SDDM + KDE Plasma Wayland Frozen After Login

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I could not log into my computer this morning. I was using SDDM + KDE Plasma with Wayland. When I typed my password and pressed ENTER, it stopped responding. The screen remained on the log-in screen, but the mouse cursor disappeared. I tried to switch terminal using CTRL+ALT+Fx (x = 2, 3, …, 8), but that didn’t work, either. What was wrong with it? I kept trying and eventually got the answer.

Rescue

Since CTRL+ALT+Fx didn’t work, I tried to log into my machine from another machine via SSH. It worked. I managed to get a shell, and restarted SDDM using sudo systemctl restart sddm. Then my computer became responsive again.

What was wrong?

Many things could go wrong. I updated quite some packages yesterday, and it was ArchLinux. As a rolling Linux distribution, any update could render the system unusable. But when I visited https://archlinux.org/, no recent news mentioned anything that needed manual intervention.

I was using NVidia’s open source driver (the nvidia-open package). Could that be a problem? I uninstalled that package and rebooted. It would still be frozen after logging in. So the display driver was not the problem.

I was using KDE Plasma Wayland session. Could that be a problem? I tried to start the session by typing startplasma-wayland on a TTY, and that started the session successfully for me. So Plasma Wayland may not be the problem.

I was using SDDM, the default display manager for KDE Plasma. Could that be a problem? I installed LXDM. LXDM could start the Plasma X11 session, but it was incapable of starting any Wayland sessions. I installed LightDM, but for some reasons it failed to start. I installed GDM. It could start the GNOME session, but it didn’t detect the presence of Plasma Wayland. In the end, I couldn’t find any display manager that could start the Plasma Wayland session besides SDDM. It could be SDDM’s problem, but I couldn’t confirm it.

I tried to created another user, and tried to log in and start the Plasma Wayland session via SDDM. Surprisingly, it worked. It gave my new user a desktop with default settings. This meant the problem was probably with my personal configuration.

What configuration?

I was using Fish as my default shell. I used chsh to change my default shell to Bash, and tried to log in. It worked. It logged into the Plasma Wayland session successfully. So it narrowed down the problem with my Fish configuration.

I moved my entire Fish configuration directory ~/.config/fish away, and attempted to login. It worked. That further confirmed that one (or more) of my Fish configuration file was wrong. Which one was it?

I tried to move my configuration files one by one from my backup location back to ~/.config/fish. It ended up that the file fish_variables was the culprit.

I then tried to gradually remove the lines from fish_variables. In the end, the culprit was the following line:

SETUVAR --export UID:1000

It is a universal variable. It means it could have been set at any moment in the past, and it persisted until today and caused the failure.

Solution

Removing the universal variable UID solved the problem, but I still don’t understand why that variable was set in the first place. And I don’t know why it was frozen when the UID environment variable was set. Actually I don’t know exactly what was frozen, the SDDM, or something in the Plasma Wayland session.