static File installOrUpdateYouTubeDL() {
  File file = youTubeDLExecutable();
  if (!file.exists()) ret forceInstallYouTubeDL();
  pcall {
    long size = urlSizeByHEAD(youTubeDL_URL());
    print("youtube-dl size on server: " + size);
    if (size > 0 && size != fileSize(file))
      ret forceInstallYouTubeDL();
  }
  ret file;
}