feat: Allow using kubectl built-in kustomize when separate kustomize binary is missing

[yxxhero]

Implements #63: Automatically fallback to kubectl's built-in kustomize when the standalone kustomize binary is not available.

Changes:

• Modified kustomizeBin() to detect and use kubectl kustomize as fallback
• Fixed runBytes() to properly handle commands with subcommands like 'kubectl kustomize'
• Added isUsingKubectlKustomize() helper to detect when using kubectl kustomize
• Updated kustomizeEnableAlphaPluginsFlag() and kustomizeLoadRestrictionsNoneFlag() to skip version detection when using kubectl kustomize
• Added clear error messages for unsupported features (edit subcommands) when using kubectl kustomize
• Modified KustomizeBuild() to omit 'build' argument when using kubectl kustomize
• Modified patch.go to omit 'build' argument and tempDir when using kubectl kustomize
• Added TestKustomizeBin test to verify fallback behavior
• Added TestKubectlKustomizeFallback integration test

[Copilot]

Pull request overview

Adds an automatic fallback to kubectl’s built-in kustomize implementation when a standalone kustomize binary is not available, and updates command execution to support multi-word “commands” like kubectl kustomize.

Changes:

• Update Runner.kustomizeBin() to prefer kustomize if present, otherwise fall back to kubectl kustomize.
• Fix Runner.runBytes() to correctly handle commands with subcommands embedded in the cmd string.
• Adjust kustomize build/patch flows and add tests around the new behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
util_test.go	Adds unit tests for kustomizeBin() selection logic (option, PATH lookup, fallback).
runner.go	Implements the fallback selection in kustomizeBin() and improves command parsing for multi-word commands.
patch.go	Updates patch rendering to support kubectl-kustomize mode (argument handling changes).
kustomize.go	Adds kubectl-kustomize detection, skips version probing, and adjusts build/edit behavior accordingly.
kubectl_kustomize_test.go	Adds a test intended to validate kubectl kustomize support.

Comments suppressed due to low confidence (1)

patch.go:201

• When using kubectl kustomize, this invocation omits the kustomization directory argument and also runs with dir=="" (current working directory). That means kubectl kustomize will not read the generated kustomization.yaml under tempDir, so patch rendering will fail. Pass tempDir as the final arg for the kubectl case (similar to KustomizeBuild), or switch to runInDir(tempDir, ...) and rely on the default . target.

	kustomizeArgs := []string{"--output", renderedFile}

	if !r.isUsingKubectlKustomize() {
		kustomizeArgs = append([]string{"build", tempDir}, kustomizeArgs...)
	}

	if u.EnableAlphaPlugins {
		f, err := r.kustomizeEnableAlphaPluginsFlag()
		if err != nil {
			return err
		}
		kustomizeArgs = append(kustomizeArgs, f)
	}

	_, err := r.run(nil, r.kustomizeBin(), kustomizeArgs...)
	if err != nil {

[Copilot]

This subtest name suggests that kustomizeBin() reads KUSTOMIZE_BIN directly, but the test actually passes the env var value via KustomizeBin(...) (same behavior as the first subtest). Rename the subtest to reflect that it’s testing the option value, or update kustomizeBin() to consult KUSTOMIZE_BIN if that’s intended.

[Copilot]

These PATH-dependent branches are mostly skipped depending on what binaries happen to be installed (e.g., the fallback case is skipped whenever kustomize exists), so the new fallback logic may not be exercised in CI. Consider making the test deterministic by setting PATH to a temp dir containing a fake kubectl executable (and no kustomize) so exec.LookPath reliably drives the intended branch.

[Copilot]

This test is named as a "fallback" test but it forces KustomizeBin("kubectl kustomize") and skips when kubectl isn’t installed, so it doesn’t verify the automatic fallback behavior in kustomizeBin() and may be skipped in CI. Consider making it self-contained by creating a fake kubectl on PATH (and no kustomize) and asserting New().kustomizeBin() selects it, or rename the test to reflect what it actually validates.