DSC, DSCI: webhook: implement one instance enforcing

The webhook handles only Create requests (configured in
config/webhook/manifests.yaml). Implements the logic which is done
now on reconcile time [1] (same for DSCI).

It checks for 0, not 1, since when the webhook is running the object
has not been created yet. Means if it's 1 then it handles request to
create a second one.

It could be probably possible to use generics but does not make a
lot of sense for such a simple case.

Closes: opendatahub-io#693

[1] https://github.com/opendatahub-io/opendatahub-operator/blob/incubation/controllers/datasciencecluster/datasciencecluster_controller.go#L98

Signed-off-by: Yauheni Kaliuta <ykaliuta@redhat.com>

controllers/webhook/webhook.go

@@ -59,5 +59,41 @@ func (w *OpenDataHubWebhook) InjectClient(c client.Client) error {
 }
 
 func (w *OpenDataHubWebhook) Handle(ctx context.Context, req admission.Request) admission.Response {
-	return admission.ValidationResponse(true, "")
+	var listLen int
+	var err error
+
+	if req.Operation != admissionv1.Create {
+		msg := fmt.Sprintf("ODH skipping %v request", req.Operation)
+		log.Info(msg)
+		return admission.Allowed(msg)
+	}
+
+	switch req.Kind.Kind {
+	case "DataScienceCluster":
+		instances := &dsc.DataScienceClusterList{}
+		err = w.client.List(ctx, instances)
+		listLen = len(instances.Items)
+
+	case "DSCInitialization":
+		instances := &dsci.DSCInitializationList{}
+		err = w.client.List(ctx, instances)
+		listLen = len(instances.Items)
+
+	default:
+		log.Info("Got wrong kind %s", req.Kind.Kind)
+		return admission.Errored(http.StatusBadRequest, nil)
+	}
+
+	if err != nil {
+		return admission.Errored(http.StatusBadRequest, err)
+	}
+
+	// if listLen == 1 now creation of #2 is being handled
+	if listLen > 0 {
+		msg := fmt.Sprintf("Only one instance of %s object is allowed", req.Kind.Kind)
+		return admission.Denied(msg)
+	}
+
+	msg := fmt.Sprintf("%s allowed", req.Kind.Kind)
+	return admission.Allowed(msg)
 }