add check that kustomization is empty

[koba1t]

• check the error at kustomize build fails silently when first line is invalid yaml #4861
• add check for a struct that is empty while unmarshal kustomization.yaml.
  • If we exec kustomize build for an empty kustomization.yaml file after merging this PR, we'll get the error.
  • maybe this is a breaking change.

[koba1t]

cc @KnVerey

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/lifecycle stale

[koba1t]

/remove-lifecycle stale

[natasha41575]

/triage accepted
/priority important-soon

[KnVerey]

Am I right to think that we want to throw an error on any file that does not contain any transformers? The code below effectively excludes files that specifically contain only Kustomization type metadata, but I suspect it would actually allow fully empty files or files with partial type meta (which is currently allowed, for better or for worse). I suggest changing the tests to be against raw yaml input rather than struct input to demonstrate the various cases more thoroughly. I also suggest trying the opposite approach for the comparison: deep copy the input Kustomization, clear its type meta, and then compare it to a completely empty object (in object form, not as yaml). LMK what you think of that approach.

[koba1t]

Thanks for your advice!
I agree with your opinions.

I fixed these codes.

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[natasha41575]

/lgtm
/approve

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[Striar-Yunis]

This was a breaking change with kustomize localize (though the issue could be argued to be the localize command). Localize strips empty resources to save space.

apiVersion: kustomize.config.k8s.io/v1alpha1
kind: Component
resources: []

Gets localized as:

apiVersion: kustomize.config.k8s.io/v1alpha1
kind: Component

The localized version in 5.1.0 is unable to build with this change.