Set X-User-Agent in gRPC-Web clients

connect_ext_test.go

@@ -2020,6 +2020,31 @@ func TestAllowCustomUserAgent(t *testing.T) {
 	}
 }
 
+func TestWebXUserAgent(t *testing.T) {
+	t.Parallel()
+
+	mux := http.NewServeMux()
+	mux.Handle(pingv1connect.NewPingServiceHandler(&pluggablePingServer{
+		ping: func(_ context.Context, req *connect.Request[pingv1.PingRequest]) (*connect.Response[pingv1.PingResponse], error) {
+			agent := req.Header().Get("User-Agent")
+			assert.NotZero(t, agent)
+			assert.Equal(
+				t,
+				req.Header().Get("X-User-Agent"),
+				agent,
+			)
+			return connect.NewResponse(&pingv1.PingResponse{Number: req.Msg.Number}), nil
+		},
+	}))
+	server := httptest.NewServer(mux)
+	t.Cleanup(server.Close)
+
+	client := pingv1connect.NewPingServiceClient(server.Client(), server.URL, connect.WithGRPCWeb())
+	req := connect.NewRequest(&pingv1.PingRequest{Number: 42})
+	_, err := client.Ping(context.Background(), req)
+	assert.Nil(t, err)
+}
+
 func TestBidiOverHTTP1(t *testing.T) {
 	t.Parallel()
 	mux := http.NewServeMux()

protocol_grpc.go

@@ -49,6 +49,8 @@ const (
 	grpcWebContentTypeDefault = "application/grpc-web"
 	grpcContentTypePrefix     = grpcContentTypeDefault + "+"
 	grpcWebContentTypePrefix  = grpcWebContentTypeDefault + "+"
+
+	headerXUserAgent = "X-User-Agent"
 )
 
 var (
@@ -252,6 +254,13 @@ func (g *grpcClient) WriteRequestHeader(_ StreamType, header http.Header) {
 	if getHeaderCanonical(header, headerUserAgent) == "" {
 		header[headerUserAgent] = []string{defaultGrpcUserAgent}
 	}
+	if g.web && getHeaderCanonical(header, headerXUserAgent) == "" {
+		// The gRPC-Web pseudo-specification seems to require X-User-Agent rather
+		// than User-Agent for all clients, even if they're not browser-based. This
+		// is very odd for a backend client, so we'll split the difference and set
+		// both.
+		header[headerXUserAgent] = []string{defaultGrpcUserAgent}
+	}
 	header[headerContentType] = []string{grpcContentTypeFromCodecName(g.web, g.Codec.Name())}
 	// gRPC handles compression on a per-message basis, so we don't want to
 	// compress the whole stream. By default, http.Client will ask the server