code: func (s *Server) runtimeHandler(req *pb.RunPodSandboxRequest) (string, error) {
  	handler := req.GetRuntimeHandler()
  	if handler == "" {
  		return handler, nil
  	}
  
  	runtime, ok := s.Runtime().(*oci.Runtime)
  	if !ok {
  		return "", fmt.Errorf("runtime interface conversion error")
  	}
  
  	if _, err := runtime.ValidateRuntimeHandler(handler); err != nil {
  		return "", err
  	}
  
  	return handler, nil
  }
docs: // runtimeHandler returns the runtime handler key provided by CRI if the key
  // does exist and the associated data are valid. If the key is empty, there
  // is nothing to do, and the empty key is returned. For every other case, this
  // function will return an empty string with the error associated.