/**
   * Tag interface marking trusted subclasses. This enables some optimizations. The implementation
   * of this interface must also be an AbstractFuture and must not override or expose for overriding
   * any of the public methods of ListenableFuture.
   */
  interface Trusted<V extends @Nullable Object> extends ListenableFuture<V> {}

  /**
   * This test is for a case where two Service.Listener callbacks for the same service would call
   * transitionService in the wrong order due to a race. Due to the fact that it is a race this test
   * isn't guaranteed to expose the issue, but it is at least likely to become flaky if the race
   * sneaks back in, and in this case flaky means something is definitely wrong.
   *
   * <p>Before the bug was fixed this test would fail at least 30% of the time.

         * Lock to ensure that a task cannot be rescheduled while a cancel is ongoing.
         *
         * In theory, cancel() could execute arbitrary listeners -- bad to do while holding a lock.
         * However, we don't expose currentFuture to users, so they can't attach listeners. And the
         * Future might not even be a ListenableFuture, just a plain Future. That said, similar

 *       However, utilities that <i>use</i> that executor have the ability to interrupt tasks
 *       running on it. This class, by contrast, does not expose an {@code Executor} API.)
 * </ul>
 *
 * <p>If you don't need the features of this class, you may prefer {@code newSequentialExecutor} for
 * its simplicity and ability to accommodate interruption.
 *
 * @since 26.0

  /**
   * This test is for a case where two Service.Listener callbacks for the same service would call
   * transitionService in the wrong order due to a race. Due to the fact that it is a race this test
   * isn't guaranteed to expose the issue, but it is at least likely to become flaky if the race
   * sneaks back in, and in this case flaky means something is definitely wrong.
   *
   * <p>Before the bug was fixed this test would fail at least 30% of the time.

   */
  /*
   * TODO(cpovirk): Change to Comparator<? super B> to permit Comparator<@Nullable ...> and
   * Comparator<SupertypeOfB>? What we have here matches the immutable collections, but those also
   * expose a public Builder constructor that accepts "? super." So maybe we should do *that*
   * instead.
   */
  public static <B> Builder<B> orderedBy(Comparator<B> comparator) {
    return new Builder<>(comparator);
  }

    // ReentrantReadWriteLock superclass. They are simply wrappers around the
    // internal Sync object, so this is safe since the shadowed locks are never
    // exposed or used.
    private final CycleDetectingReentrantReadLock readLock;
    private final CycleDetectingReentrantWriteLock writeLock;

    private final LockGraphNode lockGraphNode;

   *
   * <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation
   * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} will copy the data only once.
   * This reduces the expense of habitually making defensive copies at API boundaries. However, the
   * precise conditions for skipping the copy operation are undefined.
   *
   * @throws NullPointerException if any of {@code elements} is null

            + "The correctness of CycleDetectingReentrantReadWriteLock depends on "
            + "the fact that the shadowed ReadLock and WriteLock are never used or "
            + "exposed by the superclass implementation. If the implementation has "
            + "changed, the code must be re-inspected to ensure that the "
            + "assumption is still valid.",
        24,

     * requireNonNull is safe because this is called from the constructor after `futures` is set but
     * before releaseResources could be called (because we have not yet set up any of the listeners
     * that could call it, nor exposed this Future for users to call cancel() on).
     */
    requireNonNull(futures);

    // Corner case: List is empty.
    if (futures.isEmpty()) {
      handleAllCompleted();
      return;
    }