Help resolving PySide6 install issues on Rocky Linux 8

morimo

I am trying to get my PySide6 app running in a poetry environment on a Rocky Linux 8.10 server (accessed via ssh with X11 forwarding). According to the Qt for python docs, the only install step should be the pip install of PySide6. However, when running the application, I get the following error:

[...]
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found
qt.qpa.plugin: from 6.5.0, xcb-cursor0 or libxcb-cursor0 is needed to load the Qt xcb platform plugin.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

I can't find exact matches for any of xcb-cursor0 or libxcb-cursor0 in any of the Rocky/RHEL appstreams, and I tried installing everything I could find related to libxcb and xcb-utils, to no avail. Does anyone have suggestions for troubleshooting steps?

I've verified x11 forwarding works with xcalc, so that shouldn't be the source of the issue.

Any help would be greatly appreciated.

JonB

@morimo said in Help resolving PySide6 install issues on Rocky Linux 8:

xcb-cursor0 or libxcb-cursor0

https://stackoverflow.com/a/79066259/489865 ?

For anyone running into this on a Red Hat platform (RHEL in my case, also Fedora), the package name you'll need to install is xcb-util-cursor, and xcb-util-cursor-devel if you need to build with the library.

sudo dnf install xcb-util-cursor*

https://rockylinux.pkgs.org/9/rockylinux-appstream-x86_64/xcb-util-cursor-0.1.4-3.el9.i686.rpm.html
https://pkgs.org/download/libxcb-cursor.so.0()(64bit)
?

morimo

@JonB both are already installed.

JonB

@morimo
And you ran the suggested command?

I do not know whether the mention of the xcb cursor stuff in the message means that is really the issue or whether that comes whenever you get "Could not load the Qt platform plugin ..." for any reason. You could try export QT_DEBUG_PLUGINS=1 and then run your application and look at the diagnostic output to see whether that shows the reason clearly.

morimo

@JonB yes, I ran the command and dnf told me nothing to do.

I've also already set the debug env variable, the plugin loader finds linuxfb, libqvnc, libqvkkhrdisplay, libwayland-generic, libqminimal, libqeglfs, libqoffscreen, libqwayland-egl, libqxcb and libqminimalegl, all not in debug mode and all located in the .cache directory of the (active) poetry-managed venv. Sorry I can't post the actual output right now.

edit: is it possible that I need to force-reinstall the python package after installing the xcb dependencies?

JonB

@morimo
I think it should be loading the
Have a look through the thread https://forum.qt.io/post/758603 is in. I know nothing about RHEL. Can you run some equivalent of

ldd /home/user/.local/lib/python3.9/site-packages/PyQt6/Qt6/plugins/platforms/libqxcb.so

to find whether that still seems to be missing the cursor stuff per the error message?

Does anything in https://forums.rockylinux.org/t/about-xcb-util-cursor-availability-in-rocky-9/11761 help your situation?

morimo

ldd finds libxcb-cursor.so.0

I found more packages matching xcb-utils* (xcb-util-keysyms and xcb-util-wm) that I was missing.

After installing them, the application launches successfully.

when installing qt, should one generally just install xcb-util* and be done with it? I tried looking for documentation regarding platform plugin dependencies but didn't find anything that pointed me in the right direction.

Thank you for your time @JonB

JonB

@morimo said in Help resolving PySide6 install issues on Rocky Linux 8:

I found more packages matching xcb-utils* (xcb-util-keysyms and xcb-util-wm) that I was missing.

Sounds like sudo dnf install xcb-util-* would cover those as well as the xcb-util-cursor* ones from the original problem, and would be the suggested command for RHEL distro?

Those of us on Debian have to do something similar to pick up all the dotted around, possibly required packages too.

morimo

@JonB I don't know, I haven't been able to find any RHEL-specific docs for either the platform plugins or PySide6 itself. I would ideally only install what is actually required recommended but I couldn't find anything. Do you know somwhere where this information should/would be documented? I am homelabbing this because I will need to deploy a PySide6-based application in a secure envronment at some point in the future, so I am trying to make my installation/deployment steps as precise as I can.

jsulm

@morimo said in Help resolving PySide6 install issues on Rocky Linux 8:

Do you know somwhere where this information should/would be documented?

If your distribution provides PySide package you can check its dependencies.

morimo

@jsulm that's the thing, the distro doesn't provide the pyside package, after installing pyside from pypi finding the right dependencies involved more guesswork than I would've expected.

CristianMaureira

Hey @morimo
The dependencies will be Qt ones, that can be found here: https://doc.qt.io/qt-6/linux-requirements.html

morimo

@CristianMaureira Thank you, this is exactly what I was looking for. I have no idea how I didn't find it while googling.