Add XDG_SESSION_ID

[OJFord]

necessary for example for soteria polkit agent:
https://github.com/imvaskel/soteria?tab=readme-ov-file#installation

Closes #40.

[Copilot]

Pull request overview

This pull request adds XDG_SESSION_ID to the list of environment variables that are propagated to systemd and dbus activation environments. This is necessary for tools like the soteria polkit agent that require this variable to function properly.

Changes:

• Added XDG_SESSION_ID to the VARIABLES list in src/session.sh

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[alebastr]

Hm... GNOME developers believe that it's not safe to pass vars like XDG_SESSION_ID or XDG_VTNR, because it may break other desktop environments, or even the same DE after relogin: https://gitlab.gnome.org/GNOME/gnome-session/-/commit/646b9bc0584d02033ab9600c39b77d6f99bfc4a6.
I tend to trust GNOME in such things, because they had more users and years to encounter all kinds of interesting bugs.

I'm curious though, why soteria insists on requiring XDG_SESSION_ID? Pretty sure that no other polkit agent has such requirement, and the agents I was able to check were using polkit_unix_session_new_for_process instead.

[OJFord]

That doesn't seem right to me - if it's wrong on re-login then surely it means it wasn't correctly passed through again on the second login? Or linger is enabled, so it was already set and did nothing, as here:

sway-systemd/src/session.sh

Line 96 in f8b36c5

echo "Another session found; refusing to overwrite the variables"

More interesting case would be two concurrent logins of the same user in different ttys - i.e. two seats, two sessions, one user - this would fail the same check since it's already set in the first session, and neither's more correct than the other. Proper multi-seat handling would be a different beast though, and afaik it's usual to have the greeter/dm launch sway only in the tty, to avoid exactly this sort of collision (as well as an escape to fix wm startup issues), like:

if [ -z "$WAYLAND_DISPLAY" ] && [ -n "$XDG_VTNR" ] && [ "$XDG_VTNR" -eq 1 ] ; then
    exec sway
fi

(https://wiki.archlinux.org/title/Sway#Automatically_on_TTY_login) or as I have for greetd:

[terminal]
vt = 1

[default_session]
command = "agreety --cmd=/bin/sh"
user = "greeter"

[initial_session]
command = "systemd-cat --identifier=sway sway"
user = "ojford"

To me, we have a session target, if setting the session ID in it causes a problem in some case; the problem is not that we set it, the problem is it not being correct.