Fix panic for some invalid custom metric requests

This refactors the handling of metrics related to namespaces, by moving
the logic from the storage part to the handler. This allows the handler
to perform validation of the request.

pkg/apiserver/endpoints/handlers/get.go

@@ -41,6 +41,29 @@ func ListResourceWithOptions(r cm_rest.ListerWithOptions, scope handlers.Request
 		// For performance tracking purposes.
 		trace := utiltrace.New("List " + req.URL.Path)
 
+		requestInfo, ok := request.RequestInfoFrom(req.Context())
+		if !ok {
+			err := errors.NewBadRequest("missing requestInfo")
+			writeError(&scope, err, w, req)
+			return
+		}
+
+		// handle metrics describing namespaces
+		if requestInfo.Namespace != "" && requestInfo.Resource == "metrics" {
+			requestInfo.Subresource = requestInfo.Name
+			requestInfo.Name = requestInfo.Namespace
+			requestInfo.Resource = "namespaces"
+			requestInfo.Namespace = ""
+			requestInfo.Parts = append([]string{"namespaces", requestInfo.Name}, requestInfo.Parts[1:]...)
+		}
+
+		// handle invalid requests, e.g. /namespaces/name/foo
+		if len(requestInfo.Parts) < 3 {
+			err := errors.NewBadRequest("invalid request path")
+			writeError(&scope, err, w, req)
+			return
+		}
+
 		namespace, err := scope.Namer.Namespace(req)
 		if err != nil {
 			writeError(&scope, err, w, req)

pkg/registry/custom_metrics/reststorage.go

@@ -112,15 +112,6 @@ func (r *REST) List(ctx context.Context, options *metainternalversion.ListOption
 
 	groupResource := schema.ParseGroupResource(resourceRaw)
 
-	// handle metrics describing namespaces
-	if namespace != "" && resourceRaw == "metrics" {
-		// namespace-describing metrics have a path of /namespaces/$NS/metrics/$metric,
-		groupResource = schema.GroupResource{Resource: "namespaces"}
-		metricName = name
-		name = namespace
-		namespace = ""
-	}
-
 	var res *custom_metrics.MetricValueList
 	var err error