Correctly handle empty watch event cache

staging/src/k8s.io/apiserver/pkg/storage/tests/cacher_test.go

@@ -537,6 +537,70 @@ func TestStartingResourceVersion(t *testing.T) {
 	}
 }
 
+func TestEmptyWatchEventCache(t *testing.T) {
+	server, etcdStorage := newEtcdTestStorage(t, etcdtest.PathPrefix())
+	defer server.Terminate(t)
+
+	// add a few objects
+	updatePod(t, etcdStorage, makeTestPod("pod1"), nil)
+	updatePod(t, etcdStorage, makeTestPod("pod2"), nil)
+	updatePod(t, etcdStorage, makeTestPod("pod3"), nil)
+	updatePod(t, etcdStorage, makeTestPod("pod4"), nil)
+	updatePod(t, etcdStorage, makeTestPod("pod5"), nil)
+
+	fooCreated := updatePod(t, etcdStorage, makeTestPod("foo"), nil)
+
+	// get rv of last pod created
+	rv, err := storage.ParseWatchResourceVersion(fooCreated.ResourceVersion)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	cacher := newTestCacher(etcdStorage, 10)
+	defer cacher.Stop()
+
+	// We now have a cacher with an empty cache of watch events and a resourceVersion of rv.
+	// It should support establishing watches from rv and higher, but not older.
+
+	{
+		watcher, err := cacher.Watch(context.TODO(), "pods/ns", strconv.Itoa(int(rv-1)), storage.Everything)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+		defer watcher.Stop()
+		expectedGoneError := errors.NewGone("").ErrStatus
+		verifyWatchEvent(t, watcher, watch.Error, &expectedGoneError)
+	}
+
+	{
+		watcher, err := cacher.Watch(context.TODO(), "pods/ns", strconv.Itoa(int(rv+1)), storage.Everything)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+		defer watcher.Stop()
+		select {
+		case e := <-watcher.ResultChan():
+			t.Errorf("unexpected event %#v", e)
+		case <-time.After(3 * time.Second):
+			// watch from rv+1 remained established successfully
+		}
+	}
+
+	{
+		watcher, err := cacher.Watch(context.TODO(), "pods/ns", strconv.Itoa(int(rv)), storage.Everything)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+		defer watcher.Stop()
+		select {
+		case e := <-watcher.ResultChan():
+			t.Errorf("unexpected event %#v", e)
+		case <-time.After(3 * time.Second):
+			// watch from rv remained established successfully
+		}
+	}
+}
+
 func TestRandomWatchDeliver(t *testing.T) {
 	server, etcdStorage := newEtcdTestStorage(t, etcdtest.PathPrefix())
 	defer server.Terminate(t)

staging/src/k8s.io/apiserver/pkg/storage/watch_cache.go

@@ -412,7 +412,9 @@ func (w *watchCache) SetOnEvent(onEvent func(*watchCacheEvent)) {
 
 func (w *watchCache) GetAllEventsSinceThreadUnsafe(resourceVersion uint64) ([]*watchCacheEvent, error) {
 	size := w.endIndex - w.startIndex
-	oldest := w.resourceVersion
+	// if we have no watch events in our cache, the oldest one we can successfully deliver to a watcher
+	// is the *next* event we'll receive, which will be at least one greater than our current resourceVersion
+	oldest := w.resourceVersion + 1
 	if size > 0 {
 		oldest = w.cache[w.startIndex%w.capacity].resourceVersion
 	}