From 4bb2bd05322e127658e55a06995c021b3c5fbc41 Mon Sep 17 00:00:00 2001
From: Lukasz Szaszkiewicz <lukasz.szaszkiewicz@gmail.com>
Date: Thu, 25 Sep 2025 15:36:17 +0200
Subject: [PATCH] client-go/tools/cache/reflector: supports
 watchlist.DoesClientNotSupportWatchListSemantics

---
 .../k8s.io/client-go/tools/cache/reflector.go | 11 ++++
 .../tools/cache/reflector_watchlist_test.go   | 56 +++++++++++++++++++
 2 files changed, 67 insertions(+)

diff --git a/staging/src/k8s.io/client-go/tools/cache/reflector.go b/staging/src/k8s.io/client-go/tools/cache/reflector.go
index 9b33e01ef2d..9cb76a2f5b6 100644
--- a/staging/src/k8s.io/client-go/tools/cache/reflector.go
+++ b/staging/src/k8s.io/client-go/tools/cache/reflector.go
@@ -41,6 +41,7 @@ import (
 	"k8s.io/apimachinery/pkg/watch"
 	clientfeatures "k8s.io/client-go/features"
 	"k8s.io/client-go/tools/pager"
+	"k8s.io/client-go/util/watchlist"
 	"k8s.io/klog/v2"
 	"k8s.io/utils/clock"
 	"k8s.io/utils/ptr"
@@ -297,6 +298,16 @@ func NewReflectorWithOptions(lw ListerWatcher, expectedType interface{}, store R
 	}
 
 	r.useWatchList = clientfeatures.FeatureGates().Enabled(clientfeatures.WatchListClient)
+	if r.useWatchList && watchlist.DoesClientNotSupportWatchListSemantics(lw) {
+		// Using klog.TODO() here because switching to a caller-provided contextual logger
+		// would require an API change and updating all existing call sites.
+		klog.TODO().V(2).Info(
+			"The provided ListWatcher doesn't support WatchList semantics. The feature will be disabled. If you are using a custom client, check the documentation of watchlist.DoesClientNotSupportWatchListSemantics() method",
+			"listWatcherType", fmt.Sprintf("%T", lw),
+			"feature", clientfeatures.WatchListClient,
+		)
+		r.useWatchList = false
+	}
 
 	return r
 }
diff --git a/staging/src/k8s.io/client-go/tools/cache/reflector_watchlist_test.go b/staging/src/k8s.io/client-go/tools/cache/reflector_watchlist_test.go
index 0a9260c4c75..50ebe3e620c 100644
--- a/staging/src/k8s.io/client-go/tools/cache/reflector_watchlist_test.go
+++ b/staging/src/k8s.io/client-go/tools/cache/reflector_watchlist_test.go
@@ -43,6 +43,62 @@ import (
 	"k8s.io/utils/ptr"
 )
 
+type lwSupportsWatchListSemantics struct{ fakeListWatcher }
+
+func (lw *lwSupportsWatchListSemantics) IsWatchListSemanticsUnSupported() bool { return false }
+
+type lwDoesNotSupportWatchListSemantics struct{ fakeListWatcher }
+
+func (lw *lwDoesNotSupportWatchListSemantics) IsWatchListSemanticsUnSupported() bool { return true }
+
+type lwNoWatchListSemanticsUnSupportedExposed struct{ fakeListWatcher }
+
+func TestNewReflectorWithDisablementWatchList(t *testing.T) {
+	scenarios := []struct {
+		name                    string
+		enableWatchListClientFG bool
+		lw                      ListerWatcher
+		expectUseWatchListValue bool
+	}{
+		{
+			name:                    "WatchListClient feature gate off, client supports WatchList semantics",
+			enableWatchListClientFG: false,
+			lw:                      &lwSupportsWatchListSemantics{},
+			expectUseWatchListValue: false,
+		},
+		{
+			name:                    "WatchListClient feature gate on, client supports WatchList semantics",
+			enableWatchListClientFG: true,
+			lw:                      &lwSupportsWatchListSemantics{},
+			expectUseWatchListValue: true,
+		},
+		{
+			name:                    "WatchListClient feature gate on, client doesn't support the WatchList semantics",
+			enableWatchListClientFG: true,
+			lw:                      &lwDoesNotSupportWatchListSemantics{},
+			expectUseWatchListValue: false,
+		},
+		{
+			name:                    "WatchListClient feature gate on, client doesn't expose the IsWatchListSemanticsUnSupported method",
+			enableWatchListClientFG: true,
+			lw:                      &lwNoWatchListSemanticsUnSupportedExposed{},
+			expectUseWatchListValue: true,
+		},
+	}
+
+	for _, scenario := range scenarios {
+		t.Run(scenario.name, func(t *testing.T) {
+			clientfeaturestesting.SetFeatureDuringTest(t, clientfeatures.WatchListClient, scenario.enableWatchListClientFG)
+
+			r := NewReflectorWithOptions(scenario.lw, struct{}{}, &fakeStore{}, ReflectorOptions{})
+
+			if r.useWatchList != scenario.expectUseWatchListValue {
+				t.Fatalf("got: %v, want: %v", r.useWatchList, scenario.expectUseWatchListValue)
+			}
+		})
+	}
+}
+
 func TestInitialEventsEndBookmarkTicker(t *testing.T) {
 	assertNoEvents := func(t *testing.T, c <-chan time.Time) {
 		select {