OpenAPIResourcesGetter allows lazy-loading OpenAPI V2

staging/src/k8s.io/kubectl/pkg/cmd/explain/explain.go

@@ -74,7 +74,7 @@ type ExplainOptions struct {
 	args []string
 
 	Mapper        meta.RESTMapper
-	Schema openapi.Resources
+	openAPIGetter openapi.OpenAPIResourcesGetter
 
 	// Name of the template to use with the openapiv3 template renderer.
 	OutputFormat string
@@ -123,17 +123,14 @@ func (o *ExplainOptions) Complete(f cmdutil.Factory, cmd *cobra.Command, args []
 		return err
 	}
 
-	o.Schema, err = f.OpenAPISchema()
-	if err != nil {
-		return err
-	}
-
 	// Only openapi v3 needs the discovery client.
 	o.OpenAPIV3Client, err = f.OpenAPIV3Client()
 	if err != nil {
 		return err
 	}
 
+	// Lazy-load the OpenAPI V2 Resources, so they're not loaded when using OpenAPI V3.
+	o.openAPIGetter = f
 	o.args = args
 	return nil
 }
@@ -224,7 +221,11 @@ func (o *ExplainOptions) renderOpenAPIV2(
 		gvk = apiVersion.WithKind(gvk.Kind)
 	}
 
-	schema := o.Schema.LookupResource(gvk)
+	resources, err := o.openAPIGetter.OpenAPISchema()
+	if err != nil {
+		return err
+	}
+	schema := resources.LookupResource(gvk)
 	if schema == nil {
 		return fmt.Errorf("couldn't find resource for %q", gvk)
 	}

staging/src/k8s.io/kubectl/pkg/cmd/explain/explain_test.go

@@ -275,3 +275,44 @@ func runExplainTestCases(t *testing.T, cases []explainTestCase) {
 		buf.Reset()
 	}
 }
+
+// OpenAPI V2 specifications retrieval -- should never be called.
+func panicOpenAPISchemaFn() (openapi.Resources, error) {
+	panic("should never be called")
+}
+
+// OpenAPI V3 specifications retrieval does *not* retrieve V2 specifications.
+func TestExplainOpenAPIV3DoesNotLoadOpenAPIV2Specs(t *testing.T) {
+	// Set up OpenAPI V3 specifications endpoint for explain.
+	fakeServer, err := clienttestutil.NewFakeOpenAPIV3Server(filepath.Join(testDataPath, "openapi", "v3"))
+	if err != nil {
+		t.Fatalf("error starting fake openapi server: %v", err.Error())
+	}
+	defer fakeServer.HttpServer.Close()
+	tf := cmdtesting.NewTestFactory()
+	defer tf.Cleanup()
+	tf.OpenAPIV3ClientFunc = func() (openapiclient.Client, error) {
+		fakeDiscoveryClient := discovery.NewDiscoveryClientForConfigOrDie(&rest.Config{Host: fakeServer.HttpServer.URL})
+		return fakeDiscoveryClient.OpenAPIV3(), nil
+	}
+	// OpenAPI V2 specifications retrieval will panic if called.
+	tf.OpenAPISchemaFunc = panicOpenAPISchemaFn
+
+	// Explain the following resources, validating the command does not panic.
+	cmd := explain.NewCmdExplain("kubectl", tf, genericiooptions.NewTestIOStreamsDiscard())
+	resources := []string{"pods", "services", "endpoints", "configmaps"}
+	for _, resource := range resources {
+		cmd.Run(cmd, []string{resource})
+	}
+	// Verify retrieving OpenAPI V2 specifications will panic.
+	defer func() {
+		if panicErr := recover(); panicErr == nil {
+			t.Fatal("expecting panic for openapi v2 retrieval")
+		}
+	}()
+	// Set OpenAPI V2 output flag for explain.
+	if err := cmd.Flags().Set("output", "plaintext-openapiv2"); err != nil {
+		t.Fatal(err)
+	}
+	cmd.Run(cmd, []string{"pods"})
+}