code: void internalPropagateException(Throwable ex) {
      CountedCompleter<?> a = this, s = a;
      while (a.onExceptionalCompletion(ex, s) && (a = (s = a).completer) != null && a.status >= 0
          && a.recordExceptionalCompletion(ex) == EXCEPTIONAL);
    }
docs: Supports ForkJoinTask exception propagation.