fix: allow JSON-RPC error data in responses (#56)

Passes JSON-RPC error `Data` through server responses when a method
returns it.

Adds a new method `writeJSONRPCErrorWithData`.

Alternatives:
This is simpler than changing `writeJSONRPCError` everywhere and passing
Data as `nil`.

Author: shanejonas

rpcserver/jsonrpc_server.go

@@ -157,14 +157,22 @@ func (h *JSONRPCHandler) writeJSONRPCResponse(w http.ResponseWriter, response JS
 }
 
 func (h *JSONRPCHandler) writeJSONRPCError(w http.ResponseWriter, id any, code int, msg string) {
+	h.writeJSONRPCErrorWithData(w, id, code, msg, nil)
+}
+
+func (h *JSONRPCHandler) writeJSONRPCErrorWithData(w http.ResponseWriter, id any, code int, msg string, data any) {
+	var dataPtr *any
+	if data != nil {
+		dataPtr = &data
+	}
 	res := JSONRPCResponse{
 		JSONRPC: "2.0",
 		ID:      id,
 		Result:  nil,
 		Error: &JSONRPCError{
 			Code:    code,
 			Message: msg,
-			Data:    nil,
+			Data:    dataPtr,
 		},
 	}
 	h.writeJSONRPCResponse(w, res)
@@ -351,7 +359,7 @@ func (h *JSONRPCHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	result, err := method.call(ctx, req.Params)
 	if err != nil {
 		if jsonRPCErr, ok := err.(*JSONRPCError); ok {
-			h.writeJSONRPCError(w, req.ID, jsonRPCErr.Code, jsonRPCErr.Message)
+			h.writeJSONRPCErrorWithData(w, req.ID, jsonRPCErr.Code, jsonRPCErr.Message, jsonRPCErr.Data)
 		} else {
 			h.writeJSONRPCError(w, req.ID, CodeCustomError, err.Error())
 		}

rpcserver/jsonrpc_server_test.go

@@ -143,6 +143,33 @@ func TestJSONRPCServerDefaultLiveAndReady(t *testing.T) {
 	require.Equal(t, http.StatusNotFound, rr.Code)
 }
 
+func TestJSONRPCErrorDataIsPreserved(t *testing.T) {
+	handlerMethod := func(ctx context.Context, arg int) (int, error) {
+		errorData := any("some error data")
+		return 0, &JSONRPCError{
+			Code:    1234,
+			Message: "test error",
+			Data:    &errorData,
+		}
+	}
+
+	handler, err := NewJSONRPCHandler(map[string]interface{}{
+		"testError": handlerMethod,
+	}, JSONRPCHandlerOpts{})
+	require.NoError(t, err)
+
+	httpServer := httptest.NewServer(handler)
+	defer httpServer.Close()
+
+	client := rpcclient.NewClient(httpServer.URL)
+	resp, err := client.Call(context.Background(), "testError", 1)
+	require.NoError(t, err)
+	require.NotNil(t, resp.Error)
+	require.Equal(t, 1234, resp.Error.Code)
+	require.Equal(t, "test error", resp.Error.Message)
+	require.Equal(t, "some error data", resp.Error.Data)
+}
+
 func TestJSONRPCServerReadyzOK(t *testing.T) {
 	handler := testHandler(JSONRPCHandlerOpts{
 		ReadyHandler: func(w http.ResponseWriter, r *http.Request) error {