fix matchClusterConfiguration

controlplane/kubeadm/internal/machinefilters/machine_filters.go

@@ -277,8 +277,14 @@ func matchClusterConfiguration(kcp *controlplanev1.KubeadmControlPlane, machine
 	if ok {
 		machineClusterConfig := &kubeadmv1.ClusterConfiguration{}
 		// ClusterConfiguration annotation is not correct, only solution is to rollout.
-		if err := json.Unmarshal([]byte(machineClusterConfigStr), machineClusterConfig); err != nil {
-			return false
+		// NOTE: it is required to handle a nil ClusterConfiguration (that is serialized into "null") as a special case,
+		// because Unmarshal returns &kubeadmv1.ClusterConfiguration{}, not nil; see https://github.com/kubernetes-sigs/cluster-api/issues/3353
+		if machineClusterConfigStr != "null" {
+			if err := json.Unmarshal([]byte(machineClusterConfigStr), machineClusterConfig); err != nil {
+				return false
+			}
+		} else {
+			machineClusterConfig = nil
 		}
 		return reflect.DeepEqual(machineClusterConfig, kcp.Spec.KubeadmConfigSpec.ClusterConfiguration)
 	}

controlplane/kubeadm/internal/machinefilters/util_test.go

@@ -90,6 +90,22 @@ func TestMatchClusterConfiguration(t *testing.T) {
 		}
 		g.Expect(matchClusterConfiguration(kcp, m)).To(gomega.BeFalse())
 	})
+	t.Run("Return true if cluster configuration is nil (special case)", func(t *testing.T) {
+		g := gomega.NewWithT(t)
+		kcp := &controlplanev1.KubeadmControlPlane{
+			Spec: controlplanev1.KubeadmControlPlaneSpec{
+				KubeadmConfigSpec: bootstrapv1.KubeadmConfigSpec{},
+			},
+		}
+		m := &clusterv1.Machine{
+			ObjectMeta: metav1.ObjectMeta{
+				Annotations: map[string]string{
+					controlplanev1.KubeadmClusterConfigurationAnnotation: "null",
+				},
+			},
+		}
+		g.Expect(matchClusterConfiguration(kcp, m)).To(gomega.BeTrue())
+	})
 }
 
 func TestGetAdjustedKcpConfig(t *testing.T) {