etcdserver: resend ReadIndex request on empty apply request

Empty apply indicates first commit in current term. It is first time when follower is sure, that it's ReadIndex request can be processed.

server/etcdserver/server.go

@@ -291,6 +291,9 @@ type EtcdServer struct {
 	leadTimeMu      sync.RWMutex
 	leadElectedTime time.Time
 
+	firstCommitInTermMu sync.RWMutex
+	firstCommitInTermC  chan struct{}
+
 	*AccessController
 }
 
@@ -517,17 +520,18 @@ func NewServer(cfg config.ServerConfig) (srv *EtcdServer, err error) {
 				storage:     NewStorage(w, ss),
 			},
 		),
-		id:               id,
-		attributes:       membership.Attributes{Name: cfg.Name, ClientURLs: cfg.ClientURLs.StringSlice()},
-		cluster:          cl,
-		stats:            sstats,
-		lstats:           lstats,
-		SyncTicker:       time.NewTicker(500 * time.Millisecond),
-		peerRt:           prt,
-		reqIDGen:         idutil.NewGenerator(uint16(id), time.Now()),
-		forceVersionC:    make(chan struct{}),
-		AccessController: &AccessController{CORS: cfg.CORS, HostWhitelist: cfg.HostWhitelist},
-		consistIndex:     cindex.NewConsistentIndex(be.BatchTx()),
+		id:                 id,
+		attributes:         membership.Attributes{Name: cfg.Name, ClientURLs: cfg.ClientURLs.StringSlice()},
+		cluster:            cl,
+		stats:              sstats,
+		lstats:             lstats,
+		SyncTicker:         time.NewTicker(500 * time.Millisecond),
+		peerRt:             prt,
+		reqIDGen:           idutil.NewGenerator(uint16(id), time.Now()),
+		forceVersionC:      make(chan struct{}),
+		AccessController:   &AccessController{CORS: cfg.CORS, HostWhitelist: cfg.HostWhitelist},
+		consistIndex:       cindex.NewConsistentIndex(be.BatchTx()),
+		firstCommitInTermC: make(chan struct{}),
 	}
 	serverID.With(prometheus.Labels{"server_id": id.String()}).Set(1)
 
@@ -1770,6 +1774,16 @@ func (s *EtcdServer) LeaderChangedNotify() <-chan struct{} {
 	return s.leaderChanged
 }
 
+// FirstCommitInTermNotify returns channel that will be unlocked on first
+// entry committed in new term, which is necessary for new leader to answer
+// read-only requests (leader is not able to respond any read-only requests
+// as long as linearizable semantic is required)
+func (s *EtcdServer) FirstCommitInTermNotify() <-chan struct{} {
+	s.firstCommitInTermMu.RLock()
+	defer s.firstCommitInTermMu.RUnlock()
+	return s.firstCommitInTermC
+}
+
 // RaftStatusGetter represents etcd server and Raft progress.
 type RaftStatusGetter interface {
 	ID() types.ID
@@ -2068,6 +2082,8 @@ func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
 	// raft state machine may generate noop entry when leader confirmation.
 	// skip it in advance to avoid some potential bug in the future
 	if len(e.Data) == 0 {
+		s.notifyAboutFirstCommitInTerm()
+
 		select {
 		case s.forceVersionC <- struct{}{}:
 		default:
@@ -2140,6 +2156,15 @@ func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
 	})
 }
 
+func (s *EtcdServer) notifyAboutFirstCommitInTerm() {
+	newNotifier := make(chan struct{})
+	s.firstCommitInTermMu.Lock()
+	notifierToClose := s.firstCommitInTermC
+	s.firstCommitInTermC = newNotifier
+	s.firstCommitInTermMu.Unlock()
+	close(notifierToClose)
+}
+
 // applyConfChange applies a ConfChange to the server. It is only
 // invoked with a ConfChange that has already passed through Raft
 func (s *EtcdServer) applyConfChange(cc raftpb.ConfChange, confState *raftpb.ConfState) (bool, error) {