DSC, DSCI: webhook: implement one instance enforcing

The webhook is written with the idea to handle both Create and
Update requests (configured in config/webhook/manifests.yaml), but
at the moment only duplication check on Create is implemented.

Implements the logic which is done now on reconcile time [1] (same
for DSCI).

It checks for 0 instances, 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

@@ -59,6 +59,56 @@ func (w *OpenDataHubWebhook) InjectClient(c client.Client) error {
 	return nil
 }
 
+func (w *OpenDataHubWebhook) checkDupCreation(ctx context.Context, req admission.Request) admission.Response {
+	var listLen int
+	var err error
+
+	if req.Operation != admissionv1.Create {
+		return admission.Allowed(fmt.Sprintf("duplication check: skipping %v request", req.Operation))
+	}
+
+	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 king", "king", 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 {
+		return admission.Denied(fmt.Sprintf("Only one instance of %s object is allowed", req.Kind.Kind))
+	}
+
+	return admission.Allowed(fmt.Sprintf("%s duplication check passed", req.Kind.Kind))
+}
+
 func (w *OpenDataHubWebhook) Handle(ctx context.Context, req admission.Request) admission.Response {
-	return admission.ValidationResponse(true, "")
+	var resp admission.Response
+
+	// Handle only Create and Update
+	if req.Operation == admissionv1.Delete || req.Operation == admissionv1.Connect {
+		msg := fmt.Sprintf("ODH skipping %v request", req.Operation)
+		log.Info(msg)
+		return admission.Allowed(msg)
+	}
+
+	resp = w.checkDupCreation(ctx, req)
+	if !resp.Allowed {
+		return resp
+	}
+
+	return admission.Allowed(fmt.Sprintf("%s allowed", req.Kind.Kind))
 }