code: private File findFile(NameNodeDirType dirType, String name) {
      for (StorageDirectory sd : dirIterable(dirType)) {
        File candidate = new File(sd.getCurrentDir(), name);
        if (sd.getCurrentDir().canRead() &&
            candidate.exists()) {
          return candidate;
        }
      }
      return null;
    }
docs: Return the first readable storage file of the given name
  across any of the 'current' directories in SDs of the
  given type, or null if no such file exists.