Additionally, we MUST accept incoming traffic during this time, both new and existing.
Typically, it is rare to get new connections after shutdown has started - once shutdown is triggered the endpoint is marked `NotReady` and removed from load balancing consideration.
However, some applications do not utilize Service readiness (connecting directly to the pod, etc).

 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
abstract class AbstractIterator<T extends @Nullable Object> implements Iterator<T> {
  private State state = State.NOT_READY;

  protected AbstractIterator() {}

  private enum State {
    READY,
    NOT_READY,
    DONE,
    FAILED,
  }

  @CheckForNull private T next;

  @CheckForNull
  protected abstract T computeNext();

  @CanIgnoreReturnValue

  private State state = State.NOT_READY;

  /** Constructor for use by subclasses. */
  protected AbstractIterator() {}

  private enum State {
    /** We have computed the next element and haven't returned it yet. */
    READY,

    /** We haven't yet computed or have already returned the element. */
    NOT_READY,

    /** We have reached the end of the data and are finished. */