informer: fix race against Run and SetTransform/SetWatchErrorHandler

`SetWatchErrorHandler` claims it will fail if Run() has already started.
But if they are called concurrently, it will actually trigger a data
race.

With this PR:
```
62702 runs so far, 0 failures (100.00% pass rate). 59.152682ms avg, 189.068387ms max, 26.623785ms min
```

Without this PR:
```
5012 runs so far, 38 failures (99.25% pass rate). 58.675502ms avg, 186.018084ms max, 29.468104ms min
```

Kubernetes-commit: 35d2431b3a89c5bd693846952e9d27ce4e3a0754

tools/cache/shared_informer_test.go

@@ -393,6 +393,33 @@ func TestSharedInformerErrorHandling(t *testing.T) {
 	close(stop)
 }
 
+// TestSharedInformerStartRace is a regression test to ensure there is no race between
+// Run and SetWatchErrorHandler, and Run and SetTransform.
+func TestSharedInformerStartRace(t *testing.T) {
+	source := fcache.NewFakeControllerSource()
+	informer := NewSharedInformer(source, &v1.Pod{}, 1*time.Second).(*sharedIndexInformer)
+	stop := make(chan struct{})
+	go func() {
+		for {
+			select {
+			case <-stop:
+				return
+			default:
+			}
+			// Set dummy functions, just to test for race
+			informer.SetTransform(func(i interface{}) (interface{}, error) {
+				return i, nil
+			})
+			informer.SetWatchErrorHandler(func(r *Reflector, err error) {
+			})
+		}
+	}()
+
+	go informer.Run(stop)
+
+	close(stop)
+}
+
 func TestSharedInformerTransformer(t *testing.T) {
 	// source simulates an apiserver object endpoint.
 	source := fcache.NewFakeControllerSource()