lsps0: handle requests in a goroutine

lsps0/server.go

@@ -20,6 +20,7 @@ var ErrServerStopped = errors.New("lsps0: the server has been stopped")
 var Lsps0MessageType uint32 = 37913
 var BadMessageFormatError string = "bad message format"
 var InternalError string = "internal error"
+var MaxSimultaneousRequests = 25
 
 // ServiceDesc and is constructed from it for internal purposes.
 type serviceInfo struct {
@@ -61,6 +62,7 @@ func (s *Server) Serve(lis lightning.CustomMsgClient) error {
 		s.mu.Unlock()
 	}()
 
+	guard := make(chan struct{}, MaxSimultaneousRequests)
 	for {
 		msg, err := lis.Recv()
 		if err != nil {
@@ -113,6 +115,9 @@ func (s *Server) Serve(lis lightning.CustomMsgClient) error {
 			continue
 		}
 
+		// Deserialization step of the request params. This function is called
+		// by method handlers of service implementations to deserialize the
+		// typed request object.
 		df := func(v interface{}) error {
 			if err := json.Unmarshal(req.Params, v); err != nil {
 				return status.Newf(codes.InvalidParams, "invalid params").Err()
@@ -121,9 +126,19 @@ func (s *Server) Serve(lis lightning.CustomMsgClient) error {
 			return nil
 		}
 
-		// NOTE: The handler is being called synchonously. There's an option to
-		// do this in a goroutine instead. Also, there's the option to put the
-		// goroutine in the method desc for specific methods instead.
+		// Will block if the guard queue is already filled to ensure
+		// MaxSimultaneousRequests is not exceeded.
+		guard <- struct{}{}
+
+		// NOTE: The handler is being called asynchonously. This may cause the
+		// order of messages handled to be different from the order in which
+		// they were received.
+		go func() {
+			// Releases a queued item in the guard, to release a spot for
+			// another simultaneous request.
+			defer func() { <-guard }()
+
+			// Call the method handler for the requested method.
 		r, err := m.method.Handler(m.service.serviceImpl, context.TODO(), df)
 		if err != nil {
 			s, ok := status.FromError(err)
@@ -132,11 +147,12 @@ func (s *Server) Serve(lis lightning.CustomMsgClient) error {
 				s = status.New(codes.InternalError, InternalError)
 			}
 
-			go sendError(lis, msg, req, s)
-			continue
+				sendError(lis, msg, req, s)
+				return
 		}
 
-		go sendResponse(lis, msg, req, r)
+			sendResponse(lis, msg, req, r)
+		}()
 	}
 }