Error if image platform does not match desired

[benmoss]

Fixes #699

If the user specifies an image manifest and its platform does not match the platforms specified or defaulted, error out.

With a .ko.yaml specifying the linux/amd64 image manifest instead of the multi-platform index:

baseImageOverrides:
  github.com/google/ko: golang@sha256:a4081692fa3015104d84b125cbf623c566a0c9e2a39b9a29f6d939225d5687a4

Prior behavior

❯ go run main.go build --platform=linux/arm64 .
2022/08/04 10:36:41 No matching credentials were found, falling back on anonymous
2022/08/04 10:36:42 Using base golang@sha256:a4081692fa3015104d84b125cbf623c566a0c9e2a39b9a29f6d939225d5687a4 for github.com/google/ko
2022/08/04 10:36:42 Building github.com/google/ko for linux/amd64
...

New behavior

❯ go run main.go build --platform=linux/arm64 .
2022/08/04 10:43:52 No matching credentials were found, falling back on anonymous
2022/08/04 10:43:53 Using base golang@sha256:a4081692fa3015104d84b125cbf623c566a0c9e2a39b9a29f6d939225d5687a4 for github.com/google/ko
Error: failed to publish images: error building "ko://github.com/google/ko": base image platform "linux/amd64" does not match desired platforms [linux/arm64]
2022/08/04 10:43:53 error during command execution:failed to publish images: error building "ko://github.com/google/ko": base image platform "linux/amd64" does not match desired platforms [linux/arm64]
exit status 1

As mentioned in #699 --platform=all is a lil ambiguous so we're keeping that behavior:

❯ go run main.go build --platform=all .
2022/08/04 10:45:40 No matching credentials were found, falling back on anonymous
2022/08/04 10:45:41 Using base golang@sha256:a4081692fa3015104d84b125cbf623c566a0c9e2a39b9a29f6d939225d5687a4 for github.com/google/ko
2022/08/04 10:45:41 Building github.com/google/ko for linux/amd64
...

[imjasonh]

This mostly looks good, thanks for the contribution!

If there was an additional test in the e2e tests that checked that ko build --platform=linux/fake resulted in an error, I'd award extra bonus points 😁🏅

[imjasonh]

I'm confused a bit why this addition is needed for tests. I would have expected random.Image to produce a linux/amd64 image by default, and for NewGo to assume that platform if none is provided, so that platform matcher check you added wouldn't be necessary.

Does this change require callers of the Go API to explicitly pass WithPlatforms?

[benmoss]

random.Image has no platform (or any config at all), so you get this somewhat odd error

gobuild_test.go:448: Build() = base image platform "" does not match desired platforms []

It looks like from empty.Image it just gets an empty Platform struct:

*v1.Platform{Architecture: "", OS: "", OSVersion: "", OSFeatures: []string len: 0, cap: 0, nil, Variant: "", Features: []string len: 0, cap: 0, nil}

The CLI calls gobuildOptions which handles the defaulting of desired platforms.

My understanding from reading over the code is that the Go API (NewGo) doesn't do any defaulting, so if the user didn't set any desired platforms the current behavior would be:

• if given an image index/manifest list, no platforms would match
• if given an image manifest, it would copy the platform from the base image

The tests were relying on that latter behavior, so the resulting image would also have a blank platform.

[benmoss]

If there was an additional test in the e2e tests that checked that ko build --platform=linux/fake resulted in an error, I'd award extra bonus points 😁🏅

We'd have to set KO_DEFAULTBASEIMAGE as well, since it defaults to an image index (go:1.18), and that already handles erroring for unsupported platforms. This is behavior that only triggers when you use image manifests as your base image.