Remove dependency on the `docker_content_digest` header

[yeikel]

This is a follow up for #90 and #83

The spec is clear about these headers :

From : https://github.com/opencontainers/distribution-spec/blob/6bc87156eacf3b73362db343eb6b63d7abeedf7e/spec.md?plain=1#L51-L52

Because of the origins this specification, the client MAY encounter Docker-specific headers, such as Docker-Content-Digest, or Docker-Distribution-API-Version. These headers are OPTIONAL and clients SHOULD NOT depend on them.

Currently, we have usages of this header in at least two places :

docker_registry2/lib/registry/registry.rb

Line 131 in 204a6c8

dohead(tag_path).headers[:docker_content_digest]

docker_registry2/lib/registry/registry.rb

Line 134 in 204a6c8

doget(tag_path).headers[:docker_content_digest]

My environment is one example implementation where this header is not present at all, and as stated above, that falls within the spec. We should either :

1. Have a fallback mechanism for environments where this header is not present
2. Follow the advise shared above and remove this dependency as quoted "These headers are OPTIONAL and clients SHOULD NOT depend on them"

[deitch]

I would lean towards the first of the two options. I know they are optional, but Docker Hub still is the biggest and most well-known, and people do depend on it here already. If we can try to use it, and fall back if it does not exist, let's go for that.

[yeikel]

I would lean towards the first of the two options. I know they are optional, but Docker Hub still is the biggest and most well-known, and people do depend on it here already. If we can try to use it, and fall back if it does not exist, let's go for that.

I understand the reasoning but doesn't that add two requests in the worst case scenario as opposed to just one?

Perhaps, the argument is that one of the existing requests is a HEAD request that is usually very fast so it won't matter as much?

[deitch]

Make a single request, but read the headers twice.

[yeikel]

Make a single request, but read the headers twice.

Can we get what we need using headers? I think that most headers are optional

[deitch]

I think so

[yeikel]

As soon as we have an implementation, I can test it in my environment and confirm