incus-simplestreams : missing prune features

[Salamandar]

incus-simplestreams is a nice tool, but it's missing two features :

• Removal of orphan image tarballs (not referenced by images.json) -> This might be triggered by a incus-simplestreams add with an image with identical properties and same date, but different checksum
• Removal of old versions of a product : When publishing a new image, the old one stays and we need manual tooling to clean it up.

I implemented those two features in a python script : https://github.com/YunoHost/lxd_img_builder/blob/master/prune_incus_simplestreams.py but it would be really nice to have a incus-simplestreams prune command that does both jobs.

EDIT: considering the lack of those features, i'm wondering about the intended workflow : maybe it would be better to have a "staging" repository that i'd fill with new versions of the images and then swap the real repo and the staging repo…

[Salamandar]

Considering this comment: #1373 (comment), we should only prune images that are orphan for more than 1 hour to prevent 404s on clients that haven't yet updated their index cache.