code: final int doExec() {
          int s; boolean completed;
          if ((s = status) >= 0) {
              try {
                  completed = exec();
              } catch (Throwable rex) {
                  return setExceptionalCompletion(rex);
              }
              if (completed)
                  s = setCompletion(NORMAL);
          }
          return s;
      }
docs: Primary execution method for stolen tasks. Unless done, calls
  exec and records status if completed, but doesn't wait for
  completion otherwise.
  
  @return status on exit from this method