private static void useDaemonThreadFactory(ThreadPoolExecutor executor) {
    executor.setThreadFactory(
        new ThreadFactoryBuilder()
            .setDaemon(true)
            .setThreadFactory(executor.getThreadFactory())
            .build());
  }

  // See newDirectExecutorService javadoc for behavioral notes.
  @J2ktIncompatible
  @GwtIncompatible // TODO