etcdserver: fix racey StopNotify

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

etcdserver/server.go

@@ -197,7 +197,8 @@ type EtcdServer struct {
 	// stopping is closed by run goroutine on shutdown.
 	stopping chan struct{}
 	// done is closed when all goroutines from start() complete.
-	done chan struct{}
+	doneMu sync.Mutex
+	done   chan struct{}
 
 	errorc     chan error
 	id         types.ID
@@ -549,7 +550,9 @@ func (s *EtcdServer) start() {
 	}
 	s.w = wait.New()
 	s.applyWait = wait.NewTimeList()
+	s.doneMu.Lock()
 	s.done = make(chan struct{})
+	s.doneMu.Unlock()
 	s.stop = make(chan struct{})
 	s.stopping = make(chan struct{})
 	s.ctx, s.cancel = context.WithCancel(context.Background())
@@ -1043,7 +1046,11 @@ func (s *EtcdServer) stopWithDelay(d time.Duration, err error) {
 
 // StopNotify returns a channel that receives a empty struct
 // when the server is stopped.
-func (s *EtcdServer) StopNotify() <-chan struct{} { return s.done }
+func (s *EtcdServer) StopNotify() <-chan struct{} {
+	s.doneMu.Lock()
+	defer s.doneMu.Unlock()
+	return s.done
+}
 
 func (s *EtcdServer) SelfStats() []byte { return s.stats.JSON() }