Function applySubPath

go/core/internal/skillsinit/git.go:65–112

applySubPath replaces dest with the contents of dest/subPath. The result is that dest contains only the requested subdirectory. We materialize the content into a sibling tmp dir under the same parent so the final rename is atomic on the same filesystem.

(dest, subPath string)

// content into a sibling tmp dir under the same parent so the final rename is
// atomic on the same filesystem.
func applySubPath(dest, subPath string) error {
	// Defense in depth: filepath.IsLocal rejects absolute paths, ".."
	// segments, and reserved names — exactly the set we want to refuse.
	if !filepath.IsLocal(subPath) {
		return fmt.Errorf("invalid subPath %q", subPath)
	}
	clean := filepath.Clean(subPath)
	src := filepath.Join(dest, clean)
	info, err := os.Stat(src)
	if err != nil {
		return fmt.Errorf("stat subPath: %w", err)
	}
	if !info.IsDir() {
		return fmt.Errorf("subPath %q is not a directory", subPath)
	}

	parent := filepath.Dir(dest)
	tmp, err := os.MkdirTemp(parent, ".skill-subpath-*")
	if err != nil {
		return fmt.Errorf("mktemp: %w", err)
	}
	// Best-effort cleanup if we fail before the rename.
	cleanupTmp := tmp
	defer func() {
		if cleanupTmp != "" {
			os.RemoveAll(cleanupTmp)
		}
	}()

	// cp -rL: follow symlinks (matches the original behavior); both paths
	// are constructed by us, never user-supplied, and are passed as argv —
	// no shell, no metacharacter risk. Trailing "/." copies *contents* of
	// src into tmp, not src itself.
	cp := exec.Command("cp", "-rL", "--", src+"/.", tmp)
	cp.Stdout = os.Stdout
	cp.Stderr = os.Stderr
	if err := cp.Run(); err != nil {
		return fmt.Errorf("cp subPath contents: %w", err)
	}
	if err := os.RemoveAll(dest); err != nil {
		return err
	}
	if err := os.Rename(tmp, dest); err != nil {
		return err
	}
	cleanupTmp = ""
	return nil
}