Catch invalid use of Server.RegisterService after Register.Serve (#828)

server.go

@@ -86,6 +86,7 @@ type Server struct {
 	mu     sync.Mutex // guards following
 	lis    map[net.Listener]bool
 	conns  map[io.Closer]bool
+	serve  bool
 	drain  bool
 	ctx    context.Context
 	cancel context.CancelFunc
@@ -329,6 +330,9 @@ func (s *Server) register(sd *ServiceDesc, ss interface{}) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	s.printf("RegisterService(%q)", sd.ServiceName)
+	if s.serve {
+		grpclog.Fatalf("grpc: Server.RegisterService after Server.Serve for %q", sd.ServiceName)
+	}
 	if _, ok := s.m[sd.ServiceName]; ok {
 		grpclog.Fatalf("grpc: Server.RegisterService found duplicate service registration for %q", sd.ServiceName)
 	}
@@ -417,6 +421,7 @@ func (s *Server) useTransportAuthenticator(rawConn net.Conn) (net.Conn, credenti
 func (s *Server) Serve(lis net.Listener) error {
 	s.mu.Lock()
 	s.printf("serving")
+	s.serve = true
 	if s.lis == nil {
 		s.mu.Unlock()
 		lis.Close()