assertThat(thirdCallable.called).isFalse();
    firstFuture.set(null);
    assertThat(secondCallable.called).isFalse();
    assertThat(thirdCallable.called).isTrue();
  }

  public void testCancellationMultipleThreads() throws Exception {
    BlockingCallable blockingCallable = new BlockingCallable();
    ListenableFuture<@Nullable Void> unused = serializer.submit(blockingCallable, executor);
    ListenableFuture<Boolean> future2 =