archiver: extract Readdirnames to fs package

internal/fs/file.go

@@ -1,6 +1,7 @@
 package fs
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"time"
@@ -138,3 +139,24 @@ func ResetPermissions(path string) error {
 	}
 	return nil
 }
+
+// Readdirnames returns a list of file in a directory. Flags are passed to fs.OpenFile. O_RDONLY is implied.
+func Readdirnames(filesystem FS, dir string, flags int) ([]string, error) {
+	f, err := filesystem.OpenFile(dir, O_RDONLY|flags, 0)
+	if err != nil {
+		return nil, fmt.Errorf("openfile for readdirnames failed: %w", err)
+	}
+
+	entries, err := f.Readdirnames(-1)
+	if err != nil {
+		_ = f.Close()
+		return nil, fmt.Errorf("readdirnames %v failed: %w", dir, err)
+	}
+
+	err = f.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	return entries, nil
+}