fix: return empty list instead of nil to prevent panic. Fixes #25189 (#25192)

Signed-off-by: Ivan Pedersen <ivan.pedersen@volvocars.com>

gitops-engine/pkg/cache/cluster.go

@@ -606,6 +606,10 @@ func (c *clusterCache) listResources(ctx context.Context, resClient dynamic.Reso
 					retryCount++
 					c.log.Info(fmt.Sprintf("Error while listing resources: %v (try %d/%d)", ierr, retryCount, c.listRetryLimit))
 				}
+				// Ensure res is never nil even when there's an error
+				if res == nil {
+					res = &unstructured.UnstructuredList{}
+				}
 				//nolint:wrapcheck // wrap outside the retry
 				return ierr
 			}

gitops-engine/pkg/cache/cluster_nil_fix_test.go

@@ -0,0 +1,109 @@
+package cache
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"golang.org/x/sync/semaphore"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apimachinery/pkg/types"
+	"k8s.io/apimachinery/pkg/watch"
+	"k8s.io/client-go/dynamic"
+	"k8s.io/client-go/tools/pager"
+	"k8s.io/klog/v2/textlogger"
+)
+
+// mockResourceInterface simulates a dynamic resource interface that returns (nil, error)
+type mockResourceInterface struct{}
+
+func (m *mockResourceInterface) Create(ctx context.Context, obj *unstructured.Unstructured, options metav1.CreateOptions, subresources ...string) (*unstructured.Unstructured, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) Update(ctx context.Context, obj *unstructured.Unstructured, options metav1.UpdateOptions, subresources ...string) (*unstructured.Unstructured, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) UpdateStatus(ctx context.Context, obj *unstructured.Unstructured, options metav1.UpdateOptions) (*unstructured.Unstructured, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) Delete(ctx context.Context, name string, options metav1.DeleteOptions, subresources ...string) error {
+	return errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) DeleteCollection(ctx context.Context, options metav1.DeleteOptions, listOptions metav1.ListOptions) error {
+	return errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) Get(ctx context.Context, name string, options metav1.GetOptions, subresources ...string) (*unstructured.Unstructured, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) List(ctx context.Context, opts metav1.ListOptions) (*unstructured.UnstructuredList, error) {
+	// This simulates the problematic behavior that caused the original panic:
+	// returning (nil, error) instead of (emptyList, error)
+	return nil, errors.New("simulated list failure")
+}
+
+func (m *mockResourceInterface) Watch(ctx context.Context, opts metav1.ListOptions) (watch.Interface, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) Patch(ctx context.Context, name string, pt types.PatchType, data []byte, options metav1.PatchOptions, subresources ...string) (*unstructured.Unstructured, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) Apply(ctx context.Context, name string, obj *unstructured.Unstructured, options metav1.ApplyOptions, subresources ...string) (*unstructured.Unstructured, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) ApplyStatus(ctx context.Context, name string, obj *unstructured.Unstructured, options metav1.ApplyOptions) (*unstructured.Unstructured, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *mockResourceInterface) Namespace(string) dynamic.ResourceInterface {
+	return m
+}
+
+// TestListResourcesNilPointerFix tests that our fix prevents panic when
+// resClient.List() returns (nil, error)
+func TestListResourcesNilPointerFix(t *testing.T) {
+	// Create a cluster cache with proper configuration
+	cache := &clusterCache{
+		listSemaphore:       semaphore.NewWeighted(1),
+		listPageSize:        100,
+		listPageBufferSize:  1,
+		listRetryLimit:      1,
+		listRetryUseBackoff: false,
+		listRetryFunc:       ListRetryFuncNever,
+		log:                 textlogger.NewLogger(textlogger.NewConfig()),
+	}
+
+	// Use our mock that returns (nil, error)
+	mockClient := &mockResourceInterface{}
+
+	// This should not panic even though mockClient.List() returns (nil, error)
+	_, err := cache.listResources(context.Background(), mockClient, func(listPager *pager.ListPager) error {
+		// The fix ensures the pager receives a non-nil UnstructuredList
+		// preventing panic in GetContinue()
+		return listPager.EachListItem(context.Background(), metav1.ListOptions{}, func(obj runtime.Object) error {
+			return nil
+		})
+	})
+
+	// We expect an error (wrapped), but no panic should occur
+	if err == nil {
+		t.Fatal("Expected error from listResources due to simulated failure")
+	}
+
+	// Check that the error is properly wrapped
+	if err.Error() != "failed to list resources: simulated list failure" {
+		t.Fatalf("Unexpected error message: %v", err.Error())
+	}
+
+	t.Log("Test passed: no panic occurred despite (nil, error) from resClient.List()")
+}