passwd: Use check-* file json config. entries to migrate data across commits

[james-antill]

Needs some docs. also a bit more testing before merging, as a bunch of code got moved about and reused.

Basically the migrate code will now look at the files pointed to by check-passwd/check-group so that they can be migrated across branches.

[cgwalters]

Keep in mind too the use case I need in one downstream case is migrating between repositories, not between branches. The idea is that the "release repository" doesn't expose internal history.

[james-antill]

Right, you can extract the passwd/group files and put those in the JSON for the first build. Or am I missing something?

[james-antill]

Updated the kind test and fixed the perform_migrate test.

[cgwalters]

Hm, so for someone doing their first compose, they'd need to:

• Make a tree using a treefile without the check-passwd config
• Use say ostree cat branch /usr/lib/passwd > passwd to extract the files
• Commit those to the git repo holding the treefile
• Edit the treefile to add check-passwd pointing to that content

If so, I'm fine with that as I cannot think of a better way without doing #49. I have been thinking recently that we could pretty easily write a custom program outside of systemd that implemented sysusers, suitable for backporting to older systems.

But in the short term I think this is OK actually, because people who have existing trees and are bit by uid/gid variance can more easily extract their existing config and bake it.

[james-antill]

Yeh, that's it. The only bit that's kind of ugly is the "ostree cat" part ... so we could add something to make that easier. But hopefully not many people will need to do this anyway. You want to merge?

[cgwalters]

Only one minor comment. Otherwise I think this is OK. 👍

Long term though we are going to need a better story, and also particularly one that works with containers too. Which gets interesting for Docker containers without systemd...we have no way to provide on-boot hooks at the moment.