2.5-alpha won't start on Uuntu 22.04 due to missing Qt platform plugins

[ronso0]

Bug Description

reported here https://mixxx.discourse.group/t/ubuntu-22-04-update-mixxx-not-working/28743

Similiar reports exist for other apps and it was fixed by installing some xcb packages.
Do we need to update dependencies?

Version

2.5-alpha

OS

Ubuntu 22.04

[naught101]

Related? #12284

[naught101]

A work around for me was removing the mixxx-nightlies repos, adding https://launchpad.net/~mixxx/+archive/ubuntu/mixxxbetas, and downgrading to 2.4.0-beta-whatever.

[ronso0]

So, since

• similiar issues where reported for other Qt apps much earlier
• 2.5-alpha worked until recently for @stefan-franz @naught101 and other alpha testers

I assume it's not an Qt update / Qt packaging issue but indeed related to #12284

Everyone affected: please test the CI build from #12545
Instruction for testing and backing up your data are here https://github.com/mixxxdj/mixxx/wiki/Testing

[daschuer]

This is the warning from the original report:

azz@Dazz-VPCEH3N6E:~$ mixxx
Using preferences ScaleFactor 1
Warning: Ignoring WAYLAND_DISPLAY on Gnome. Use QT_QPA_PLATFORM=wayland to run on Wayland anyway.
qt.qpa.plugin: Could not find the Qt platform plugin “xcb” in “”
qt.qpa.plugin: Could not find the Qt platform plugin “wayland” in “”

[daschuer]

Does starting via

QT_DEBUG_PLUGINS=1 mixxx

Reveals an issue?

[ronso0]

I justed with 2.5alpha193gb14c624a98-1jammy in VMs

• Ubuntu Studio 22.04: worked once, then I installed VirtualBox Guest Additions to enable bi-directional clipboard and it failed exactly like plain Ubuntu
• Ubuntu 22.04: fails with the mentioned message
  starting via QT_DEBUG_PLUGINS=1 mixxx:

QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib/x86_64-linux-gnu/qt6/plugins/platforms" ...
QFactoryLoader::QFactoryLoader() checking directory path "/usr/bin/platforms" ...
qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Aborted (core dumped)

FWIW the dependencies installed in Ubuntu / Ubuntu Studio are identical

libb2-1                 0.98.1-1.1
libqt6core5compat6      6.2.4-1
libqt6core6             6.2.4+dfsg-2ubuntu1
libqt6dbus6             6.2.4+dfsg-2ubuntu1
libqt6gui6              6.2.4+dfsg-2ubuntu1
libqt6keychain1         0.13.2-5
libqt6network6          6.2.4+dfsg-2ubuntu1
libqt6opengl6           6.2.4+dfsg-2ubuntu1
libqt6qml6              6.2.4+dfsg-3ubuntu1
libqt6qmlmodels6        6.2.4+dfsg-3ubuntu1
libqt6quick6            6.2.4+dfsg-3ubuntu1
libqt6sql6              6.2.4+dfsg-2ubuntu1
libqt6sql6-sqlite       6.2.4+dfsg-2ubuntu1
libqt6svg6              6.2.4-1ubuntu1
libqt6svgwidgets6       6.2.4-1ubuntu1
libqt6widgets6          6.2.4+dfsg-2ubuntu1
libqt6xml6              6.2.4+dfsg-2ubuntu1
qt6-gtk-platformtheme   6.2.4+dfsg-2ubuntu1

[daschuer]

Normally 'QT_DEBUG_PLUGINS=1 mixxx' should produce extra log info like this:

...
Got keys from plugin meta data ("ico", "cur")
QFactoryLoader::QFactoryLoader() looking at ...
Found metadata in lib ...
metadata=
{
    "IID": "org.qt-project.Qt.QImageIOHandlerFactoryInterface",
    "MetaData": {
        "Keys": [
            "jpg",
            "jpeg"
        ],
        "MimeTypes": [
            "image/jpeg",
            "image/jpeg"
        ]
    },
    "className": "QJpegPlugin",
    "debug": true,
    "version": 329993
}
...

Is this info shown with --developer, or does the crash happen before the debug info is generated?

[daschuer]

I just found this new warning in Qt v6.7.0-beta1
"From 6.5.0, xcb-cursor0 or libxcb-cursor0 is needed to load the Qt xcb platform plugin.";
https://github.com/qt/qtbase/blob/5b9ce5d1298f5dffd986b422857cd1624693cabd/src/gui/kernel/qguiapplication.cpp#L1254

This is interesting, because your crash happens with Qt 6.2 so probably another issue?

[daschuer]

This is not our issue because the error does not state: "even though it was found"

[daschuer]

Oh, form Qt6 the xcb plugin is bundled to qt6-qpa-plugins
https://packages.ubuntu.com/jammy/qt6-qpa-plugins

[ronso0]

qt6-qpa-plugins fixes it for me, new PR is #12549