code: func validateVolumeAttachmentSource(source *storage.VolumeAttachmentSource, fldPath *field.Path) field.ErrorList {
  	allErrs := field.ErrorList{}
  	if source.PersistentVolumeName == nil || len(*source.PersistentVolumeName) == 0 {
  		allErrs = append(allErrs, field.Required(fldPath, ""))
  	}
  	return allErrs
  }
docs: // validateSource tests if the source is valid for VolumeAttachment.