From 181db664a659df0ca68e874c6ccd14bbe755cccc Mon Sep 17 00:00:00 2001
From: Philipp Matthes <p.matthes@sap.com>
Date: Mon, 9 Feb 2026 11:27:35 +0100
Subject: [PATCH] Fix multicluster indexing

---
 docs/guides/multicluster/readme.md            |   4 +
 internal/scheduling/explanation/controller.go |  19 ++-
 pkg/multicluster/client.go                    |  26 +++++
 pkg/multicluster/client_test.go               | 110 ++++++++++++++++++
 4 files changed, 148 insertions(+), 11 deletions(-)

diff --git a/docs/guides/multicluster/readme.md b/docs/guides/multicluster/readme.md
index 99b1ed0cd..2b421b443 100644
--- a/docs/guides/multicluster/readme.md
+++ b/docs/guides/multicluster/readme.md
@@ -79,6 +79,10 @@ tee $TILT_OVERRIDES_PATH <<EOF
 global:
   conf:
     apiServerOverrides:
+    - gvk: cortex.cloud/v1alpha1/DecisionList
+      host: https://host.docker.internal:8444
+      caCert: |
+$(cat /tmp/root-ca-remote.pem | sed 's/^/        /')
     - gvk: cortex.cloud/v1alpha1/Decision
       host: https://host.docker.internal:8444
       caCert: |
diff --git a/internal/scheduling/explanation/controller.go b/internal/scheduling/explanation/controller.go
index 720b8aa17..1c7edae15 100644
--- a/internal/scheduling/explanation/controller.go
+++ b/internal/scheduling/explanation/controller.go
@@ -188,18 +188,15 @@ func (c *Controller) StartupCallback(ctx context.Context) error {
 // This function sets up the controller with the provided manager.
 func (c *Controller) SetupWithManager(mgr manager.Manager, mcl *multicluster.Client) error {
 	if !c.SkipIndexFields {
-		gvk, err := mcl.GVKFromHomeScheme(&v1alpha1.Decision{})
-		if err != nil {
-			return err
+		ctx := context.Background()
+		obj := &v1alpha1.Decision{}
+		lst := &v1alpha1.DecisionList{}
+		idx := "spec.resourceID"
+		fnc := func(obj client.Object) []string {
+			decision := obj.(*v1alpha1.Decision)
+			return []string{decision.Spec.ResourceID}
 		}
-		cluster := mcl.ClusterForResource(gvk)
-		if err := cluster.GetCache().IndexField(
-			context.Background(), &v1alpha1.Decision{}, "spec.resourceID",
-			func(obj client.Object) []string {
-				decision := obj.(*v1alpha1.Decision)
-				return []string{decision.Spec.ResourceID}
-			},
-		); err != nil {
+		if err := mcl.IndexField(ctx, obj, lst, idx, fnc); err != nil {
 			return err
 		}
 	}
diff --git a/pkg/multicluster/client.go b/pkg/multicluster/client.go
index fb2cd3cf6..bcc8b6a2a 100644
--- a/pkg/multicluster/client.go
+++ b/pkg/multicluster/client.go
@@ -334,3 +334,29 @@ func (c *subResourceClient) Patch(ctx context.Context, obj client.Object, patch
 func (c *subResourceClient) Apply(ctx context.Context, obj runtime.ApplyConfiguration, opts ...client.SubResourceApplyOption) error {
 	return errors.New("apply operation is not supported in multicluster subresource client")
 }
+
+// Index a field for a resource in the matching cluster's cache.
+// Usually, you want to index the same field in both the object and list type,
+// as both would be mapped to individual clients based on their GVK.
+func (c *Client) IndexField(ctx context.Context, obj client.Object, list client.ObjectList, field string, extractValue client.IndexerFunc) error {
+	gvkObj, err := c.GVKFromHomeScheme(obj)
+	if err != nil {
+		return err
+	}
+	if err := c.ClusterForResource(gvkObj).
+		GetCache().
+		IndexField(ctx, obj, field, extractValue); err != nil {
+		return err
+	}
+	// Index the object in the list cluster as well.
+	gvkList, err := c.GVKFromHomeScheme(list)
+	if err != nil {
+		return err
+	}
+	if err := c.ClusterForResource(gvkList).
+		GetCache().
+		IndexField(ctx, obj, field, extractValue); err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/pkg/multicluster/client_test.go b/pkg/multicluster/client_test.go
index 8ee9373d7..4d815028a 100644
--- a/pkg/multicluster/client_test.go
+++ b/pkg/multicluster/client_test.go
@@ -12,6 +12,7 @@ import (
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
+	"sigs.k8s.io/controller-runtime/pkg/cache"
 	"sigs.k8s.io/controller-runtime/pkg/client"
 	"sigs.k8s.io/controller-runtime/pkg/client/fake"
 	"sigs.k8s.io/controller-runtime/pkg/cluster"
@@ -38,19 +39,62 @@ func (u *unknownType) DeepCopyObject() runtime.Object {
 	return &unknownType{TypeMeta: u.TypeMeta, ObjectMeta: u.ObjectMeta}
 }
 
