fixup! refactor: set CoreDNS based on struct not nil

Author: dkoshkin

docs/content/customization/generic/dns.md

@@ -48,3 +48,32 @@ Applying this configuration will result in the following value being set:
             imageRepository: "my-registry.io/my-org/my-repo"
             imageTag: "v1.11.3_custom.0"
     ```
+
+The CoreDNS version can also be updated automatically. To do this, set `coreDNS` to an empty object:
+
+```yaml
+apiVersion: cluster.x-k8s.io/v1beta1
+kind: Cluster
+metadata:
+  name: <NAME>
+spec:
+  topology:
+    variables:
+      - name: clusterConfig
+        value:
+          dns:
+            coreDNS: {}
+```
+
+Applying this configuration will result in the following value being set,
+with the version of the CoreDNS image being set based on the cluster's Kubernetes version:
+
+- `KubeadmControlPlaneTemplate`:
+
+  - ```yaml
+    spec:
+      kubeadmConfigSpec:
+        clusterConfiguration:
+          dns:
+            imageTag: "v1.11.3"
+    ```

pkg/handlers/generic/mutation/coredns/inject.go

@@ -71,10 +71,11 @@ func (h *coreDNSPatchHandler) Mutate(
 		h.variableFieldPath...,
 	)
 	if err != nil {
-		if !variables.IsNotFoundError(err) {
-			return err
+		if variables.IsNotFoundError(err) {
+			log.V(5).Info("coreDNS variable not defined")
+			return nil
 		}
-		log.V(5).Info("coreDNS variable not defined")
+		return err
 	}
 
 	log = log.WithValues(

pkg/handlers/generic/mutation/coredns/inject_test.go

@@ -46,7 +46,34 @@ var _ = Describe("Generate CoreDNS patches", func() {
 	testDefs := []testObj{
 		{
 			patchTest: capitest.PatchTestDef{
-				Name:        "unset variable",
+				Name: "unset variable",
+			},
+			cluster: clusterv1.Cluster{
+				ObjectMeta: metav1.ObjectMeta{
+					Name:      "test-cluster",
+					Namespace: request.Namespace,
+					Labels: map[string]string{
+						clusterv1.ProviderNameLabel: "nutanix",
+					},
+				},
+				Spec: clusterv1.ClusterSpec{
+					Topology: &clusterv1.Topology{
+						Version: "1.30.100",
+					},
+				},
+			},
+		},
+		{
+			patchTest: capitest.PatchTestDef{
+				Name: "variable with defaults",
+				Vars: []runtimehooksv1.Variable{
+					capitest.VariableWithValue(
+						v1alpha1.ClusterConfigVariableName,
+						v1alpha1.CoreDNS{},
+						v1alpha1.DNSVariableName,
+						VariableName,
+					),
+				},
 				RequestItem: request.NewKubeadmControlPlaneTemplateRequestItem(""),
 				ExpectedPatchMatchers: []capitest.JSONPatchMatcher{{
 					Operation: "add",
@@ -205,8 +232,16 @@ var _ = Describe("Generate CoreDNS patches", func() {
 		},
 		{
 			patchTest: capitest.PatchTestDef{
-				Name:            "error if cannot find default CoreDNS version",
-				RequestItem:     request.NewKubeadmControlPlaneTemplateRequestItem(""),
+				Name:        "error if cannot find default CoreDNS version",
+				RequestItem: request.NewKubeadmControlPlaneTemplateRequestItem(""),
+				Vars: []runtimehooksv1.Variable{
+					capitest.VariableWithValue(
+						v1alpha1.ClusterConfigVariableName,
+						v1alpha1.CoreDNS{},
+						v1alpha1.DNSVariableName,
+						VariableName,
+					),
+				},
 				ExpectedFailure: true,
 			},
 			cluster: clusterv1.Cluster{