Fail to run in Linux

[chavezheras]

I followed the installation instructions but could not get it to run in Linux. Here's the error:

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: xcb, linuxfb, minimal, minimalegl, wayland-egl, wayland, vnc, vkkhrdisplay, offscreen.

fish: Job 1, './tilia-0.5.15-linux' terminated by signal SIGABRT (Abort)

My Linux install:
x86_64 Ubuntu 24.04.3 LTS
Kernel version: 6.14.0-33-generic

[FelipeDefensor]

Hello Daniel! Thanks for the issue. I am still settling in Linz, so it may be a week or two until I can get my hands on a Linux to try and reproduce. In the meantime: - Could you try installing the `libxcb-cursor0` library and see if that fixes it? - Could you run the executable with the `QT_DEBUG_PLUGINS` environment variable set to 1 and send the resulting output?

[FelipeDefensor]

@azfoo could you maybe take a quick look?

[FelipeDefensor]

Looking at the Qt source code, it seems like installing libxcb-cursor0 or xcb-cursor0 could fix the problem.

if (name == QStringLiteral("xcb") && QVersionNumber::compare(QLibraryInfo::version(), QVersionNumber(6, 5, 0)) >= 0) {
          qCWarning(lcQpaPluginLoading).nospace().noquote()
                  << "From 6.5.0, xcb-cursor0 or libxcb-cursor0 is needed to load the Qt xcb platform plugin.";
      }
      qCInfo(lcQpaPluginLoading).nospace().noquote()
              << "Could not load the Qt platform plugin \"" << name << "\" in \""
              << QDir::toNativeSeparators(platformPluginPath) << "\" even though it was found.";

[FelipeDefensor]

I was able to reproduce this in a Docker container, but installing those libraries there did not fix it. I'm not sure if it would work in an installation with a graphical interface, I was not able to reproduce it outside of Docker yet.

Will continue debugging on Monday.

[FelipeDefensor]

#370 seems to be related to this.

[FelipeDefensor]

This may be relevant: https://stackoverflow.com/a/55485079/15862653

[azfoo]

After playing around with the binaries in Docker this week, it seems we probably will need to have a few more packages bundled in to have things up and running. Getting Qt to display in a container has been a nightmare, but with archlinux, you will need at least python-pyqt6 + wayland-docs with QT_QPA_PLATFORM=wayland set. (Haven’t yet figured out what the X equivalent is, but something similar to python-pyqt6 across the other Linux platforms will cover the necessary missing libraries. According to the docs, Qt runs through all of its screen options automatically and picks the most appropriate, but I’m not sure why that is the error that everyone is getting…)

X11docker was a potential solution to get the app containerised (which might be the easier solution to the Linux distribution issues) but set up has been more difficult than I anticipated.

[azfoo]

ps. we might be missing many steps regarding including packages and setting flags in our build process for Linux. https://github.com/musescore/MuseScore/blob/master/buildscripts/ci/linux/setup.sh

[FelipeDefensor]

ps. we might be missing many steps regarding including packages and setting flags in our build process for Linux. https://github.com/musescore/MuseScore/blob/master/buildscripts/ci/linux/setup.sh

Does comparing the build process for a C++ Qt application tells us much? I never deployed anything in C++, so I wouldn't know.

[FelipeDefensor]

I wasn't able to reproduce in Ubuntu 24.04.3 LTS in a graphical environment. TiLiA runs fine on a fresh installation.

[FelipeDefensor]

After playing around with the binaries in Docker this week, it seems we probably will need to have a few more packages bundled in to have things up and running. Getting Qt to display in a container has been a nightmare, but with archlinux, you will need at least python-pyqt6 + wayland-docs with QT_QPA_PLATFORM=wayland set. (Haven’t yet figured out what the X equivalent is, but something similar to python-pyqt6 across the other Linux platforms will cover the necessary missing libraries. According to the docs, Qt runs through all of its screen options automatically and picks the most appropriate, but I’m not sure why that is the error that everyone is getting…)

X11docker was a potential solution to get the app containerised (which might be the easier solution to the Linux distribution issues) but set up has been more difficult than I anticipated.

Looking at the source code, the empty string in qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found. draws my attention, as that should be platformPluginPath. I wouldn't expect the platform plugin, whatever that might be, to be on the working directory.

Not having access to the underlying C++ makes this very frustrating. Can we make PyQt use a Qt we built from source with debug code? Or could we somehow step through the C++ code?

Edit: it seems possible with PySide - https://doc.qt.io/qtforpython-6/tutorials/debugging/mixed_debugging.html

[FelipeDefensor]

I could find very little about deploying PyQt application to Linux. The PySide docs, on the other hand, have a section on deployment. Here's what they say about Linux:

As of March 2021, Qt 6 is not supported yet. PyInstaller is unable to properly deploy Qt; the Qt plugins are not copied. With that, using --onefile is not possible.
It is possible to use PyInstaller for the non --onefile case though by manually copying the Qt plugins, QML imports and translations into the dist directory after running PyInstaller.

That is not entirely true for PyQt, as it seems to work perfectly in Windows and Mac with --onefile. However, PySide (which means the Qt company itself) provides the pyside6-deploy for cross-platform deployment. Switching from PyQt to PySide shouldn't be hard. Should we give it a go?

[azfoo]

Maybe this first?

[FelipeDefensor]

Maybe this first?

I did find it before but the homepage leading to a 500 Error is a red flag. The docs are also offline and the GitHub page is not very inspiring. I would bet it is actively maintained, as Riverbank Computing seems to be invested in keeping the PyQt ecosystem healthy, but I think the extra effort to migrate the codebase to PySide is worth it to get a more robust solution and better docs. Being able to debug the C++ code like I mentioned above is also a big plus.

[azfoo]

(Double checked and they don't support pyqt6 anyway, oops)