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Unable to Start Plasma as Different User from TTY (lemmy.kde.social)

submitted 1 month ago* (last edited 1 month ago) by [email protected] to c/[email protected]

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I am trying to run a Plasma session as a different user from bash by running sudo -u $TARGET_USER dbus-launch --exit-with-session startplasma-wayland.

However, when trying to do so, I get an error "Could not create wayland socket".

My goal is to start a Plasma session as a specified user, wait for the user to log out and continue running the script.

Does anyone know how I might be able to fix this error?
Do you know of another way to start a Plasma session as a different user and wait for the session to exit?

I also tried to do the same using the sudo --preserve-env --set-home XDG_RUNTIME_DIR=/run/users/$(id -u $TARGET_USER) switches.

When doing so, I get the following messages repeatedly:

No backend specified, automatically choosing drm
kwin_core: Failed to take control of /org/freedesktop/login1/session/_311 session. Maybe another compositor is running?
kwin_wayland_drm: Failed to open drm device at "/dev/dri/card1"
kwin_wayland_drm: No suitable DRM devices have been found

However, logging in to the tty with the $TARGET_USER directly and running dbus-launch --exit-with-session startplasma-wayland works properly as expected.

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[–] [email protected] 2 points 1 month ago (1 children)

Why not just log in as the user in TTY and then start it?

I'n not sure I understand the use-case of why it needs a Plasma session to start a script that needs to keep running afterwards. If the script itself does not need a plasma session, then you can just start it as a user service with systemd.

[–] [email protected] 1 points 1 month ago* (last edited 1 month ago)

Thanks for your answer! I am writing a setup script which installs and configures my system automatically. As the creation of rclone configurations requires passwords, I wanted to boot up a Plasma Wayland session (for each user which needs to be created) and prompt them to create the necessary rclone configurations and sign out afterwards.

My current (non functioning) draft of the script looks like this: https://git.nuth.ch/manuth/PortValhalla/src/commit/c88be78a7bce6b5d4d3bb64456b58366d1a1305d/scripts/Common/Software/rclone/main.fish#L10-L60

[–] [email protected] 1 points 1 month ago* (last edited 1 month ago)

Alright, so I found an absolute trainwreck of a solution. All the ways I have tested (sudo, login, openvt, machinectl, systemd-run) don't work because important variables like XDG_SESSION_ID are not created. So probably these methods don't open up a real session or something...?

What I did is to automatically open up a session for the specified user - in this case using getty. Using these code lines, I created a systemd-configuration and booted it up by running systemctl daemon-reload:

{
    printf %s\n \
        "[Service]" \
        "ExecStart=" \
        "ExecStart=-/sbin/agetty -o '-p -f -- \\u' --noclear --autologin $TARGET_USER %I \$TERM"
} | sudo tee "/etc/systemd/system/[email protected]/autologin.conf"

sudo systemctl daemon-reload
sudo systemctl restart getty@tty2

source: https://wiki.archlinux.org/title/Getty

After this, the user has a running session and proper environment variables set up. I use pgrep to wait for the expected process to show up:

sessionPid=""

while [ -z "$sessionPid" ]; do
    sessionPid=$(sudo pgrep -t tty2 -u "$TARGET_USER")
done

The environment variables of the process with the PID $sessionPid can then be borrowed to run startplasma-wayland properly as the desired user:

sudo -u "$TARGET_USER" \
    env \
        $(sudo -u "$TARGET_USER" fish -c "cat /proc/$sessionPid/environ | string split0)") \
        /usr/lib/plasma-dbus-run-session-if-needed startplasma-wayland

The fact, that this command would always end up focussing on tty2 after exiting the plasma session, I kind of abused the openvt-command to force the focus to return to the previous tty:

openvt -sw -- \
    sudo -u "$TARGET_USER" \
        env \
            $(sudo -u "$TARGET_USER" fish -c "cat /proc/$sessionPid/environ | string split0)") \
            /usr/lib/plasma-dbus-run-session-if-needed startplasma-wayland

I also did some quirks like using openvt to find an unused tty. If you want to see the whole thing in action - script is here:

https://git.nuth.ch/manuth/PortValhalla/src/commit/b6329693d6fd717c8ca6f7a278a864930b7f705d/scripts/Common/Software/rclone/main.fish#L10-L60