overlay: Bind ostree-prepare-root.service to ignition-complete.target

[cgwalters]

This unit comes from ostree upstream, but forms part of our
Ignition sequence. Make it required by ignition-complete.target
so that if it fails we get the same OnFailure= behavior.

Making this change as during development ignition-ostree-mount-sysroot.service
wasn't running which caused this unit to fail; the root cause would have been
a bit more obvious with this change.

[jlebon]

Not totally against, though ignition-complete.target is meant to be a master on/off switch for whether to run Ignition (see coreos/ignition-dracut#55). So it's only really useful on first boot. Adding non-firstboot-only services under it means there will be different behaviours wrt failures between first and subsequent boots for that unit, and can also give off the impression that ostree-prepare-root.service is a firstboot-only service too.

Hmm actually, ostree-prepare-root.service already has OnFailure=emergency.target. So maybe we just need:

diff --git a/src/boot/ostree-prepare-root.service b/src/boot/ostree-prepare-root.service
index 63357581..1017c2fd 100644
--- a/src/boot/ostree-prepare-root.service
+++ b/src/boot/ostree-prepare-root.service
@@ -22,6 +22,7 @@ DefaultDependencies=no
 ConditionKernelCommandLine=ostree
 ConditionPathExists=/etc/initrd-release
 OnFailure=emergency.target
+OnFailureJobMode=isolate
 After=sysroot.mount
 Before=initrd-root-fs.target

for the same reason as ignition-complete.target?

[cgwalters]

Yep, you're totally right. Will do.