etcdserver: export method EtcdServer.leaderChangedNotify (#12378)

server/etcdserver/server.go

@@ -185,6 +185,15 @@ type Server interface {
 	ClusterVersion() *semver.Version
 	Cluster() api.Cluster
 	Alarms() []*pb.AlarmMember
+
+	// LeaderChangedNotify returns a channel for application level code to be notified
+	// when etcd leader changes, this function is intend to be used only in application
+	// which embed etcd.
+	// Caution:
+	// 1. the returned channel is being closed when the leadership changes.
+	// 2. so the new channel needs to be obtained for each raft term.
+	// 3. user can loose some consecutive channel changes using this API.
+	LeaderChangedNotify() <-chan struct{}
 }
 
 // EtcdServer is the production implementation of the Server interface
@@ -1743,7 +1752,7 @@ func (s *EtcdServer) getLead() uint64 {
 	return atomic.LoadUint64(&s.lead)
 }
 
-func (s *EtcdServer) leaderChangedNotify() <-chan struct{} {
+func (s *EtcdServer) LeaderChangedNotify() <-chan struct{} {
 	s.leaderChangedMu.RLock()
 	defer s.leaderChangedMu.RUnlock()
 	return s.leaderChanged

server/etcdserver/v3_server.go

@@ -712,7 +712,7 @@ func (s *EtcdServer) linearizableReadLoop() {
 		ctxToSend := make([]byte, 8)
 		id1 := s.reqIDGen.Next()
 		binary.BigEndian.PutUint64(ctxToSend, id1)
-		leaderChangedNotifier := s.leaderChangedNotify()
+		leaderChangedNotifier := s.LeaderChangedNotify()
 		select {
 		case <-leaderChangedNotifier:
 			continue