add KUBECTL_EXPLAIN_OPENAPIV3 envar to gate new explain implementation

update

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

@@ -18,14 +18,17 @@ package explain
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/spf13/cobra"
 
 	"k8s.io/apimachinery/pkg/api/meta"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/cli-runtime/pkg/genericclioptions"
+	"k8s.io/client-go/discovery"
 	cmdutil "k8s.io/kubectl/pkg/cmd/util"
 	"k8s.io/kubectl/pkg/explain"
+	explainv2 "k8s.io/kubectl/pkg/explain/v2"
 	"k8s.io/kubectl/pkg/util/i18n"
 	"k8s.io/kubectl/pkg/util/openapi"
 	"k8s.io/kubectl/pkg/util/templates"
@@ -62,12 +65,25 @@ type ExplainOptions struct {
 
 	Mapper meta.RESTMapper
 	Schema openapi.Resources
+
+	// Toggles whether the OpenAPI v3 template-based renderer should be used to show
+	// output.
+	EnableOpenAPIV3 bool
+
+	// Name of the template to use with the openapiv3 template renderer. If
+	// `EnableOpenAPIV3` is disabled, this does nothing
+	OutputFormat string
+
+	// Client capable of fetching openapi documents from the user's cluster
+	DiscoveryClient discovery.DiscoveryInterface
 }
 
 func NewExplainOptions(parent string, streams genericclioptions.IOStreams) *ExplainOptions {
 	return &ExplainOptions{
-		IOStreams: streams,
+		IOStreams:       streams,
-		CmdParent: parent,
+		CmdParent:       parent,
+		EnableOpenAPIV3: os.Getenv("KUBECTL_EXPLAIN_OPENAPIV3") == "true",
+		OutputFormat:    "plaintext",
 	}
 }
 
@@ -89,6 +105,12 @@ func NewCmdExplain(parent string, f cmdutil.Factory, streams genericclioptions.I
 	}
 	cmd.Flags().BoolVar(&o.Recursive, "recursive", o.Recursive, "Print the fields of fields (Currently only 1 level deep)")
 	cmd.Flags().StringVar(&o.APIVersion, "api-version", o.APIVersion, "Get different explanations for particular API version (API group/version)")
+
+	// Only enable --output as a valid flag if the feature is enabled
+	if o.EnableOpenAPIV3 {
+		cmd.Flags().StringVar(&o.OutputFormat, "output", o.OutputFormat, "Format in which to render the schema")
+	}
+
 	return cmd
 }
 
@@ -104,6 +126,15 @@ func (o *ExplainOptions) Complete(f cmdutil.Factory, cmd *cobra.Command, args []
 		return err
 	}
 
+	// Only openapi v3 needs the discovery client.
+	if o.EnableOpenAPIV3 {
+		clientset, err := f.KubernetesClientSet()
+		if err != nil {
+			return err
+		}
+		o.DiscoveryClient = clientset.DiscoveryClient
+	}
+
 	o.args = args
 	return nil
 }
@@ -142,6 +173,17 @@ func (o *ExplainOptions) Run() error {
 		}
 	}
 
+	if o.EnableOpenAPIV3 {
+		return explainv2.PrintModelDescription(
+			fieldsPath,
+			o.Out,
+			o.DiscoveryClient.OpenAPIV3(),
+			fullySpecifiedGVR,
+			recursive,
+			o.OutputFormat,
+		)
+	}
+
 	gvk, _ := o.Mapper.KindFor(fullySpecifiedGVR)
 	if gvk.Empty() {
 		gvk, err = o.Mapper.KindFor(fullySpecifiedGVR.GroupResource().WithVersion(""))

staging/src/k8s.io/kubectl/pkg/explain/v2/explain.go

@@ -0,0 +1,84 @@
+/*
+Copyright 2022 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package v2
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/client-go/openapi"
+)
+
+// PrintModelDescription prints the description of a specific model or dot path.
+// If recursive, all components nested within the fields of the schema will be
+// printed.
+func PrintModelDescription(
+	fieldsPath []string,
+	w io.Writer,
+	client openapi.Client,
+	gvr schema.GroupVersionResource,
+	recursive bool,
+	outputFormat string,
+) error {
+	return printModelDescriptionWithGenerator(
+		NewGenerator(), fieldsPath, w, client, gvr, recursive, outputFormat)
+}
+
+// Factored out for testability
+func printModelDescriptionWithGenerator(
+	generator *generator,
+	fieldsPath []string,
+	w io.Writer,
+	client openapi.Client,
+	gvr schema.GroupVersionResource,
+	recursive bool,
+	outputFormat string,
+) error {
+	paths, err := client.Paths()
+
+	if err != nil {
+		return fmt.Errorf("failed to fetch list of groupVersions: %w", err)
+	}
+
+	var resourcePath string
+	if len(gvr.Group) == 0 {
+		resourcePath = fmt.Sprintf("api/%s", gvr.Version)
+	} else {
+		resourcePath = fmt.Sprintf("apis/%s/%s", gvr.Group, gvr.Version)
+	}
+
+	gv, exists := paths[resourcePath]
+
+	if !exists {
+		return fmt.Errorf("could not locate schema for %s", resourcePath)
+	}
+
+	openAPISchemaBytes, err := gv.Schema(runtime.ContentTypeJSON)
+	if err != nil {
+		return fmt.Errorf("failed to fetch openapi schema for %s: %w", resourcePath, err)
+	}
+
+	var parsedV3Schema map[string]interface{}
+	if err := json.Unmarshal(openAPISchemaBytes, &parsedV3Schema); err != nil {
+		return fmt.Errorf("failed to parse openapi schema for %s: %w", resourcePath, err)
+	}
+
+	return generator.Render(outputFormat, parsedV3Schema, gvr, fieldsPath, recursive, w)
+}