support building from a scratch image

[tzneal]

Fixes #207

[imjasonh]

You might have a better time using pkg/v1/empty and pkg/v1/mutate to do this:

idx := mutate.IndexMediaType(empty.Index, types.OCIImageIndex)
for _, p := range platforms {
  plat, err := v1.ParsePlatform(p)
  // handle err
  img := mutate.MediaType(
    mutate.ConfigMediaType(empty.Image, types.OCIConfigJSON),
    types.OCIManifestSchema1,
  )
  idx = mutate.AppendManifests(idx, mutate.IndexAddendum{
    Add: img,
    Descriptor: v1.Descriptor{
      Platform: plat,
    },
  })
}
return idx, nil

This will handle most of the manual hash handling for you, and should be usable has a multi-arch zero-layer base image.

I wonder what we should do when the base is scratch and --platform=all 🤔

[tzneal]

"all" failed to parse with the previous platform parsing code, I added an explicit error for it now. I concluded it didn't make sense for a scratch image case, and you can work around it by just providing all platforms if you really do want them.

[imjasonh]

This looks really good! Thanks for doing this.

[codecov-commenter]

Codecov Report

Merging #1038 (5239c55) into main (175820e) will increase coverage by 0.07%.
The diff coverage is 58.97%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
+ Coverage   52.82%   52.89%   +0.07%     
==========================================
  Files          43       44       +1     
  Lines        3360     3399      +39     
==========================================
+ Hits         1775     1798      +23     
- Misses       1359     1371      +12     
- Partials      226      230       +4     

Impacted Files	Coverage Δ
pkg/commands/config.go	53.01% <0.00%> (-4.89%)	⬇️
pkg/commands/scratch.go	71.87% <71.87%> (ø)

[codecov-commenter]

Codecov Report

Merging #1038 (d9566fd) into main (175820e) will increase coverage by 0.36%.
The diff coverage is 60.97%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
+ Coverage   52.82%   53.19%   +0.36%     
==========================================
  Files          43       44       +1     
  Lines        3360     3401      +41     
==========================================
+ Hits         1775     1809      +34     
- Misses       1359     1362       +3     
- Partials      226      230       +4     

Impacted Files	Coverage Δ
pkg/commands/config.go	53.01% <0.00%> (-4.89%)	⬇️
pkg/build/scratch.go	73.52% <73.52%> (ø)

... and 1 file with indirect coverage changes

[tzneal]

@imjasonh Was there something else remaining for this?

[imjasonh]

Sorry for taking so long to get back to this. This change still looks good, and thanks for adding a docs section as well.

I think it would be good to have an e2e test in https://github.com/ko-build/ko/blob/main/.github/workflows/e2e.yaml that does KO_DEFAULTBASEIMAGE=scratch ko build --platform=linux/amd64,linux/s390x and checks that the resulting image is as expected (using crane and jq probably). I'd be happy to help with that if you're still interested. If not I can do it as a followup.

Thanks again for making this change, and sorry again for the delay.

[tzneal]

If not I can do it as a followup.

I'd be happy for you to take over the e2e test part. I'm swamped with some other work and certain that you can do it faster than me 😆

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[cprivitere]

Can this be re-opened? I think it was only closed because it was stale, not because nobody wants it.

[cprivitere]

Dunno what your current situation is @imjasonh maybe you can do the e2e stuff if you have time now?