Implemented flow-control on message forwarder.

internal/msgpackrouter/router.go

@@ -33,15 +33,26 @@ type Router struct {
 	routesLock     sync.Mutex
 	routes         map[string]*msgpackrpc.Connection
 	routesInternal map[string]RouterRequestHandler
+	sendQueueSize  int
 }
 
-func New() *Router {
+func New(perConnSendQueueSize int) *Router {
 	return &Router{
 		routes:         make(map[string]*msgpackrpc.Connection),
 		routesInternal: make(map[string]RouterRequestHandler),
+		sendQueueSize:  perConnSendQueueSize,
 	}
 }
 
+// SetSendQueueSize sets the size of the send queue for each connection.
+// Only new connections will be affected by this change, existing connections
+// will keep their current send queue size.
+func (r *Router) SetSendQueueSize(size int) {
+	r.routesLock.Lock()
+	defer r.routesLock.Unlock()
+	r.sendQueueSize = size
+}
+
 func (r *Router) Accept(conn io.ReadWriteCloser) <-chan struct{} {
 	res := make(chan struct{})
 	go func() {
@@ -70,7 +81,7 @@ func (r *Router) connectionLoop(conn io.ReadWriteCloser) {
 	defer conn.Close()
 
 	var msgpackconn *msgpackrpc.Connection
-	msgpackconn = msgpackrpc.NewConnection(conn, conn,
+	msgpackconn = msgpackrpc.NewConnectionWithMaxWorkers(conn, conn,
 		func(ctx context.Context, _ msgpackrpc.FunctionLogger, method string, params []any) (_result any, _err any) {
 			// This handler is called when a request is received from the client
 			slog.Debug("Received request", "method", method, "params", params)
@@ -149,6 +160,7 @@ func (r *Router) connectionLoop(conn io.ReadWriteCloser) {
 			}
 			slog.Error("Error in connection", "err", err)
 		},
+		r.sendQueueSize,
 	)
 
 	msgpackconn.Run()

internal/msgpackrouter/router_test.go

@@ -92,7 +92,7 @@ func TestBasicRouterFunctionality(t *testing.T) {
 	})
 	go cl2.Run()
 
-	router := msgpackrouter.New()
+	router := msgpackrouter.New(0)
 	router.Accept(ch1b)
 	router.Accept(ch2b)
 
@@ -160,3 +160,55 @@ func TestBasicRouterFunctionality(t *testing.T) {
 	require.Contains(t, cl1Notifications.String(), "notification: ping [b 14 true true]")
 	cl1NotificationsMux.Unlock()
 }
+
+func TestMessageForwarderCongestionControl(t *testing.T) {
+	// Test parameters
+	queueSize := 5
+	msgLatency := 100 * time.Millisecond
+	// Run a batch of 20 requests, and expect them to take more than 400 ms
+	// in total because the router should throttle requests in batch of 5.
+	batchSize := queueSize * 4
+	expectedLatency := msgLatency * time.Duration(batchSize/queueSize)
+
+	// Make a client that simulates a slow response
+	ch1a, ch1b := newFullPipe()
+	cl1 := msgpackrpc.NewConnection(ch1a, ch1a, func(ctx context.Context, logger msgpackrpc.FunctionLogger, method string, params []any) (_result any, _err any) {
+		time.Sleep(msgLatency)
+		return true, nil
+	}, nil, nil)
+	go cl1.Run()
+
+	// Make a second client to send requests, without any delay
+	ch2a, ch2b := newFullPipe()
+	cl2 := msgpackrpc.NewConnection(ch2a, ch2a, nil, nil, nil)
+	go cl2.Run()
+
+	// Setup router
+	router := msgpackrouter.New(queueSize) // max 5 pending messages per connection
+	router.Accept(ch1b)
+	router.Accept(ch2b)
+
+	{
+		// Register a method on the first client
+		result, reqErr, err := cl1.SendRequest(context.Background(), "$/register", []any{"test"})
+		require.Equal(t, true, result)
+		require.Nil(t, reqErr)
+		require.NoError(t, err)
+	}
+
+	// Run batch of requests from cl2 to cl1
+	start := time.Now()
+	var wg sync.WaitGroup
+	for range batchSize {
+		wg.Go(func() {
+			_, _, err := cl2.SendRequest(t.Context(), "test", []any{})
+			require.NoError(t, err)
+		})
+	}
+	wg.Wait()
+	elapsed := time.Since(start)
+
+	// Check that the elapsed time is greater than expectedLatency
+	fmt.Println("Elapsed time for requests:", elapsed)
+	require.Greater(t, elapsed, expectedLatency, "Expected elapsed time to be greater than %s", expectedLatency)
+}

main.go

@@ -45,12 +45,13 @@ var Version string = "0.0.0-dev"
 
