sbool tailFile_verbose;

// raise flag to stop
static Flag tailFile(final File file, final int interval, fO onData) {
  final new Flag flag;
  thread "Tail File" {
    long l = 0;
    repeat with ms sleep interval {
      if (flag.isUp()) break;
      long l2 = l(file);
      if (tailFile_verbose) print("tailFile: l=" + l + ", l2=" + l2);
      if (l2 < l) l = 0;
      if (l2 != l) pcall {
        S text = null;
        pcall {
          text = loadTextFilePart(file, l, l2);
        }
        if (nempty(text))
          callF(onData, text);
        l = l2;
      }
    }
  }
  ret flag;
}