(fix): Preserve immutable fields during upgrade (llamastack#131)

During upgrades, if we do server-side apply without preserving the selectors field, we get 'field is immutable' error which blocks deployment updates and also causes continuous operator reconciliation. Preserving existing selector values fixes the issue.

Approved-by: rhdedgar

Approved-by: mfleader
(cherry picked from commit e2cc886)

Author: VaishnaviHire

pkg/deploy/deploy.go

@@ -35,7 +35,7 @@ func ApplyDeployment(ctx context.Context, cli client.Client, scheme *runtime.Sch
 	if !reflect.DeepEqual(found.Spec, deployment.Spec) {
 		logger.Info("Updating Deployment", "deployment", deployment.Name)
 
-		// Preserve the existing selector to avoid immutable field error
+		// Preserve the existing selector to avoid immutable field error during upgrades
 		deployment.Spec.Selector = found.Spec.Selector
 
 		// Use server-side apply to merge changes properly

pkg/deploy/deploy_test.go

@@ -0,0 +1,106 @@
+package deploy
+
+import (
+	"context"
+	"testing"
+
+	llamav1alpha1 "github.com/llamastack/llama-stack-k8s-operator/api/v1alpha1"
+	"github.com/stretchr/testify/require"
+	appsv1 "k8s.io/api/apps/v1"
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/types"
+	logf "sigs.k8s.io/controller-runtime/pkg/log"
+)
+
+func TestApplyDeploymentPreservesSelector(t *testing.T) {
+	ctx := context.Background()
+	logger := logf.Log.WithName("test-apply-deployment")
+
+	instance := &llamav1alpha1.LlamaStackDistribution{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      "test-instance",
+			Namespace: "default",
+			UID:       "test-uid",
+		},
+	}
+
+	deploymentName := "test-deployment-selector"
+	namespace := "default"
+
+	// Initial deployment with a specific selector
+	initialDeployment := &appsv1.Deployment{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      deploymentName,
+			Namespace: namespace,
+		},
+		Spec: appsv1.DeploymentSpec{
+			Selector: &metav1.LabelSelector{
+				MatchLabels: map[string]string{"app": "initial"},
+			},
+			Template: corev1.PodTemplateSpec{
+				ObjectMeta: metav1.ObjectMeta{
+					Labels: map[string]string{"app": "initial"},
+				},
+				Spec: corev1.PodSpec{
+					Containers: []corev1.Container{
+						{
+							Name:  "llamastack",
+							Image: "quay.io/llamastack/llama-stack-k8s-operator:v0.0.1",
+						},
+					},
+				},
+			},
+		},
+	}
+
+	err := ApplyDeployment(ctx, k8sClient, k8sClient.Scheme(), instance, initialDeployment.DeepCopy(), logger)
+	require.NoError(t, err)
+
+	// Verify the deployment was created
+	foundDeployment := &appsv1.Deployment{}
+	err = k8sClient.Get(ctx, types.NamespacedName{Name: deploymentName, Namespace: namespace}, foundDeployment)
+	require.NoError(t, err)
+	require.NotNil(t, foundDeployment.Spec.Selector)
+	require.Equal(t, "initial", foundDeployment.Spec.Selector.MatchLabels["app"])
+
+	// Updated deployment with changes.
+	// The fix should preserve the original selector.
+	updatedDeployment := &appsv1.Deployment{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      deploymentName,
+			Namespace: namespace,
+		},
+		Spec: appsv1.DeploymentSpec{
+			Selector: &metav1.LabelSelector{
+				MatchLabels: map[string]string{"app": "initial"}, // Must match existing selector
+			},
+			Template: corev1.PodTemplateSpec{
+				ObjectMeta: metav1.ObjectMeta{
+					Labels: map[string]string{"app": "initial"},
+				},
+				Spec: corev1.PodSpec{
+					Containers: []corev1.Container{
+						{
+							Name:  "llamastack",
+							Image: "quay.io/llamastack/llama-stack-k8s-operator:v0.0.2",
+						},
+					},
+				},
+			},
+		},
+	}
+
+	err = ApplyDeployment(ctx, k8sClient, k8sClient.Scheme(), instance, updatedDeployment.DeepCopy(), logger)
+	require.NoError(t, err)
+
+	err = k8sClient.Get(ctx, types.NamespacedName{Name: deploymentName, Namespace: namespace}, foundDeployment)
+	require.NoError(t, err)
+
+	// The selector should be preserved from the initial deployment
+	require.NotNil(t, foundDeployment.Spec.Selector)
+	require.Equal(t, "initial", foundDeployment.Spec.Selector.MatchLabels["app"])
+
+	// And the other updates should be applied
+	require.Equal(t, "quay.io/llamastack/llama-stack-k8s-operator:v0.0.2", foundDeployment.Spec.Template.Spec.Containers[0].Image)
+}