make sure unlock safe even if listeners panic (#383)

* make sure unlock safe even if listeners panic

* fix #378

* fix #378

core/discov/internal/statewatcher.go

@@ -18,6 +18,7 @@ type (
 		disconnected bool
 		currentState connectivity.State
 		listeners    []func()
+		// lock only guards listeners, because only listens can be accessed by other goroutines.
 		lock sync.Mutex
 	}
 )
@@ -32,27 +33,33 @@ func (sw *stateWatcher) addListener(l func()) {
 	sw.lock.Unlock()
 }
 
-func (sw *stateWatcher) watch(conn etcdConn) {
-	sw.currentState = conn.GetState()
-	for {
-		if conn.WaitForStateChange(context.Background(), sw.currentState) {
-			newState := conn.GetState()
+func (sw *stateWatcher) notifyListeners() {
 	sw.lock.Lock()
-			sw.currentState = newState
+	defer sw.lock.Unlock()
+
+	for _, l := range sw.listeners {
+		l()
+	}
+}
 
-			switch newState {
+func (sw *stateWatcher) updateState(conn etcdConn) {
+	sw.currentState = conn.GetState()
+	switch sw.currentState {
 	case connectivity.TransientFailure, connectivity.Shutdown:
 		sw.disconnected = true
 	case connectivity.Ready:
 		if sw.disconnected {
 			sw.disconnected = false
-					for _, l := range sw.listeners {
-						l()
-					}
+			sw.notifyListeners()
 		}
 	}
+}
 
-			sw.lock.Unlock()
+func (sw *stateWatcher) watch(conn etcdConn) {
+	sw.currentState = conn.GetState()
+	for {
+		if conn.WaitForStateChange(context.Background(), sw.currentState) {
+			sw.updateState(conn)
 		}
 	}
 }

core/proc/shutdown+polyfill.go

@@ -4,10 +4,12 @@ package proc
 
 import "time"
 
+// AddShutdownListener returns fn itself on windows, lets callers call fn on their own.
 func AddShutdownListener(fn func()) func() {
 	return fn
 }
 
+// AddWrapUpListener returns fn itself on windows, lets callers call fn on their own.
 func AddWrapUpListener(fn func()) func() {
 	return fn
 }