code: func getPVSourceFromSpec(spec *volume.Spec) (*api.CSIPersistentVolumeSource, error) {
  	volSrc, pvSrc, err := getSourceFromSpec(spec)
  	if err != nil {
  		return nil, err
  	}
  	if volSrc != nil {
  		return nil, fmt.Errorf("unexpected api.CSIVolumeSource found in volume.Spec")
  	}
  	return pvSrc, nil
  }
docs: // getPVSourceFromSpec ensures only CSIPersistentVolumeSource is present in volume.Spec