converger: Wait till the timer exits

Wait till the timer loop exits, otherwise we might race and panic if
ConvergedTimer gets called after we've asked for an exit, but before it
does.

converger/converger.go

@@ -81,6 +81,7 @@ type convergerUID struct {
 	mutex     sync.Mutex
 	timer     chan struct{}
 	running   bool // is the above timer running?
+	wg        sync.WaitGroup
 }
 
 // NewConverger builds a new converger struct
@@ -335,7 +336,9 @@ func (obj *convergerUID) StartTimer() (func() error, error) {
 		return obj.StopTimer, fmt.Errorf("Timer already started!")
 	}
 	obj.mutex.Unlock()
+	obj.wg.Add(1)
 	go func() {
+		defer obj.wg.Done()
 		for {
 			select {
 			case _, ok := <-obj.timer: // reset signal channel
@@ -377,6 +380,7 @@ func (obj *convergerUID) StopTimer() error {
 		return fmt.Errorf("Timer isn't running!")
 	}
 	close(obj.timer)
+	obj.wg.Wait()
 	obj.running = false
 	return nil
 }