internal/pkg: parallelise scrub

This significantly improves scrubbing performance. Since the cache directory structure is friendly to simultaneous access, this is possible without synchronisation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/dir.go

@@ -196,15 +196,16 @@ func Flatten(fsys fs.FS, root string, w io.Writer) (n int, err error) {
 }
 
 // HashFS returns a checksum produced by hashing the result of [Flatten].
-func HashFS(fsys fs.FS, root string) (Checksum, error) {
+func HashFS(buf *Checksum, fsys fs.FS, root string) error {
 	h := sha512.New384()
 	if _, err := Flatten(fsys, root, h); err != nil {
-		return Checksum{}, err
+		return err
 	}
-	return (Checksum)(h.Sum(nil)), nil
+	h.Sum(buf[:0])
+	return nil
 }
 
 // HashDir returns a checksum produced by hashing the result of [Flatten].
-func HashDir(pathname *check.Absolute) (Checksum, error) {
-	return HashFS(os.DirFS(pathname.String()), ".")
+func HashDir(buf *Checksum, pathname *check.Absolute) error {
+	return HashFS(buf, os.DirFS(pathname.String()), ".")
 }