test: add test for #8312

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

test/e2e/main_test.go

@@ -321,6 +321,7 @@ func testAllNSPrometheus(t *testing.T) {
 		"PrometheusUTF8MetricsSupport":              testPrometheusUTF8MetricsSupport,
 		"PrometheusUTF8LabelSupport":                testPrometheusUTF8LabelSupport,
 		"StuckStatefulSetRollout":                   testStuckStatefulSetRollout,
+		"PromScaleUpWithoutLabels":                  testPromScaleUpWithoutLabels,
 	}
 
 	for name, f := range testFuncs {

test/e2e/prometheus_test.go

@@ -6111,3 +6111,33 @@ type prometheusAlertmanagerAPIResponse struct {
 	Status string                 `json:"status"`
 	Data   *alertmanagerDiscovery `json:"data"`
 }
+
+func testPromScaleUpWithoutLabels(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	testCtx := framework.NewTestCtx(t)
+	defer testCtx.Cleanup(t)
+	ns := framework.CreateNamespace(ctx, t, testCtx)
+	framework.SetupPrometheusRBAC(ctx, t, testCtx, ns)
+
+	name := "test"
+
+	// Create a Prometheus resource with 1 replica
+	p, err := framework.CreatePrometheusAndWaitUntilReady(ctx, ns, framework.MakeBasicPrometheus(ns, name, name, 1))
+	require.NoError(t, err)
+
+	// Remove all labels on the StatefulSet using Patch
+	stsName := fmt.Sprintf("prometheus-%s", name)
+	err = framework.RemoveAllLabelsFromStatefulSet(ctx, stsName, ns)
+	require.NoError(t, err)
+
+	// Scale up the Prometheus resource to 2 replicas
+	_, err = framework.UpdatePrometheusReplicasAndWaitUntilReady(ctx, p.Name, ns, 2)
+	require.NoError(t, err)
+
+	// Verify the StatefulSet now has labels again (restored by the operator)
+	stsClient := framework.KubeClient.AppsV1().StatefulSets(ns)
+	sts, err := stsClient.Get(ctx, stsName, metav1.GetOptions{})
+	require.NoError(t, err)
+	require.NotEmpty(t, sts.GetLabels(), "expected labels to be restored on the StatefulSet by the operator")
+}

test/framework/framework.go

@@ -859,3 +859,18 @@ func (f *Framework) CreateOrUpdateAdmissionWebhookServer(
 
 	return service, certBytes, nil
 }
+
+func removeLabelsPatch(labels ...string) ([]byte, error) {
+	type patch struct {
+		Op   string `json:"op"`
+		Path string `json:"path"`
+	}
+
+	var patches []patch
+	encoder := strings.NewReplacer("/", "~1", "~", "~0")
+	for _, label := range labels {
+		patches = append(patches, patch{Op: "remove", Path: "/metadata/labels/" + encoder.Replace(label)})
+	}
+
+	return json.Marshal(patches)
+}

test/framework/namespace.go

@@ -16,7 +16,6 @@ package framework
 
 import (
 	"context"
-	"encoding/json"
 	"maps"
 	"testing"
 
@@ -81,25 +80,7 @@ func (f *Framework) AddLabelsToNamespace(ctx context.Context, name string, addit
 }
 
 func (f *Framework) RemoveLabelsFromNamespace(ctx context.Context, name string, labels ...string) error {
-	ns, err := f.KubeClient.CoreV1().Namespaces().Get(ctx, name, metav1.GetOptions{})
-	if err != nil {
-		return err
-	}
-
-	if len(ns.Labels) == 0 {
-		return nil
-	}
-
-	type patch struct {
-		Op   string `json:"op"`
-		Path string `json:"path"`
-	}
-
-	var patches []patch
-	for _, l := range labels {
-		patches = append(patches, patch{Op: "remove", Path: "/metadata/labels/" + l})
-	}
-	b, err := json.Marshal(patches)
+	b, err := removeLabelsPatch(labels...)
 	if err != nil {
 		return err
 	}

test/framework/statefulset.go

@@ -0,0 +1,53 @@
+// Copyright 2026 The prometheus-operator Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package framework
+
+import (
+	"context"
+	"fmt"
+	"maps"
+	"slices"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/types"
+)
+
+// RemoveAllLabelsFromStatefulSet removes all labels from a StatefulSet using JSON Patch.
+func (f *Framework) RemoveAllLabelsFromStatefulSet(ctx context.Context, name, namespace string) error {
+	sts, err := f.KubeClient.AppsV1().StatefulSets(namespace).Get(ctx, name, metav1.GetOptions{})
+	if err != nil {
+		return fmt.Errorf("failed to get StatefulSet: %w", err)
+	}
+
+	if len(sts.Labels) == 0 {
+		return nil
+	}
+
+	b, err := removeLabelsPatch(slices.Sorted(maps.Keys(sts.GetLabels()))...)
+	if err != nil {
+		return fmt.Errorf("failed to marshal patch: %w", err)
+	}
+
+	updatedSts, err := f.KubeClient.AppsV1().StatefulSets(namespace).Patch(ctx, name, types.JSONPatchType, b, metav1.PatchOptions{})
+	if err != nil {
+		return fmt.Errorf("failed to patch StatefulSet: %w", err)
+	}
+
+	if len(updatedSts.Labels) != 0 {
+		return fmt.Errorf("expected all labels to be removed from StatefulSet, but got %d labels: %v", len(updatedSts.Labels), updatedSts.Labels)
+	}
+
+	return nil
+}