Let GC print specific message for RESTMapping failure

pkg/controller/garbagecollector/operations.go

@@ -30,13 +30,36 @@ import (
 	"k8s.io/kubernetes/pkg/client/retry"
 )
 
+type restMappingError struct {
+	kind    string
+	version string
+}
+
+func (r *restMappingError) Error() string {
+	versionKind := fmt.Sprintf("%s/%s", r.version, r.kind)
+	return fmt.Sprintf("unable to get REST mapping for %s.", versionKind)
+}
+
+// Message prints more details
+func (r *restMappingError) Message() string {
+	versionKind := fmt.Sprintf("%s/%s", r.version, r.kind)
+	errMsg := fmt.Sprintf("unable to get REST mapping for %s.", versionKind)
+	errMsg += fmt.Sprintf(" If %s is a thirdparty resource (tpr), please note that the garbage collector doesn't support tpr yet. Once tpr is supported, object with ownerReferences referring non-existing tpr objects will be deleted by the garbage collector.", versionKind)
+	errMsg += fmt.Sprintf(" If %s is not a tpr, then you should remove ownerReferences that refer %s objects manually.", versionKind, versionKind)
+	return errMsg
+}
+
+func newRESTMappingError(kind, version string) *restMappingError {
+	return &restMappingError{kind: kind, version: version}
+}
+
 // apiResource consults the REST mapper to translate an <apiVersion, kind,
 // namespace> tuple to a unversioned.APIResource struct.
 func (gc *GarbageCollector) apiResource(apiVersion, kind string, namespaced bool) (*metav1.APIResource, error) {
 	fqKind := schema.FromAPIVersionAndKind(apiVersion, kind)
 	mapping, err := gc.restMapper.RESTMapping(fqKind.GroupKind(), apiVersion)
 	if err != nil {
-		return nil, fmt.Errorf("unable to get REST mapping for kind: %s, version: %s", kind, apiVersion)
+		return nil, newRESTMappingError(kind, apiVersion)
 	}
 	glog.V(5).Infof("map kind %s, version %s to resource %s", kind, apiVersion, mapping.Resource)
 	resource := metav1.APIResource{