client-go/reflector: make UseWatchList a pointer

until #115478(use streaming against the etcd storage)
is resolved the cacher need a way to disable the streaming.

Kubernetes-commit: 41e706600aea7468f486150d951d3b8948ce89d5

tools/cache/reflector.go

@@ -43,6 +43,7 @@ import (
 	"k8s.io/klog/v2"
 	"k8s.io/utils/clock"
 	"k8s.io/utils/pointer"
+	"k8s.io/utils/ptr"
 	"k8s.io/utils/trace"
 )
 
@@ -107,7 +108,9 @@ type Reflector struct {
 	// might result in an increased memory consumption of the APIServer.
 	//
 	// See https://github.com/kubernetes/enhancements/tree/master/keps/sig-api-machinery/3157-watch-list#design-details
-	UseWatchList bool
+	//
+	// TODO(#115478): Consider making reflector.UseWatchList a private field. Since we implemented "api streaming" on the etcd storage layer it should work.
+	UseWatchList *bool
 }
 
 // ResourceVersionUpdater is an interface that allows store implementation to
@@ -237,8 +240,12 @@ func NewReflectorWithOptions(lw ListerWatcher, expectedType interface{}, store S
 		r.expectedGVK = getExpectedGVKFromObject(expectedType)
 	}
 
-	if s := os.Getenv("ENABLE_CLIENT_GO_WATCH_LIST_ALPHA"); len(s) > 0 {
-		r.UseWatchList = true
+	// don't overwrite UseWatchList if already set
+	// because the higher layers (e.g. storage/cacher) disabled it on purpose
+	if r.UseWatchList == nil {
+		if s := os.Getenv("ENABLE_CLIENT_GO_WATCH_LIST_ALPHA"); len(s) > 0 {
+			r.UseWatchList = ptr.To(true)
+		}
 	}
 
 	return r
@@ -325,9 +332,10 @@ func (r *Reflector) ListAndWatch(stopCh <-chan struct{}) error {
 	klog.V(3).Infof("Listing and watching %v from %s", r.typeDescription, r.name)
 	var err error
 	var w watch.Interface
-	fallbackToList := !r.UseWatchList
+	useWatchList := ptr.Deref(r.UseWatchList, false)
+	fallbackToList := !useWatchList
 
-	if r.UseWatchList {
+	if useWatchList {
 		w, err = r.watchList(stopCh)
 		if w == nil && err == nil {
 			// stopCh was closed