* snapshot of the state and therefore it can be used to answer simple queries without needing to
   * grab a lock.
   */
  // @Immutable except that Throwable is mutable (initCause(), setStackTrace(), mutable subclasses).
  private static final class StateSnapshot {
    /**
     * The internal state, which equals external state unless shutdownWhenStartupFinishes is true.
     */
    final State state;