automatically capture last mutating systemd unit, highlight as "agent"

[cgwalters]

This is very similar to coreos/rpm-ostree@83c942d and later coreos/rpm-ostree#2459

Basically...it'd be really useful to be able to see from bootc status the basic information about which calling process (systemd unit e.g.) last mutated the system. Maybe even a history of such changes?

And perhaps like the rpm-ostree change there should be an explicit API to claim "I am the active driver" and require disabling that active driver to make other changes.

An architectural thing to debate here is whether this history canonically lives in the journal or whether it's bound e.g. to deployment metadata. (I think probably the latter, it's a tiny amount of information and some edge systems e.g. will want to turn the local journal off to reduce persistent writes, only doing select remote logging).

I would also really like to figure out how to better wire up our default simplistic systemd unit+timer into this.

[cgwalters]

Also on this topic, I think we should ship a systemd generator that checks for our own automatic update unit being enabled, and if it is immediately queues a registration for that unit.

Hmm, in general maybe the argument here is there should be a generic way for us to find metadata about systemd units that affect us? Or maybe we should actually ship a generic bootc-controller.service that should be a symlink to the agent?

[jlebon]

And perhaps like the rpm-ostree change there should be an explicit API to claim "I am the active driver" and require disabling that active driver to make other changes.

I think ideally there'd be some mechanism by which bootc upgrade actually proxies to the driver instead of flat out erroring out (using the word "proxies" loosely here, basically could just fork out to a configured command). That allows bootc to remain the primary UX.