I've been thinking more about the ostree/composefs integration and longer term, I think composefs should have its own opinionated management tooling for backing store files and checkouts.

Basically we move the "GC problem" from higher level tools into a shared composefs layer - and that will greatly "thin out" what ostree needs to do, and the same for container/storage type things. And more generally, it would help drive unifying these two things which I think we do want long term. Related to this, a mounted composefs shouldn't have backing store files deleted underneath it.

Maybe we could get away with having this just be a directory, e.g. /composefs (like /ostree/repo) or perhaps /usr/.composefs. Call this a $composefsdir.

Vaguely thinking perhaps we could have then $composefsdir/roots.d with namespaced subdirectories, like $composefsdir/roots.d/ostree and $composefsdir/roots.d/containers. Finally there'd be $composefsdir/files which would hold the regular files.

Then we'd have a CLI tool like /usr/libexec/composefsctl --root /composefs gc that would iterate over all composefs filesystems and GC any unreferenced regular files. In order to ensure GC doesn't race with addition we'd also need "API" operations like /usr/libexec/composefsctl add container/foo.composefs that did locking. And a corresponding composefsctl delete.