code: func (i *PendingItem) SetErr(err error) {
  	if err == nil {
  		panic("internal error")
  	}
  	i.lock.Lock()
  	defer i.lock.Unlock()
  	if i.err == nil {
  		i.err = err
  		for _, child := range i.linked {
  			child.lock.Lock()
  			child.err = err
  			child.lock.Unlock()
  		}
  	}
  }
docs: // SetErr forcibly set an item as failed. Normally not used by callers.