@Test
  public void testCancelled() throws InterruptedException, ExecutionException {
    Task<List<?>> t =
        Tasks.sequential(sayTask("1"), sayTask("2a", Duration.THIRTY_SECONDS, "2b"), sayTask("3"));
    ec.submit(t);
    synchronized (messages) {
      while (messages.size() <= 1) messages.wait();
    }
    Assert.assertEquals(messages, Arrays.asList("1", "2a"));
    Time.sleep(Duration.millis(50));
    t.cancel(true);
    Assert.assertTrue(t.isDone());
    // 2 should get cancelled, and invoke the cancellation semaphore
    // 3 should get cancelled and not run at all
    Assert.assertEquals(messages, Arrays.asList("1", "2a"));

    // Need to ensure that 2 has been started; race where we might cancel it before its run method
    // is even begun. Hence doing "2a; pause; 2b" where nothing is interruptable before pause.
    Assert.assertTrue(cancellations.tryAcquire(10, TimeUnit.SECONDS));

    Iterator<Task<?>> ci = ((HasTaskChildren) t).getChildren().iterator();
    Assert.assertEquals(ci.next().get(), "1");
    Task<?> task2 = ci.next();
    Assert.assertTrue(task2.isBegun());
    Assert.assertTrue(task2.isDone());
    Assert.assertTrue(task2.isCancelled());

    Task<?> task3 = ci.next();
    Assert.assertFalse(task3.isBegun());
    Assert.assertTrue(task2.isDone());
    Assert.assertTrue(task2.isCancelled());

    // but we do _not_ get a mutex from task3 as it does not run (is not interrupted)
    Assert.assertEquals(cancellations.availablePermits(), 0);
  }
 @Test
 public void testComplex() throws InterruptedException, ExecutionException {
   Task<List<?>> t =
       Tasks.sequential(
           sayTask("1"), sayTask("2"), Tasks.parallel(sayTask("4"), sayTask("3")), sayTask("5"));
   ec.submit(t);
   Assert.assertEquals(t.get().size(), 4);
   Asserts.assertEqualsIgnoringOrder((List<?>) t.get().get(2), ImmutableSet.of("3", "4"));
   Assert.assertTrue(
       messages.equals(Arrays.asList("1", "2", "3", "4", "5"))
           || messages.equals(Arrays.asList("1", "2", "4", "3", "5")),
       "messages=" + messages);
 }