📖 update webhooks for core types to match controller-runtime v0.15

Signed-off-by: Mikko Ylinen <mikko.ylinen@intel.com>

docs/book/src/reference/webhook-for-core-types.md

@@ -7,47 +7,58 @@ There is an [example](https://github.com/kubernetes-sigs/controller-runtime/tree
 in controller-runtime.
 
 It is suggested to use kubebuilder to initialize a project, and then you can
-follow the steps below to add admission webhooks for core types.
+follow the steps below to add admission webhooks for core types. The example shows
+how to set up a mutating webhook following the controller-runtime's webhook builder.
 
-## Implement Your Handler
+## Implement Your Webhook
 
-You need to have your handler implements the
-[admission.Handler](https://pkg.go.dev/sigs.k8s.io/controller-runtime/pkg/webhook/admission?tab=doc#Handler)
+You need to have your webhook to implement the
+[admission.CustomDefaulter](https://pkg.go.dev/sigs.k8s.io/controller-runtime/pkg/webhook/admission?tab=doc#CustomDefaulter)
 interface.
 
 ```go
-type podAnnotator struct {
-	Client  client.Client
-	decoder *admission.Decoder
+type podAnnotator struct {}
+
+func (a *podAnnotator) setupWebhookWithManager(mgr ctrl.Manager) error {
+       return ctrl.NewWebhookManagedBy(mgr).
+               For(&corev1.Pod{}).
+               WithDefaulter(a).
+               Complete()
 }
 
-func (a *podAnnotator) Handle(ctx context.Context, req admission.Request) admission.Response {
-	pod := &corev1.Pod{}
-	err := a.decoder.Decode(req, pod)
-	if err != nil {
-		return admission.Errored(http.StatusBadRequest, err)
-	}
+func (a *podAnnotator) Default(ctx context.Context, obj runtime.Object) error {
+    log := logf.FromContext(ctx)
+    pod, ok := obj.(*corev1.Pod)
+    if !ok {
+        return fmt.Errorf("expected a Pod but got a %T", obj)
+    }
 
 	// mutate the fields in pod
+    if pod.Annotations == nil {
+        pod.Annotations = map[string]string{}
+    }
+    pod.Annotations["example-mutating-admission-webhook"] = "foo"
+
+    log.Info("Annotated pod")
 
-	marshaledPod, err := json.Marshal(pod)
-	if err != nil {
-		return admission.Errored(http.StatusInternalServerError, err)
-	}
-	return admission.PatchResponseFromRaw(req.Object.Raw, marshaledPod)
+	return nil
 }
 ```
 
 **Note**: in order to have controller-gen generate the webhook configuration for
 you, you need to add markers. For example,
-`// +kubebuilder:webhook:path=/mutate-v1-pod,mutating=true,failurePolicy=fail,groups="",resources=pods,verbs=create;update,versions=v1,name=mpod.kb.io`
+`// +kubebuilder:webhook:path=/mutate--v1-pod,mutating=true,failurePolicy=fail,groups="",resources=pods,verbs=create;update,versions=v1,name=mpod.kb.io`
+(for webhooks the `path` is of format `/mutate-<group>-<version>-<kind>`. Since this documentation focuses on the core types the group is an empty string).
 
 ## Update main.go
 
 Now you need to register your handler in the webhook server.
 
 ```go
-mgr.GetWebhookServer().Register("/mutate-v1-pod", &webhook.Admission{Handler: &podAnnotator{Client: mgr.GetClient()}})
+    if err := (&podAnnotator{}).setupWebhookWithManager(mgr); err != nil {
+        entryLog.Error(err, "unable to create webhook", "webhook", "Pod")
+        os.Exit(1)
+    }
 ```
 
 You need to ensure the path here match the path in the marker.
@@ -74,4 +85,4 @@ Deploying it is just like deploying a webhook server for CRD. You need to
 You can follow the [tutorial](/cronjob-tutorial/running.md).
 
 
-[cronjob-tutorial]: /cronjob-tutorial/cronjob-tutorial.md
+[cronjob-tutorial]: /cronjob-tutorial/cronjob-tutorial.md