Detecting bootc hosts

[cgwalters]

Proxying this from a chat request: What's the best practice to detect a bootc-enabled host system?

Today we have /run/ostree-booted etc. - but in the future I think we want to get away from that.

One thing I was thinking about here is leveraging /usr/lib/os-release - we could add a field like UPDATE_MECHANISM=bootc. And I'd advocate doing e.g. UPDATE_MECHANISM=dnf for package-based systems.

For now the spec suggests distro-specific fields, so for e.g. Fedora derivatives we could do FEDORA_UPDATE_MECHANISM=bootc and FEDORA_UPDATE_MECHANISM=dnf.

That said...it may be simplest to add a stub bootc.service unit here in preparation for #2

Yet another approach would be having bootc status never fail, but output a clear state value. Of these, I like this the most as far as being immediately actionable.

[beav]

I'd be +1 to the last option as well.

What would the bootc status --json output look like for a system with bootc installed but not in use? If there is a boolean or string that we can guarantee will exist, we can use that to detect if we should process the other fields or stop.

[cgwalters]

OK I've put up #244