Why do I have Wayland packages installed although my system is not using Wayland?
I am using Ubuntu MATE 17.10. The release notes never mentioned anything about Wayland, and I've always assumed this system was using the Xorg display server.
However, today I ran the usual
sudo apt update && sudo apt full-upgrade
and noticed that the output included:
The following packages will be upgraded:
libwayland-client0 libwayland-cursor0 libwayland-server0 [...]
I thought, that's odd. Is my system using Wayland after all?
As suggested by Byte Commander in chat, I looked for running processes (using pgrep Xorg and pstree), and found Xorg but no Wayland. So I'm pretty sure Wayland is not running. I looked for Wayland packages:
dpkg -l | grep [Ww]ayland
ii kwayland-data 4:5.38.0-0ubuntu1 all Qt library wrapper for Wayland libraries - data files
ii kwayland-integration:amd64 4:5.10.5-0ubuntu1 amd64 kwayland runtime integration plugins
ii libkf5waylandclient5:amd64 4:5.38.0-0ubuntu1 amd64 Qt library wrapper for Wayland libraries
ii libqt5waylandclient5:amd64 5.9.1-2 amd64 QtWayland client library
ii libqt5waylandcompositor5:amd64 5.9.1-2 amd64 QtWayland compositor library
ii libwayland-client0:amd64 1.14.0-1ubuntu0.1 amd64 wayland compositor infrastructure - client library
ii libwayland-cursor0:amd64 1.14.0-1ubuntu0.1 amd64 wayland compositor infrastructure - cursor library
ii libwayland-egl1-mesa:amd64 17.2.8-0ubuntu0~17.10.1 amd64 implementation of the Wayland EGL platform -- runtime
ii libwayland-server0:amd64 1.14.0-1ubuntu0.1 amd64 wayland compositor infrastructure - server library
ii qtwayland5:amd64 5.9.1-2 amd64 QtWayland platform plugin
Why do I have these packages related to Wayland even though I'm not using Wayland?
Thanks to Byte Commander and Eliah Kagan making suggestions in chat I was able to figure out that these packages were installed as dependencies.
As suggested I simulated removing Wayland related packages... When I tried pretending to remove all Wayland-related packages...
apt remove '.*wayland.*' -s
the result looked apocalyptic, with a full-screen terminal full of packages being removed, including the entire ubuntu-mate-desktop metapackage and all its adjuncts, the xorg package (oops!) and marco (MATE's default window manager).
But just removing kwayland-data only seemed to remove a couple of other mysterious packages kwayland-integration and libkf5waylandclient5...
So as Eliah Kagan suggested I tried asking aptitude why I had these packages, for example
$ aptitude why kwayland-data
i konsole Depends libkf5windowsystem5 (>= 4.96.0)
i A libkf5windowsystem5 Recommends kwayland-integration
i A kwayland-integration Depends libkf5waylandclient5 (>= 4:5.27.0+p16.10+git20161029.2052)
i A libkf5waylandclient5 Depends kwayland-data (= 4:5.38.0-0ubuntu1)
Ahh! That makes sense. I installed Konsole because other terminal emulators don't support CTL.
By asking aptitude why over and over like a persistent toddler, it's possible to find out why any particular package is required or has been installed, for example:
$ aptitude why libwayland-egl1-mesa
i libwebkit2gtk-4.0-37 Depends libwayland-egl1-mesa (>= 10.0.2) | libwayland-egl1
$ aptitude why libwayland-cursor0
i libgtk-3-0 Depends libwayland-cursor0 (>= 1.9.91)
$ aptitude why libgtk-3-0
i mate-utils Depends libgtk-3-0 (>= 3.16.2)
$ aptitude why mate-utils
i ubuntu-mate-desktop Depends mate-utils
Simulating removal of packages like libgtk-3-0 (be careful not to actually remove them...) reveals more about the dependency structures in play.
Conclusion
I have these packages because other packages surprisingly depend on them. Some, such as kwayland-data, installed as a dependency of a dependency of a package recommended by a dependency of a non-essential package I installed myself, could be safely removed, but others, such as libwayland-egl1-mesa are dependencies of packages integral to my system, and removing them would be disastrous! With APT's --simulate or -s flag and aptitude we can safely investigate these matters.