/** Checks that #drain() invocations behave correctly for a drained (empty) queue. */
  private void assertDrained(BlockingQueue<Object> q) {
    assertNull(q.peek());
    assertInterruptibleDrained(q);
    assertUninterruptibleDrained(q);
  }

  private void assertInterruptibleDrained(BlockingQueue<Object> q) {
    // nothing to drain, thus this should wait doing nothing
    try {