fix notifier for backward compatibility

common/signal/notifier.go

@@ -5,7 +5,8 @@ import "sync"
 // Notifier is a utility for notifying changes. The change producer may notify changes multiple time, and the consumer may get notified asynchronously.
 type Notifier struct {
 	sync.Mutex
-	waiters []chan struct{}
+	waiters    []chan struct{}
+	notCosumed bool
 }
 
 // NewNotifier creates a new Notifier.
@@ -16,19 +17,31 @@ func NewNotifier() *Notifier {
 // Signal signals a change, usually by producer. This method never blocks.
 func (n *Notifier) Signal() {
 	n.Lock()
+	defer n.Unlock()
+
+	if len(n.waiters) == 0 {
+		n.notCosumed = true
+		return
+	}
+
 	for _, w := range n.waiters {
 		close(w)
 	}
 	n.waiters = make([]chan struct{}, 0, 8)
-	n.Unlock()
 }
 
-// Wait returns a channel for waiting for changes. The returned channel never gets closed.
+// Wait returns a channel for waiting for changes.
 func (n *Notifier) Wait() <-chan struct{} {
 	n.Lock()
 	defer n.Unlock()
 
 	w := make(chan struct{})
+	if n.notCosumed {
+		n.notCosumed = false
+		close(w)
+		return w
+	}
+
 	n.waiters = append(n.waiters, w)
 	return w
 }