add audit/admin trace support for browser requests (#10947)

To support this functionality we had to fork
the gorilla/rpc package with relevant changes

pkg/rpc/json2/client.go

@@ -0,0 +1,83 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Copyright 2012 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Copyright 2020 MinIO, Inc. All rights reserved.
+// forked from https://github.com/gorilla/rpc/v2
+// modified to be used with MinIO under Apache
+// 2.0 license that can be found in the LICENSE file.
+
+package json2
+
+import (
+	"io"
+	"math/rand"
+
+	jsoniter "github.com/json-iterator/go"
+)
+
+// ----------------------------------------------------------------------------
+// Request and Response
+// ----------------------------------------------------------------------------
+
+// clientRequest represents a JSON-RPC request sent by a client.
+type clientRequest struct {
+	// JSON-RPC protocol.
+	Version string `json:"jsonrpc"`
+
+	// A String containing the name of the method to be invoked.
+	Method string `json:"method"`
+
+	// Object to pass as request parameter to the method.
+	Params interface{} `json:"params"`
+
+	// The request id. This can be of any type. It is used to match the
+	// response with the request that it is replying to.
+	Id uint64 `json:"id"`
+}
+
+// clientResponse represents a JSON-RPC response returned to a client.
+type clientResponse struct {
+	Version string               `json:"jsonrpc"`
+	Result  *jsoniter.RawMessage `json:"result"`
+	Error   *jsoniter.RawMessage `json:"error"`
+}
+
+// EncodeClientRequest encodes parameters for a JSON-RPC client request.
+func EncodeClientRequest(method string, args interface{}) ([]byte, error) {
+	c := &clientRequest{
+		Version: "2.0",
+		Method:  method,
+		Params:  args,
+		Id:      uint64(rand.Int63()),
+	}
+	var json = jsoniter.ConfigCompatibleWithStandardLibrary
+	return json.Marshal(c)
+}
+
+// DecodeClientResponse decodes the response body of a client request into
+// the interface reply.
+func DecodeClientResponse(r io.Reader, reply interface{}) error {
+	var c clientResponse
+	var json = jsoniter.ConfigCompatibleWithStandardLibrary
+	if err := json.NewDecoder(r).Decode(&c); err != nil {
+		return err
+	}
+	if c.Error != nil {
+		jsonErr := &Error{}
+		if err := json.Unmarshal(*c.Error, jsonErr); err != nil {
+			return &Error{
+				Code:    E_SERVER,
+				Message: string(*c.Error),
+			}
+		}
+		return jsonErr
+	}
+
+	if c.Result == nil {
+		return ErrNullResult
+	}
+
+	return json.Unmarshal(*c.Result, reply)
+}