rauc: rauc-mark-good: disable implicit dependencies

[js731ca]

Having implicit dependencies enabled would lead to ordering cycles when enabling 'boot assessment', through
systemd-boot-check-no-failures [1], which is a service that forces the multi-user.target to be reached before the boot-complete.target.

The ordering cycle in this case would be:
boot-complete -requires-> systemd-boot-check-no-failures systemd-boot-check-no-failures -after-> multi-user rauc-mark-good -after-> boot-complete
boot-complete -after-> multi-user
multi-user -after-> rauc-mark-good

Which systemd would break by simply not running rauc-mark-good.

The solution is to disable implicit dependencies that would otherwise add an 'After=rauc-mark-good.service' to the multi-user.target via the [Install] section.

Link: https://www.freedesktop.org/software/systemd/man/latest/systemd-boot-check-no-failures.html

[ejoerns]

Having implicit dependencies enabled would lead to ordering cycles when enabling 'boot assessment', through
systemd-boot-check-no-failures [1], which is a service that forces the multi-user.target to be reached before the boot-complete.target.

Even if not using the example systemd-boot-check-no-failures directly, it is probably a good blueprint to order multi-user.target before boot-complete.target, yes.

@js731ca thank you for diving into this.

The solution is to disable implicit dependencies that would otherwise add an 'After=rauc-mark-good.service' to the multi-user.target via the [Install] section.

I had to look this up, so for reference: this addresses systemd behavior that's described in https://www.freedesktop.org/software/systemd/man/latest/systemd.target.html#Default%20Dependencies

Setting DefaultDependencies=no sounds reasonable to me. An alternative would be not to use WantedBy=multi-user.target for installation. But I actually don't know a better option, yet.

@michaelolbrich maybe?

[js731ca]

Setting DefaultDependencies=no sounds reasonable to me. An alternative would be not to use WantedBy=multi-user.target for installation. But I actually don't know a better option, yet.

@michaelolbrich maybe?

Btw: what also worked was using [Install] WantedBy=basic.target, since the service "just needs to be queued $somehow"... but the DefaultDependencies seemed cleaner to me

[michaelolbrich]

This looks like the correct solution to me. And the equivalent service from systemd (systemd-bless-boot.service) contains DefaultDependencies=no as well.

[ejoerns]

@js731ca @michaelolbrich thanks for your opinions on this!

So we go for the DefaultDependencies=no solution.

FTR: CI fails due to the PR sitting on older master but the change itself cannot make CI fail, anyway.