you already check the bug report system ?

I tried some search terms, but didn't see much of use. I don't know whether I would recognise something relevant.

Exiting when DISPLAY isn't set sounds like the right choice to me.

@wrosecrans, yeah, I don't disagree. In that case, do you think it makes sense to crash? It might.

Axel Spoerl

Qt applications attempt to detect the backend, which can be anything from XCB, offscreen, eglfs to wayland (and more).
This fails without DISPLAY being correctly set. It triggers an assertion in debug mode, as @SGaist wrote.

Instead of crashing immediately, VLC Player brings up its CLI when DISPLAY is unset.
However, it does crash when you even attempt to add a video to the playlist:

VLC media player 3.0.21 Vetinari
Command Line Interface initialized. Type `help' for help.
> add /home/axel/Videos/IMG_0954.MP4
> [00007fb26c0013c0] gl vout display error: parent window not available
[00007fb26c0013c0] xcb vout display error: window not available
[00007fb26c0013c0] fb vout display error: cannot get terminal mode (Inappropriate ioctl for device)
Segmentation fault (core dumped)
axel@axelsqt:~>

The reason that I ask is so that I can advise the KDE Frameworks developers.

All KDE developers I know are pretty proficient in QtGui. KDE even implements its own platform theme.
On behalf of which "KDE Frameworks developers" are you speaking?
I am confused.

RokeJulianLockhart

@Axel-Spoerl, I only wanted to discuss it with them, but you've all answered everything I wanted to know. (I don't mean I was speaking on their behalf.) Thanks, all!