This is usually necessary for a graphical session. In most cases,
in Chimera elogind performs the task.
XDG_RUNTIME_DIR
This is an environment variable and path that many services and
applications rely on, particularly those that use logind but
also others.
The XDG_RUNTIME_DIR environment variable’s value is a path. The
path is created when the user logs in, and is deleted when last
login logs out, unless there are lingering services for the user.
The typical value is /run/user/UID where UID is the user ID
of the user.
In most distributions that use systemd, the env var is set by the
pam_systemd module, with logind creating it as it keeps track
of the login. In many non-systemd distributions, the same task is
done by elogind.
In Chimera, elogind has this functionality disabled. Instead, it
is created and tracked by the turnstile session tracking system.
That means whichever solution you use for seat management, you will
always have your runtime directory managed by turnstile.
The main reason for this is that when using the “linger” option with
user services, the directory needs to remain there as long as the
services are running, which is not possible to reliably ensure with
elogind unless you set the linger bit separately in elogind too
(and even that does not cover all cases).
The other main reason is convenience, as it means a single solution for all seat management daemons.
Seat daemons
In general you currently have two options as far as seat daemons
go. Note that things which use libseat will work with both options;
this includes Wayland compositors based on wlroots and Weston,
as well as Xorg (but not necessarily things using Xorg).
elogind
The elogind daemon manages user logins as well as auxiliary tasks
such as system power handling. Big desktops will require elogind;
particularly GNOME requires it.
If installed, it comes with default service links. That means most
users will get it out of box. It is also a dependency of base-full.
It is the recommended solution; avoiding it is at your own risk.
seatd
Some setups can use an alternative to elogind in form of seatd,
particularly some specific Wayland compositors such as Weston and
those based on the wlroots library.
Keep in mind that seatd very much conflicts with elogind, so
ensure that only one is running. You will need to manually enable
the seatd service. The daemon also does much less than elogind
does, so e.g. system power management is not handled by it.