[gdm-list] RFC: user session lifetimes vs. $DISPLAY

[Simon McVittie <simon mcvittie collabora co uk>]

I've recently been researching systemd's current support for user
sessions, with the goal of sorting out any remaining omissions/issues
and having GDM integrate well with it.

It looks as though the intention is that if I have overlapping sessions
like this:

* 14:00: log in to GDM on X11 display :0
* 14:10: log in via ssh or getty or something
* 14:20: log out from :0
* 14:30: log in to GDM on X11 display :1
* 14:40: log out from ssh/getty
* 14:50: log out from :1

then from 14:00 to 14:50 I have one XDG_RUNTIME_DIR, one 'systemd
--user' instance (as per systemd's TODO: started by logind using
user  service, on behalf of pam_systemd) and one 'dbus-daemon --session'
instance (presumably started by that 'systemd --user'). Is that how it's
meant to work? Or am I meant to have one 'systemd --user' instance per
login session, or one D-Bus session bus per login session?

Is there any plan for how GUI processes started via session D-Bus
activation should pick up the right value for $DISPLAY (in my example:
":0" until 14:20, nothing from 14:20 to 14:30, ":1" from 14:30 onwards)
and $XAUTHORITY, or is that something that still needs to be solved? Or
are GUI processes meant to obtain their X11 display and authority file
in some other way?

One simple example of a GUI process started by session D-Bus activation
is that /usr/bin/gnome-terminal is just a "remote control" which
activates ${libexecdir}/gnome-terminal-server, a GUI application, and
tells it to open a new window. This is currently a D-Bus session service
using traditional D-Bus activation, but it should presumably become a
systemd user service, with the dbus-daemon handing off activation to the
user instance of systemd.

If used in my example above, a gnome-terminal-server started at 14:05
ought to use display :0, and exit at 14:20 when it loses its connection
to the X11 server; if I then run gnome-terminal at (say) 14:35, the
desired result is a new gnome-terminal-server on display :1. If the
overlapping sessions share a 'systemd --user' and a 'dbus-daemon
--session', then that would even work if I ran gnome-terminal from the
text-mode session, which is a nice improvement over how it currently
works with a session-scoped dbus-daemon (it'd fail because the text-mode
session either doesn't have a dbus-daemon, or has a dbus-daemon with no
$DISPLAY).

If there is not currently a plan for how to deal with DISPLAY and
XAUTHORITY, I think they could be solved by having 'systemd --user'
watch logind, and include those variables (or perhaps only DISPLAY?)
from the corresponding uid's most-recently-started X11 session? (I
believe GDM only supports one simultaneous X11 session per uid anyway.)

I don't know much about Wayland, but it appears that it normally has one
socket "wayland-0" in the XDG_RUNTIME_DIR, and only needs to use further
environment variables if there's more than one Wayland compositor
sharing an XDG_RUNTIME_DIR.

    S