 // Server configuration
 type Config struct {
-	LogLevel        slog.Level
-	ListenTCPAddr   string
-	ListenUnixAddr  string
-	SerialPortAddr  string
-	SerialBaudRate  int
-	MonitorPortAddr string
+	LogLevel                    slog.Level
+	ListenTCPAddr               string
+	ListenUnixAddr              string
+	SerialPortAddr              string
+	SerialBaudRate              int
+	MonitorPortAddr             string
+	MaxPendingRequestsPerClient int
 }
 
 func main() {
@@ -80,7 +81,7 @@ func main() {
 	cmd.Flags().StringVarP(&cfg.SerialPortAddr, "serial-port", "p", "", "Serial port address")
 	cmd.Flags().IntVarP(&cfg.SerialBaudRate, "serial-baudrate", "b", 115200, "Serial port baud rate")
 	cmd.Flags().StringVarP(&cfg.MonitorPortAddr, "monitor-port", "m", "127.0.0.1:7500", "Listening port for MCU monitor proxy")
-
+	cmd.Flags().IntVarP(&cfg.MaxPendingRequestsPerClient, "max-pending-requests", "", 25, "Maximum number of pending requests per client connection")
 	cmd.AddCommand(&cobra.Command{
 		Use:  "version",
 		Long: "Print version information",
@@ -155,7 +156,7 @@ func startRouter(cfg Config) error {
 	}
 
 	// Run router
-	router := msgpackrouter.New()
+	router := msgpackrouter.New(cfg.MaxPendingRequestsPerClient)
 
 	// Register TCP network API methods
 	networkapi.Register(router)

msgpackrpc/connection.go

@@ -52,6 +52,8 @@ type Connection struct {
 	activeOutRequests      map[MessageID]*outRequest
 	activeOutRequestsMutex sync.Mutex
 	lastOutRequestsIndex   atomic.Uint32
+
+	workerSlots chan bool
 }
 
 type inRequest struct {
@@ -79,8 +81,14 @@ type NotificationHandler func(logger FunctionLogger, method string, params []any
 // sending a request or notification.
 type ErrorHandler func(error)
 
-// NewConnection starts a new
+// NewConnection creates a new MessagePack-RPC Connection handler.
 func NewConnection(in io.ReadCloser, out io.WriteCloser, requestHandler RequestHandler, notificationHandler NotificationHandler, errorHandler ErrorHandler) *Connection {
+	return NewConnectionWithMaxWorkers(in, out, requestHandler, notificationHandler, errorHandler, 0)
+}
+
+// NewConnectionWithMaxWorkers creates a new MessagePack-RPC Connection handler
+// with a specified maximum number of worker goroutines to handle incoming requests.
+func NewConnectionWithMaxWorkers(in io.ReadCloser, out io.WriteCloser, requestHandler RequestHandler, notificationHandler NotificationHandler, errorHandler ErrorHandler, maxWorkers int) *Connection {
 	outEncoder := msgpack.NewEncoder(out)
 	outEncoder.UseCompactInts(true)
 	if requestHandler == nil {
@@ -109,9 +117,24 @@ func NewConnection(in io.ReadCloser, out io.WriteCloser, requestHandler RequestH
 		activeOutRequests:   map[MessageID]*outRequest{},
 		logger:              NullLogger{},
 	}
+	if maxWorkers > 0 {
+		conn.workerSlots = make(chan bool, maxWorkers)
+	}
 	return conn
 }
 
+func (c *Connection) startWorker(cb func()) {
+	if c.workerSlots == nil {
+		go cb()
+		return
+	}
+	c.workerSlots <- true
+	go func() {
+		defer func() { <-c.workerSlots }()
+		cb()
+	}()
+}
+
 func (c *Connection) SetLogger(l Logger) {
 	c.loggerMutex.Lock()
 	c.logger = l
@@ -215,7 +238,7 @@ func (c *Connection) handleIncomingRequest(id MessageID, method string, params [
 	logger := c.logger.LogIncomingRequest(id, method, params)
 	c.loggerMutex.Unlock()
 
-	go func() {
+	c.startWorker(func() {
 		reqResult, reqError := c.requestHandler(ctx, logger, method, params)
 
 		var existing *inRequest
@@ -238,7 +261,7 @@ func (c *Connection) handleIncomingRequest(id MessageID, method string, params [
 			c.errorHandler(fmt.Errorf("error sending response: %w", err))
 			c.Close()
 		}
-	}()
+	})
 }
 
 func (c *Connection) handleIncomingNotification(method string, params []any) {
@@ -261,7 +284,9 @@ func (c *Connection) handleIncomingNotification(method string, params []any) {
 	logger := c.logger.LogIncomingNotification(method, params)
 	c.loggerMutex.Unlock()
 
-	go c.notificationHandler(logger, method, params)
+	c.startWorker(func() {
+		c.notificationHandler(logger, method, params)
+	})
 }
 
 func (c *Connection) handleIncomingResponse(id MessageID, reqError any, reqResult any) {