gnome-systemd: Actually create scopes for launched applications

[Fantu]

Applications launched from Cinnamon end up in the same cgroup as the desktop itself, instead of getting one of their own — linuxmint/cinnamon#12015.

The code to fix this has been here since 2023: cinnamon_global_app_launched_cb() calls gnome_start_systemd_scope() on every launch. It just never does anything, because the function gives up when sd_pid_get_user_unit() returns -ENODATA. That check is inherited from gnome-desktop, where it holds: gnome-shell is started as a systemd user unit, so it is part of the user instance. Cinnamon is not — cinnamon-session and everything it spawns live in the logind session scope — so the check fails every time:

$ cat /proc/$(pidof -s cinnamon)/cgroup
0::/user.slice/user-1000.slice/session-2.scope
sd_pid_get_user_unit(cinnamon) -> -61 (ENODATA)

Being outside the user instance does not stop us from using it. The user manager is perfectly willing to migrate a PID out of the logind session scope into a transient scope it owns:

cgroup before: /user.slice/user-1000.slice/session-2.scope
cgroup after:  /user.slice/user-1000.slice/user@1000.service/app.slice/test.scope

So this only gives up when there is no user instance at all, checking that logind knows about us and that the user manager is reachable. Systems not managed by systemd behave exactly as before: same early return, no D-Bus traffic, no new warnings.

Two related changes come with it:

• The unit is now named app-cinnamon-<ApplicationID>-<pid>.scope, following https://systemd.io/DESKTOP_ENVIRONMENTS/ and matching gnome-desktop, which renamed gnome-launched- to app-gnome- for the same reason. Tools that inspect cgroups look for the app- prefix to tell applications apart from other units, so keeping the old name would have left the reported issue effectively unfixed. The .desktop suffix is stripped, as the scheme expects the bare application ID.
• CollectMode=inactive-or-failed is set, so failed applications do not linger as failed units. Same as gnome-desktop.

What this buys us

Every application gets an identity systemd can act on: per-app resource accounting and limits, systemctl --user stop app-cinnamon-…, sane systemd-oomd targets instead of taking the whole session down, recognition by cgroup-based system monitors, and an orderly per-app shutdown at logout.

Testing

Built and installed on Mint 23 (Cinnamon 6.7.4, cinnamon-session 6.7.3), X11 and Wayland sessions:

$ systemctl --user list-units --type=scope --all --no-legend
  app-cinnamon-debian-xterm-16842.scope  loaded active running  Application launched by cinnamon
  app-cinnamon-nemo-16923.scope          loaded active running  Application launched by cinnamon

$ cat /proc/16842/cgroup
0::/user.slice/user-1000.slice/user@1000.service/app.slice/app-cinnamon-debian-xterm-16842.scope

Mission Center now lists them under Running Apps. Note that recent Mission Center releases also match applications heuristically, so this is no longer the only way it can see them — the other benefits above are the reason to do it regardless.

Related

• global: Don't create a scope for D-Bus activated applications cinnamon#13926 adds the pid == 0 guard for D-Bus activated applications. Without it, those launches create an empty app-cinnamon-<id>-0.scope as soon as this PR lands, so the two should go in together.
• session: Add a drop-in for the application scopes cinnamon creates cinnamon-session#213 ships a drop-in for the scopes created here.