Reject duplicate keys on the notifier.

Signed-off-by: Burcu Dogan <jbd@google.com>

mend

chanotify/chanotify_test.go

@@ -13,8 +13,12 @@ func TestNotifier(t *testing.T) {
 	id1 := "1"
 	id2 := "2"
 
-	s.Add(id1, ch1)
-	s.Add(id2, ch2)
+	if err := s.Add(id1, ch1); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.Add(id2, ch2); err != nil {
+		t.Fatal(err)
+	}
 	s.m.Lock()
 	if len(s.doneCh) != 2 {
 		t.Fatalf("want 2 channels, got %d", len(s.doneCh))
@@ -43,7 +47,9 @@ func TestConcurrentNotifier(t *testing.T) {
 	var chs []chan struct{}
 	for i := 0; i < 8; i++ {
 		ch := make(chan struct{}, 2)
-		s.Add(i, ch)
+		if err := s.Add(i, ch); err != nil {
+			t.Fatal(err)
+		}
 		chs = append(chs, ch)
 	}
 	testCounter := make(map[interface{}]int)
@@ -86,10 +92,26 @@ func TestAddToBlocked(t *testing.T) {
 	go func() {
 		// give some time to start first select
 		time.Sleep(1 * time.Second)
-		s.Add(id, ch)
+		if err := s.Add(id, ch); err != nil {
+			t.Fatal(err)
+		}
 		ch <- struct{}{}
 	}()
 	if got, want := <-s.Chan(), id; got != want {
 		t.Fatalf("got %v; want %v", got, want)
 	}
 }
+
+func TestAddDuplicate(t *testing.T) {
+	s := New()
+	ch1 := make(chan struct{}, 1)
+	ch2 := make(chan struct{}, 1)
+
+	if err := s.Add(1, ch1); err != nil {
+		t.Fatalf("cannot add; err = %v", err)
+	}
+
+	if err := s.Add(1, ch2); err == nil {
+		t.Fatalf("duplicate keys are not allowed; but Add succeeded")
+	}
+}