feat: write protocol acknowledgements from go gateway

go/vehicle-gateway/internal/gateway/tcp_server.go

@@ -21,11 +21,14 @@ type FrameExtractor func([]byte) (frames [][]byte, remainder []byte, err error)
 
 type FrameParser func(raw []byte, receivedAtMS int64, sourceEndpoint string) (envelope.FrameEnvelope, error)
 
+type FrameResponder func(raw []byte, env envelope.FrameEnvelope) (response []byte, ok bool, err error)
+
 type TCPProtocol struct {
 	Protocol envelope.Protocol
 	Addr     string
 	Extract  FrameExtractor
 	Parse    FrameParser
+	Respond  FrameResponder
 }
 
 type TCPServer struct {
@@ -153,7 +156,7 @@ func (s *TCPServer) handleConnection(ctx context.Context, conn net.Conn) {
 			}
 			pending = remainder
 			for _, frame := range frames {
-				s.handleFrame(ctx, frame, source)
+				s.handleFrame(ctx, conn, frame, source)
 			}
 		}
 		if err != nil {
@@ -174,7 +177,7 @@ func (s *TCPServer) handleConnection(ctx context.Context, conn net.Conn) {
 	}
 }
 
-func (s *TCPServer) handleFrame(ctx context.Context, raw []byte, source string) {
+func (s *TCPServer) handleFrame(ctx context.Context, conn net.Conn, raw []byte, source string) {
 	receivedAtMS := time.Now().UnixMilli()
 	env, err := s.protocol.Parse(raw, receivedAtMS, source)
 	if err != nil {
@@ -213,6 +216,21 @@ func (s *TCPServer) handleFrame(ctx context.Context, raw []byte, source string)
 		s.logger.Error("publish unified failed", "protocol", s.protocol.Protocol, "event_id", env.StableEventID(), "error", err)
 		return
 	}
+	if s.protocol.Respond == nil {
+		return
+	}
+	response, ok, err := s.protocol.Respond(raw, env)
+	if err != nil {
+		s.logger.Warn("build protocol response failed", "protocol", s.protocol.Protocol, "event_id", env.StableEventID(), "error", err)
+		return
+	}
+	if !ok || len(response) == 0 {
+		return
+	}
+	_ = conn.SetWriteDeadline(time.Now().Add(5 * time.Second))
+	if _, err := conn.Write(response); err != nil {
+		s.logger.Warn("write protocol response failed", "protocol", s.protocol.Protocol, "event_id", env.StableEventID(), "error", err)
+	}
 }
 
 func (p TCPProtocol) String() string {

go/vehicle-gateway/internal/gateway/tcp_server_test.go

@@ -3,6 +3,7 @@ package gateway
 import (
 	"context"
 	"encoding/hex"
+	"io"
 	"log/slog"
 	"net"
 	"testing"
@@ -73,6 +74,37 @@ func TestTCPServerPublishesBadFrameOnlyToRaw(t *testing.T) {
 	}
 }
 
+func TestTCPServerWritesProtocolResponseAfterPublish(t *testing.T) {
+	frame := buildGBFrame(0x07, 0xfe, "LNBSCB3D4R1234567", nil)
+	sink := &recordingSink{}
+	server := newTestServer(t, TCPProtocol{
+		Protocol: envelope.ProtocolGB32960,
+		Addr:     ":0",
+		Extract:  gb32960.ExtractFrames,
+		Parse:    gb32960.ParseFrame,
+		Respond: func(_ []byte, env envelope.FrameEnvelope) ([]byte, bool, error) {
+			if len(sink.unified) != 1 || sink.unified[0].EventID != env.EventID {
+				t.Fatalf("response built before publish: raw=%d unified=%d", len(sink.raw), len(sink.unified))
+			}
+			return []byte("ACK"), true, nil
+		},
+	}, sink)
+
+	client, done := runPipe(t, server)
+	if _, err := client.Write(frame); err != nil {
+		t.Fatalf("client.Write() error = %v", err)
+	}
+	buf := make([]byte, 3)
+	if _, err := io.ReadFull(client, buf); err != nil {
+		t.Fatalf("read response error = %v", err)
+	}
+	if string(buf) != "ACK" {
+		t.Fatalf("response = %q", string(buf))
+	}
+	_ = client.Close()
+	<-done
+}
+
 func newTestServer(t *testing.T, protocol TCPProtocol, sink *recordingSink) *TCPServer {
 	t.Helper()
 	server, err := NewTCPServer(TCPServerConfig{