code: func (r *controller) Close() error {
  	select {
  	case <-r.closed:
  		return r.err
  	default:
  		if r.cancelPull != nil {
  			r.cancelPull()
  		}
  
  		r.err = exec.ErrControllerClosed
  		close(r.closed)
  	}
  	return nil
  }
docs: // Close the runner and clean up any ephemeral resources.