code: func validateNodeConfigSourceSpec(source *core.NodeConfigSource, fldPath *field.Path) field.ErrorList {
  	allErrs := field.ErrorList{}
  	count := int(0)
  	if source.ConfigMap != nil {
  		count++
  		allErrs = append(allErrs, validateConfigMapNodeConfigSourceSpec(source.ConfigMap, fldPath.Child("configMap"))...)
  	}
  	// add more subfields here in the future as they are added to NodeConfigSource
  
  	// exactly one reference subfield must be non-nil
  	if count != 1 {
  		allErrs = append(allErrs, field.Invalid(fldPath, source, "exactly one reference subfield must be non-nil"))
  	}
  	return allErrs
  }
docs: // validation specific to Node.Spec.ConfigSource