allow reads on disconnected followers (#121)

controller/controller.go

@@ -84,6 +84,7 @@ type Controller struct {
 	lives     map[*liveBuffer]bool
 	lcond     *sync.Cond
 	fcup      bool                        // follow caught up
+	fcuponce  bool                        // follow caught up once
 	shrinking bool                        // aof shrinking flag
 	shrinklog [][]string                  // aof shrinking log
 	hooks     map[string]*Hook            // hook name
@@ -492,7 +493,7 @@ func (c *Controller) handleInputCommand(conn *server.Conn, msg *server.Message,
 		// read operations
 		c.mu.RLock()
 		defer c.mu.RUnlock()
-		if c.config.FollowHost != "" && !c.fcup {
+		if c.config.FollowHost != "" && !c.fcuponce {
 			return writeErr(errors.New("catching up to leader"))
 		}
 	case "follow", "readonly", "config":

controller/follow.go

@@ -218,6 +218,7 @@ func (c *Controller) followStep(host string, port int, followc uint64) error {
 	if caughtUp {
 		c.mu.Lock()
 		c.fcup = true
+		c.fcuponce = true
 		c.mu.Unlock()
 		log.Info("caught up")
 	}
@@ -241,6 +242,7 @@ func (c *Controller) followStep(host string, port int, followc uint64) error {
 				caughtUp = true
 				c.mu.Lock()
 				c.fcup = true
+				c.fcuponce = true
 				c.mu.Unlock()
 				log.Info("caught up")
 			}

controller/stats.go

@@ -80,6 +80,7 @@ func (c *Controller) cmdServer(msg *server.Message) (res string, err error) {
 	if c.config.FollowHost != "" {
 		m["following"] = fmt.Sprintf("%s:%d", c.config.FollowHost, c.config.FollowPort)
 		m["caught_up"] = c.fcup
+		m["caught_up_once"] = c.fcuponce
 	}
 	m["http_transport"] = c.http
 	m["pid"] = os.Getpid()