Drain callback channel when try to deregister it

callback() is blocked on sending into channel otherwise.

rpc.go

@@ -191,10 +191,10 @@ func (l *Libvirt) listen() {
 func (l *Libvirt) callback(id uint32, res response) {
 	l.cm.Lock()
 	c, ok := l.callbacks[id]
-	l.cm.Unlock()
 	if ok {
 		c <- res
 	}
+	l.cm.Unlock()
 }
 
 // route sends incoming packets to their listeners.
@@ -281,7 +281,19 @@ func (l *Libvirt) register(id uint32, c chan response) {
 
 // deregister destroys a method response callback
 func (l *Libvirt) deregister(id uint32) {
+	lockChan := make(chan bool)
+	go func() {
 		l.cm.Lock()
+		lockChan <- true
+	}()
+Loop:
+	for {
+		select {
+		case _ = <-l.callbacks[id]:
+		case _ = <-lockChan:
+			break Loop
+		}
+	}
 	close(l.callbacks[id])
 	delete(l.callbacks, id)
 	l.cm.Unlock()