imagebuildah: Support custom image reference lookup for cache push/pull

[aaronlehmann]

What type of PR is this?

/kind api-change

What this PR does / why we need it:

This allows callers to provide custom SourceLookupReferenceFunc and DestinationLookupReferenceFunc for cache pull/push. These can be used to implement custom blob caches, and to wrap the reference being pushed/pulled to influence the copy behavior.

How to verify it

Verification isn't really necessary since this is just plumbing through functions for other code calling into buildah, but one could write a test program that embeds buildah and supplies its own funcs here.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

n/a

Does this PR introduce a user-facing change?

None

[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[rhatdan]

Please add tests.

[aaronlehmann]

Please add tests.

I think this is a tricky thing to construct a test for since it requires pushing cache to/from a registry. The integration tests do this, but they use a real buildah binary that doesn't use these options.

I saw there is a TestConformance test which exercises imagebuildah.BuildDockerfiles, but this doesn't have a registry available to test with, and hacking either hacking that into this test or creating a similar test with a registry feels like it would involve creating a lot of infrastructure just to test something extremely simple.

One option would be to add hidden flags to the buildah build subcommand that rewrite the cache source/destination refs (basically just for testing purposes - I don't think they would have a practical use), and exercise that in integration tests. Does that sound like a good approach? Do you have any other ideas?

[rhatdan]

No after looking at the PR more closely, I think we can let it in without tests.

[aaronlehmann]

Cool, are there any other changes you'd like here?

[mtrmac]

I am just randomly passing by and I am expressing no opinion on the cost/benefit of adding these indirections.

[mtrmac]

Thanks!

Implementation LGTM. I’ll let others decide about the cost/benefit of the feature.

[rhatdan]

/lgtm
/approve
Thanks @aaronlehmann

[openshift-ci]

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