Post by Mantas Mikulėnas
one answer. (Note that stuff like $DISPLAY gets imported into systemd
--user's environment, to make it available to the services'
environment, and you can't make $DISPLAY have two values at once.)
Oh. There's a design problem here as well: if session-specific software
sets environment variables for user services, they are not removed from
the user manager env store when that session terminates.
Let's consider the following scenario:
1. A machine is booted up and is ready to establish user sessions.
2. User U logs in (maybe remotely, or locally on a VT/kmscon) and
establishes an XDG_SESSION_TYPE=tty session Id=1. A systemd --user
instance is launched.
3. Later on, user U logs into a graphical session Id=2 at a
seat on the machine. `systemctl import-environment DISPLAY
XAUTHORITY', etc. Some time afterwards user U logs out; session 1 is
intact, as well as the systemd --user instance; the now-defunct
DISPLAY is not expunged from the systemd --user environment
4. User U logs in into a graphical session Id=3 again, but with a
Wayland-based setup, e. g. to work around a bug. `systemctl
import-environment WAYLAND_DISPLAY', etc. The defunct DISPLAY is
still present in the environment!
The above was a brainstormed reproducer; I did actually encounter this
kind of bug IRL a couple of times this way.
I'd used GDM to log in to GNOME on Xorg, then logged out, then logged in
to Sway (a Wayland compositor/window manager), and some apps e. g.
telegram-desktop malfunction because they infer they're running in GNOME
from the GNOME-specific environment vars that were not unset on logout
% systemctl --user show-environment | grep -i GNOME
The _, most likely set by a unix shell, is especially funny. :)
The leaked values of XDG_CURRENT_DESKTOP and XDG_SESSION_DESKTOP also
break xdg-desktop-portal in the new session as well, etc. In short,
this is not robust enough and can break in a multitude of ways, most of
them unknown a priori.
To sum it up, if we use the user manager environment store this way,
there should be a way to automatically remove session-specific
environment vars from systemd --user when the respective session is
Possible ways to fix:
* Per-session unit managers: this approach was discussed previously and
deemed hard to implement; unclear relationships between session units
and user units.
* Tie the systemd--user environment store entries to session IDs.
When logind emits org.freedesktop.login1.Manager.SessionRemoved on
/org/freedesktop/login1, remove environment variables tied to that
* user-specific environment stores for graphical sessions in logind and
an opt-in mechanism (e. g. UseSessionEnvironment=yes) in unit files to
use them, API to modify/flush them (on the bus and via loginctl),
autoflush when the graphical session dies.
Not too hard to implement, basically 1 hash table per user.
A session starter would do
`loginctl import-environment XDG_CURRENT_DESKTOP'
`systemctl import-environment XDG_CURRENT_DESKTOP'
If I recall correctly, user managers already requires logind, so
that's not a problem.
I'm not opposed to writing a patch to fix this or just filing an issue
report, but wanted to discuss the general direction we'd like to take