code: func validateVolumeNodeAffinity(nodeAffinity *core.VolumeNodeAffinity, fldPath *field.Path) (bool, field.ErrorList) {
  	allErrs := field.ErrorList{}
  
  	if nodeAffinity == nil {
  		return false, allErrs
  	}
  
  	if nodeAffinity.Required != nil {
  		allErrs = append(allErrs, ValidateNodeSelector(nodeAffinity.Required, fldPath.Child("required"))...)
  	} else {
  		allErrs = append(allErrs, field.Required(fldPath.Child("required"), "must specify required node constraints"))
  	}
  
  	return true, allErrs
  }
docs: // validateVolumeNodeAffinity tests that the PersistentVolume.NodeAffinity has valid data
  // returns:
  // - true if volumeNodeAffinity is set
  // - errorList if there are validation errors