Consider adding Requisite/Conflict equivalents that do not trigger OnFailure

[benzea]

There are two slightly different use cases for this. The commonality in these two cases is the idea of using use OnFailure in GNOME to start the full screen "Session Failed" dialog. i.e. I have a gnome-session-failed.target which should be executed if certain services fail.

However, some of these services should only run when certain other targets are already present or not present. There are two scenarios:

• The service should only run when another target is present (not used yet, but planned). This means it should run for example when both bluetooth.target and gnome-session-initialized.target are up (i.e. both in Requisite/After/PartOf/WantedBy).
• The service conflicts with another target. An actual example is not running a specific service in the GDM session, which I right now solve using Conflicts=gnome-session@gnome-login.target (gnome-login is the session type).

I believe in both cases the Requisite or Conflicts will cause the unit to go into the failed state, triggering OnFailure. I can easily work around this by having a .target with the dependencies and a separate .service with the OnFailure entry. This works great, but it is a kind of cumbersome (and causes unnecessary logging).

[benzea]

A few observations that I was having now:

• I thought I could use an intermediate target to contain the OnFailure, seems like I got that part wrong
• Seems like Conflicts and Requisite behaves quite differently with Conflict already preventing the start itself and also the OnFailure action or so.

Either way, my solution will now probably be to use ExecStopPost with a binary that will start the OnFailure unit if SERVICE_RESULT==start-limit-hit and maybe in some other conditions.

[benzea]

The most extreme scenario I have for this is:

• gsd-xsettings.target marks whether the current session type wants the normal GNOME X11 services (configured via gnome-session@SESSION.target.d/session.conf drop-in)
• gnome-shell starts gnome-session-x11-services.target when Xwayland is required by a client. It also needs to know when the connected services are ready.
• gsd-xsettings.service should run if both gsd-xsettings.target and gnome-session-x11-services.target are started
• gsd-xsettings.service needs Before=gnome-session-x11-services.target to signal readyness