*     typically check whether it failed: See <a
     *     href="https://errorprone.info/bugpattern/FutureReturnValueIgnored">https://errorprone.info/bugpattern/FutureReturnValueIgnored</a>.
     */
    public <C extends @Nullable Object> ListenableFuture<C> callAsync(
        AsyncCallable<C> combiner, Executor executor) {

    final CountDownLatch doneLatch = new CountDownLatch(numberOfThreads);
    for (int i = numberOfThreads; i > 0; i--) {
      @SuppressWarnings("unused") // https://errorprone.info/bugpattern/FutureReturnValueIgnored
      Future<?> possiblyIgnoredError =
          executorService.submit(
              new Callable<@Nullable Void>() {
                @Override
                public @Nullable Void call() throws Exception {

     *     typically check whether it failed: See <a
     *     href="https://errorprone.info/bugpattern/FutureReturnValueIgnored">https://errorprone.info/bugpattern/FutureReturnValueIgnored</a>.
     */
    public <C extends @Nullable Object> ListenableFuture<C> callAsync(
        AsyncCallable<C> combiner, Executor executor) {

  private static void startDirectorySymlinkSwitching(
      final Path file, final Path target, ExecutorService executor) {
    @SuppressWarnings("unused") // https://errorprone.info/bugpattern/FutureReturnValueIgnored
    Future<?> possiblyIgnoredError =
        executor.submit(
            new Runnable() {
              @Override
              public void run() {
                boolean createSymlink = false;