+// fakeCache implements cache.Cache interface for testing IndexField.
+type fakeCache struct {
+	cache.Cache
+	indexFieldFunc func(ctx context.Context, obj client.Object, field string, extractValue client.IndexerFunc) error
+	// Track calls to IndexField for verification
+	indexFieldCalls []indexFieldCall
+	mu              sync.Mutex
+}
+
+type indexFieldCall struct {
+	obj   client.Object
+	field string
+}
+
+func (f *fakeCache) IndexField(ctx context.Context, obj client.Object, field string, extractValue client.IndexerFunc) error {
+	f.mu.Lock()
+	f.indexFieldCalls = append(f.indexFieldCalls, indexFieldCall{obj: obj, field: field})
+	f.mu.Unlock()
+	if f.indexFieldFunc != nil {
+		return f.indexFieldFunc(ctx, obj, field, extractValue)
+	}
+	return nil
+}
+
+func (f *fakeCache) getIndexFieldCalls() []indexFieldCall {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f.indexFieldCalls
+}
+
 // fakeCluster implements cluster.Cluster interface for testing.
 type fakeCluster struct {
 	cluster.Cluster
 	fakeClient client.Client
+	fakeCache  *fakeCache
 }
 
 func (f *fakeCluster) GetClient() client.Client {
 	return f.fakeClient
 }
 
+func (f *fakeCluster) GetCache() cache.Cache {
+	return f.fakeCache
+}
+
 func newFakeCluster(scheme *runtime.Scheme, objs ...client.Object) *fakeCluster {
 	return &fakeCluster{
 		fakeClient: fake.NewClientBuilder().WithScheme(scheme).WithObjects(objs...).Build(),
+		fakeCache:  &fakeCache{},
+	}
+}
+
+func newFakeClusterWithCache(scheme *runtime.Scheme, fakeCache *fakeCache, objs ...client.Object) *fakeCluster {
+	return &fakeCluster{
+		fakeClient: fake.NewClientBuilder().WithScheme(scheme).WithObjects(objs...).Build(),
+		fakeCache:  fakeCache,
 	}
 }
 
@@ -1449,3 +1493,69 @@ func TestClient_StatusAndSubResource_ErrorOnUnknownType(t *testing.T) {
 		t.Error("expected error for unknown type in subresource Patch")
 	}
 }
+
+// TestClient_IndexField_WithRemoteClusters tests IndexField with remote clusters configured.
+func TestClient_IndexField_WithRemoteClusters(t *testing.T) {
+	scheme := newTestScheme(t)
+
+	homeCache := &fakeCache{}
+	homeCluster := newFakeClusterWithCache(scheme, homeCache)
+
+	remoteObjCache := &fakeCache{}
+	remoteObjCluster := newFakeClusterWithCache(scheme, remoteObjCache)
+
+	remoteListCache := &fakeCache{}
+	remoteListCluster := newFakeClusterWithCache(scheme, remoteListCache)
+
+	objGVK := schema.GroupVersionKind{
+		Group:   "",
+		Version: "v1",
+		Kind:    "ConfigMap",
+	}
+	listGVK := schema.GroupVersionKind{
+		Group:   "",
+		Version: "v1",
+		Kind:    "ConfigMapList",
+	}
+
+	c := &Client{
+		HomeCluster: homeCluster,
+		HomeScheme:  scheme,
+		remoteClusters: map[schema.GroupVersionKind]cluster.Cluster{
+			objGVK:  remoteObjCluster,
+			listGVK: remoteListCluster,
+		},
+	}
+
+	ctx := context.Background()
+
+	obj := &corev1.ConfigMap{}
+	list := &corev1.ConfigMapList{}
+	field := "metadata.name"
+	extractValue := func(obj client.Object) []string {
+		return []string{obj.GetName()}
+	}
+
+	err := c.IndexField(ctx, obj, list, field, extractValue)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	// Verify IndexField was called on the remote object cluster's cache
+	objCalls := remoteObjCache.getIndexFieldCalls()
+	if len(objCalls) != 1 {
+		t.Errorf("expected 1 IndexField call on remote object cluster, got %d", len(objCalls))
+	}
+
+	// Verify IndexField was called on the remote list cluster's cache
+	listCalls := remoteListCache.getIndexFieldCalls()
+	if len(listCalls) != 1 {
+		t.Errorf("expected 1 IndexField call on remote list cluster, got %d", len(listCalls))
+	}
+
+	// Verify home cluster cache was NOT called
+	homeCalls := homeCache.getIndexFieldCalls()
+	if len(homeCalls) != 0 {
+		t.Errorf("expected 0 IndexField calls on home cluster, got %d", len(homeCalls))
+	}
+}