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
```

staging/src/k8s.io/client-go/tools/cache/shared_informer.go

@@ -459,6 +459,11 @@ func (s *sharedIndexInformer) Run(stopCh <-chan struct{}) {
 		klog.Warningf("The sharedIndexInformer has started, run more than once is not allowed")
 		return
 	}
+
+	func() {
+		s.startedLock.Lock()
+		defer s.startedLock.Unlock()
+
 		fifo := NewDeltaFIFOWithOptions(DeltaFIFOOptions{
 			KnownObjects:          s.indexer,
 			EmitDeltaTypeReplaced: true,
@@ -478,10 +483,6 @@ func (s *sharedIndexInformer) Run(stopCh <-chan struct{}) {
 			WatchErrorHandler: s.watchErrorHandler,
 		}
 
-	func() {
-		s.startedLock.Lock()
-		defer s.startedLock.Unlock()
-
 		s.controller = New(cfg)
 		s.controller.(*controller).clock = s.clock
 		s.started = true

staging/src/k8s.io/client-go/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()