fix the fake client example: how to handle a race between the fake client and informer

Note that the fake client isn't designed to work with informer. It
doesn't support resource version. It's encouraged to use a real client
in an integration/E2E test if you need to test complex behavior with
informer/controllers.

staging/src/k8s.io/client-go/examples/fake-client/main_test.go

@@ -21,11 +21,13 @@ import (
 	"testing"
 	"time"
 
-	"k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/util/wait"
+	"k8s.io/apimachinery/pkg/watch"
 	"k8s.io/client-go/informers"
 	"k8s.io/client-go/kubernetes/fake"
+	clienttesting "k8s.io/client-go/testing"
 	"k8s.io/client-go/tools/cache"
 )
 
@@ -34,8 +36,20 @@ func TestFakeClient(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
+	watcherStarted := make(chan struct{})
 	// Create the fake client.
 	client := fake.NewSimpleClientset()
+	// A catch-all watch reactor that allows us to inject the watcherStarted channel.
+	client.PrependWatchReactor("*", func(action clienttesting.Action) (handled bool, ret watch.Interface, err error) {
+		gvr := action.GetResource()
+		ns := action.GetNamespace()
+		watch, err := client.Tracker().Watch(gvr, ns)
+		if err != nil {
+			return false, nil, err
+		}
+		close(watcherStarted)
+		return true, watch, nil
+	})
 
 	// We will create an informer that writes added pods to a channel.
 	pods := make(chan *v1.Pod, 1)
@@ -57,6 +71,15 @@ func TestFakeClient(t *testing.T) {
 	// we send any events to it.
 	cache.WaitForCacheSync(ctx.Done(), podInformer.HasSynced)
 
+	// The fake client doesn't support resource version. Any writes to the client
+	// after the informer's initial LIST and before the informer establishing the
+	// watcher will be missed by the informer. Therefore we wait until the watcher
+	// starts.
+	// Note that the fake client isn't designed to work with informer. It
+	// doesn't support resource version. It's encouraged to use a real client
+	// in an integration/E2E test if you need to test complex behavior with
+	// informer/controllers.
+	<-watcherStarted
 	// Inject an event into the fake client.
 	p := &v1.Pod{ObjectMeta: metav1.ObjectMeta{Name: "my-pod"}}
 	_, err := client.CoreV1().Pods("test-ns").Create(context.TODO(), p, metav1.CreateOptions{})