Running frameworks powered applications on Wayland

After I got KWindowSystem to no longer require X11 on Linux I dared to start some of our applications which got ported to frameworks 5 with the magic “-platform wayland” command line switch. And look there: they started and were useable. But there were some obvious problems: e.g. our platform integration plugin was not loaded. But with a small change to Qt 5.3 this problem is solved. Now if you run a Wayland compositor in a KDE session you will get the KDE integration as one would expect. But this just openend a new set of problems: our default QStyle (Oxygen) was crashing any application on startup as it was still assuming that it’s on X11 if compiled with X11 support. Easy enough to fix and now Oxygen works as one would expect.

This experience highlighted an obvious problem: our applications will crash if they are compiled with X11 support and do not perform a runtime check. So I started to hunt down possible crashers in our libraries. In the perfect world there should not be any X11 specific code in the libraries (except KWindowSystems). And luckily our libraries supported non-X11 platforms for quite some time, so all the X specific code should be in HAVE_X11 blocks. So a small reminder for everybody who thinks that we should never have supported Windows in the first place: thanks to Windows support most of our applications just work on Wayland. Have a look at this nice collection of applications which are running just fine on Wayland:

Aus Weston

Right now we have reached a state where I am no longer able to crash applications at random and most applications which I give a try just work. We had a few problems in kde4support which prevented any applications which still used K(Unique)Application to start at all on Wayland and that framework had stronger usage of HAVE_X11 than I expected. Although the framework is deprecated I fixed all the issues I could find in it, to get applications to support Wayland faster. Basically if your application compiles on frameworks it will run on Wayland out of the box. Also thanks to that work our applications will just work on any proprietary windowing system (so no need to be upset that we put more work on e.g. Windows support than on your small pet project). My personal aim is to have Wayland support at least on par with Windows support in our first release of frameworks. Later on we will improve it to get it on the same level as X11 support.

Of course it’s possible that I haven’t spotten all crashes in the libraries yet. And I encourage our application developers to test their applications on Wayland. That’s quite simple in fact: just start Weston (can be nested in the X session) and prepare the setup:

source kf5_env.sh # your kf5 environment variables setup script
unset DISPLAY # to make sure that any XOpenDisplay call will fail
export WAYLAND_DISPLAY=wayland-0 # adjust to whatever you used in Weston

Now all you need is to pass “-platform wayland” to the command line args to start your application on Weston. In case you are using a KApplication and KCmdLineArgs you need to use “--platform wayland” instead. Update: as an alternative one can also export the environment variable “QT_QPA_PLATFORM” and set it to “wayland”. If you run into any problems with it, feel free to ping me on IRC. In case you get your application to crash in one of the frameworks and it’s a crash due to using X11, please report a bug and set me on CC. In case you hit a crash in your application there is a very easy pattern to solve the problem:

#if HAVE_X11
if (QX11Info::isPlatformX11()) {
    // do your X specific calls here
}
#endif

If you are not using QX11Info you can also do:

#if HAVE_X11
if (QGuiApplication::platformName() == QStringLiteral("xcb")) {
    // do your X specific calls here
}
#endif

As you can see it’s performing a string comparison so you don’t want to have that in a code path which gets called often. Recommendation: move it into a member variable which gets initialized only once in app life cycle and also ifdef the variable – no need to do these checks on e.g. Windows. If you have any questions with it, feel free to ask me.

In the workspace module I hit a few crashers in modules which make absolutely no sense on non-X11. The code is too X specific and most likely needs a complete rewrite if we want to have it on non-X11. Of course we don’t want to have modules which just crash in systemsettings. So I introduced a new method to KService to check whether the plugin makes sense on the current Qt platform. This is honored by ::noDisplay() so systemsettings (and also kcmshell5) won’t pick them. To specify one can use e.g.

X-KDE-OnlyShowOnQtPlatforms=xcb

in the plugin’s desktop file. There’s also a “X-KDE-NotShowOnQtPlatforms”.