Function ensure_delta_source_present 

pub(crate) fn ensure_delta_source_present(
    repo: &Repository<Sha512HashValue>,
    delta: &Delta,
) -> Result<()>

A delta only carries the layers that changed; the rest have to come from the image it was built against, which must therefore already be in repo.

composefs would notice this eventually, but only once it starts importing, which is pretty late for a nice experience.