[Interest] Qt 5.15 for Linux/X11 : "-qt-xcb" no longer supported?

[Florian Bruhin]

On Fri, May 22, 2020 at 06:56:32PM -0700, Thiago Macieira wrote:
> On Friday, 22 May 2020 07:54:03 PDT Florian Bruhin wrote:
> > I'm talking about running Qt's binary releases, not building from sources.
> 
> Then take the list of "not found" and pass it through your package manager 
> installer.
> 
> On an RPM-based distro:
> 
> ldd binary | awk '/not found/ { print $1 "()(64bit)" | xargs $PM install
> 
> where $PM is zypper or dnf or yum.

That works. However, initially you'll be met with an error message like this:

    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: eglfs, linuxfb, minimal, minimalegl,
    offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl,
    wayland-xcomposite-glx, xcb.

There's nothing in there that tells me:

- What exactly happened in a clear way (what does the 'in ""' mean?!)
- That there's some dependency missing
- That I need to go look at /usr/lib/qt/plugins/platforms/libqxcb.so

Especially on CI where this is likely to happen, it can be painful to find out.
Setting QT_DEBUG_PLUGINS=1 usually helps:

    [lots of output]

    Cannot load library .../Qt/plugins/platforms/libqxcb.so:
    (libxcb-randr.so.0: cannot open shared object file: No such file or directory)

But again, that's something you need to know about - if you're new to Qt and
just trying to use the binaries on CI (e.g. via PyQt), it doesn't seem to be
clear to me at all how to get started, and the vague error message doesn't
help. Thus why I think this should be documented somewhere (and/or the error
message improved, though I don't know how difficult it is to get a proper error
message when something like this happens).

Florian

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