Let the config choose protocol 2 or 3, and fall back for one session only.

The default stays v2. A reflector that refuses that version and names the other gets one session of it, then the station announces the configured protocol again.

Co-authored-by: Cursor <cursoragent@cursor.com>

station/session.go

@@ -30,11 +30,15 @@ var (
 // Run keeps the reflector session until ctx is canceled.
 func (s *Station) Run(ctx context.Context) error {
 	defer s.Close()
-	major := uint16(3)
+	var major uint16
+	holdMajor := false
 	for {
 		if ctx.Err() != nil {
 			return nil
 		}
+		if !holdMajor {
+			major = s.configuredMajor()
+		}
 		s.mu.Lock()
 		if s.fatal != "" || !s.loaded {
 			s.mu.Unlock()
@@ -61,6 +65,7 @@ func (s *Station) Run(ctx context.Context) error {
 		stale := s.epoch != epoch
 		s.mu.Unlock()
 		if stale {
+			holdMajor = false
 			if conn != nil {
 				// This dial used the config from before Save.
 				_ = conn.Close()
@@ -72,7 +77,6 @@ func (s *Station) Run(ctx context.Context) error {
 			if ctx.Err() != nil {
 				return nil
 			}
-			major = 3
 			s.mu.Lock()
 			s.noteDownLocked(s.clock.Now(), true)
 			s.mu.Unlock()
@@ -82,7 +86,7 @@ func (s *Station) Run(ctx context.Context) error {
 			continue
 		}
 
-		err = s.session(ctx, conn, major, epoch)
+		err = s.session(ctx, conn, major, epoch, major == s.configuredMajor())
 		conn.Close()
 		fallback := errors.Is(err, errFallback)
 		reconnect := errors.Is(err, errReconnect)
@@ -111,9 +115,10 @@ func (s *Station) Run(ctx context.Context) error {
 				return nil
 			}
 			if fallback {
-				major = 2
+				major = otherMajor(major)
+				holdMajor = true
 			} else {
-				major = 3
+				holdMajor = false
 			}
 			continue
 		}
@@ -124,7 +129,7 @@ func (s *Station) Run(ctx context.Context) error {
 		}
 		fatal := s.fatal != ""
 		s.mu.Unlock()
-		major = 3
+		holdMajor = false
 		if ctx.Err() != nil {
 			return nil
 		}
@@ -297,7 +302,23 @@ const (
 	phChallenge
 )
 
-func (s *Station) session(ctx context.Context, conn net.Conn, major uint16, epoch uint64) error {
+func (s *Station) configuredMajor() uint16 {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.cfg.protocolMajor == 3 {
+		return 3
+	}
+	return 2
+}
+
+func otherMajor(major uint16) uint16 {
+	if major == 2 {
+		return 3
+	}
+	return 2
+}
+
+func (s *Station) session(ctx context.Context, conn net.Conn, major uint16, epoch uint64, allowFallback bool) error {
 	s.mu.Lock()
 	s.conn = conn
 	stale := s.epoch != epoch
@@ -428,7 +449,7 @@ func (s *Station) session(ctx context.Context, conn net.Conn, major uint16, epoc
 			if udpRx.Before(ex.limit) {
 				ex.limit = udpRx
 			}
-			err := s.onFrame(ctx, wire, major, tcpf, ex)
+			err := s.onFrame(ctx, wire, major, tcpf, ex, allowFallback)
 			if err != nil {
 				return err
 			}
@@ -512,14 +533,14 @@ func (s *Station) onClock(now time.Time) {
 	}
 }
 
-func (s *Station) onFrame(ctx context.Context, wire *framed, major uint16, f frame, ex *exchange) error {
+func (s *Station) onFrame(ctx context.Context, wire *framed, major uint16, f frame, ex *exchange, allowFallback bool) error {
 	switch f.typ {
 	case tcpHeartbeat:
 		return nil
 	case tcpError:
-		return s.onReflectorError(f.body, major)
+		return s.onReflectorError(f.body, major, allowFallback)
 	case tcpProtoDown:
-		return s.onDowngrade(f.body, major)
+		return s.onDowngrade(f.body, major, allowFallback)
 	}
 	if !ex.authed && f.typ >= 100 {
 		return s.retry("Protocol error", nil)
@@ -530,25 +551,25 @@ func (s *Station) onFrame(ctx context.Context, wire *framed, major uint16, f fra
 	return s.onV3(ctx, wire, f, ex)
 }
 
-func (s *Station) onReflectorError(body []byte, major uint16) error {
+func (s *Station) onReflectorError(body []byte, major uint16, allowFallback bool) error {
 	msg, _, err := parseString(body)
 	if err != nil {
 		s.endFatalCause("could not be read", err, true)
 		return fmt.Errorf("could not be read: %w", err)
 	}
-	if major >= 3 && fallback20(msg) {
+	if allowFallback && wantsOther(major, msg) {
 		return errFallback
 	}
 	s.endFatal(msg, true)
 	return errStopped
 }
 
-func (s *Station) onDowngrade(body []byte, major uint16) error {
+func (s *Station) onDowngrade(body []byte, major uint16, allowFallback bool) error {
 	gotMajor, gotMinor, err := parseVersion(body)
 	if err != nil {
 		return err
 	}
-	if major >= 3 && gotMajor == 2 {
+	if allowFallback && acceptsOther(major, gotMajor, gotMinor) {
 		return errFallback
 	}
 	s.endFatal("Unsupported protocol version "+strconvU(gotMajor)+"."+strconvU(gotMinor), true)