butane: import as git subtree (alternative to #2235)

[prestist]

This is an alternative to #2235 that uses a git subtree merge to bring butane into ignition.
The main difference: butane stays a separate Go module with its own go.mod, and the coreos/butane
repo can keep functioning independently.

Syncing future butane changes is straightforward:

git fetch butane-remote main
git merge -s subtree --allow-unrelated-histories butane-remote/main

	#2235 (direct copy)	this PR (subtree)
git history	butane commits inlined into PR	preserved as merge parent
Go module	single module, imports rewritten	separate modules, butane unchanged
butane repo	archived	stays functional
upstream sync	manual	git merge -s subtree
build	one go build	sub-build via butane/build

This is a proof-of-concept -- it doesn't include the transpilation integration,
vendor changes, or import rewriting that #2235 has. Those would be follow-up work.

[gemini-code-assist]

Code Review

This pull request integrates the butane project as a subtree, updating the top-level build scripts and Makefile to compile and install it, alongside adding its core specs, utilities, tests, and automation workflows. The code review identified several issues: a relative path bug in the build script, a logical error in translateResource that bypasses local Ignition config validation, potential edge cases in path traversal checks, variable shadowing of the report package, inefficient regex compilation in a loop, and a reflection typo in map key validation within tests.

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[gemini-code-assist]

If BIN_PATH is a relative path (e.g., bin/amd64), changing the directory to butane will cause the sub-build to write the binary to butane/bin/amd64/butane instead of the top-level bin/amd64/butane. This will cause the top-level Makefile installation to fail because it expects the binary at the top-level directory.

To fix this, resolve BIN_PATH to an absolute path before changing the directory.

abs_bin_path=$(mkdir -p "${BIN_PATH}" && cd "${BIN_PATH}" && pwd)
(cd butane && BIN_PATH="${abs_bin_path}" ./build)

[gemini-code-assist]

The check strings.HasPrefix(c.String(), "$.ignition.config") is a logical error and will always evaluate to false. This is because c is defined locally on line 120 as c := path.New("yaml", "local"), which has a string representation of "yaml.local" (not starting with "$.ignition.config"). As a result, local Ignition configs specified via local are never validated.

To fix this, split translateResource into two translators:

1. translateResource (for general resources, without Ignition config validation).
2. translateIgnitionResource (specifically for Ignition configs, which performs the validation).

Then, register translateIgnitionResource in translateIgnition, and translateResource in translateFile.

[gemini-code-assist]

Using manual string concatenation and strings.HasPrefix to check if a path is within a base directory can lead to edge cases, such as when filesDir is the root directory / (where absBase + Separator becomes //, causing false positives). A more robust and idiomatic approach in Go is to use filepath.Rel and check if the relative path starts with ...

Suggested change

	if absPath != absBase && !strings.HasPrefix(absPath, absBase+string(filepath.Separator)) {
	return common.ErrFilesDirEscape
	}
	rel, err := filepath.Rel(absBase, absPath)
	if err != nil {
	return err
	}
	if strings.HasPrefix(rel, "..") {
	return common.ErrFilesDirEscape
	}

[gemini-code-assist]

The variable name report shadows the imported package name github.com/coreos/vcontext/report. It is better to rename it to valReport or vr to avoid shadowing and improve code clarity.

Suggested change

	report := validate.ValidateWithContext(config, rawConfig)
	r.Merge(report)
	valReport := validate.ValidateWithContext(config, rawConfig)
	r.Merge(valReport)

[gemini-code-assist]

Compiling a regular expression inside a loop or function that is called repeatedly is inefficient. Since translatePasswdUser can be called multiple times (once for each user), compiling the regex on every invocation is a performance bottleneck.

You can avoid using a regular expression entirely by using strings.Split and strings.ReplaceAll to handle both Unix and Windows line endings, which is significantly faster and more memory-efficient.

Suggested change

	for _, line := range regexp.MustCompile("\r?\n").Split(string(sshKeys), -1) {
	for _, line := range strings.Split(strings.ReplaceAll(string(sshKeys), "\r\n", "\n"), "\n") {

[gemini-code-assist]

The check if v.IsZero() is a typo and should be if !value.IsValid(). Currently, it checks the map v itself instead of the retrieved element value. If a key does not exist in the map, v.MapIndex returns an invalid reflect.Value, which bypasses this check and eventually falls through to the default case with an incorrect error message.

Suggested change

	if v.IsZero() {
	if !value.IsValid() {