Refuse `copy.Options.PreserveDigests` in `docker-archive:` (and other transports where it is impossible)

[danielerepici]

Hello, as per the title, here's my steps:

$ skopeo inspect docker://my.registry/foo/bar@sha256:68394b2d21cb73b2ef86b29149e3e1b97d619662f0a35e9bbd62014f2c7a3cd8 | grep Digest -m 1
    "Digest": "sha256:68394b2d21cb73b2ef86b29149e3e1b97d619662f0a35e9bbd62014f2c7a3cd8",
$ skopeo copy --preserve-digests docker://my.registry/foo/bar@sha256:68394b2d21cb73b2ef86b29149e3e1b97d619662f0a35e9bbd62014f2c7a3cd8 docker-archive:my-archive:foo/bar:my-archive
Getting image source signatures
Copying blob 949cb00e9eeb done   |
Copying blob 6bd7f4c9f9b3 done   |
Copying blob a87c9cb132ac done   |
Copying blob 8564620f1ca9 done   |
Copying blob bd635e478e6f done   |
Copying blob a9673d869b5a done   |
Copying blob 6e880c714c2b done   |
Copying blob 3c3a064ac0ad done   |
Copying blob 1d04f3755388 done   |
Copying blob be9fc2a15855 done   |
Copying blob 5428f6566eee done   |
Copying blob 43281a5eef5e done   |
Copying config 1ec95189a6 done   |
Writing manifest to image destination
$ skopeo inspect docker-archive:my-archive:foo/bar:my-archive | grep Digest -m 1
    "Digest": "sha256:b64d8cd83311acf6ef458440b6ef19b86db2665525c365093804267811ee3606",

Extra info: it's a single-architecture image.

[mtrmac]

Thanks for reaching out.

docker-archive doesn’t have a manifest digest because it doesn’t have a (registry-like) manifest at all. If you want to preserve the original representation of an image, docker-archive is entirely unsuitable. (Generally my strong recommendation is to use a registry for all image transfers, not any kind of archive format; registries are significantly more efficient.)

(Sadly, the docker-archive: destination is not told about the --preserve-digests option and thus has currently no mechanism to object to the use of that option.)

[mtrmac]

the docker-archive: destination is not told about the --preserve-digests option

That could be improved

[danielerepici]

Just found a similar issue which I didn't spot initially, putting a link here for reference:

containers/skopeo#1440

I understand that this should be done via a registry, thank you. Feel free to close this.

[mtrmac]

Let’s keep tracking the better reporting when given such inputs, in the code where that would be changed.