adding a noop/noop modifier with a json API (#319)

* Adding a noop modifier in its own noop package

go.mod

@@ -8,4 +8,5 @@ require (
 	golang.org/x/net v0.0.0-20190628185345-da137c7871d7
 	google.golang.org/grpc v1.37.0
 	google.golang.org/grpc/cmd/protoc-gen-go-grpc v1.1.0 // indirect
+	google.golang.org/protobuf v1.26.0 // indirect
 )

har/har.go

@@ -307,6 +307,7 @@ type contentJSON struct {
 	Encoding string `json:"encoding,omitempty"`
 }
 
+// MarshalJSON marshals the byte slice into json after encoding based on c.Encoding.
 func (c Content) MarshalJSON() ([]byte, error) {
 	var txt string
 	switch c.Encoding {
@@ -327,6 +328,7 @@ func (c Content) MarshalJSON() ([]byte, error) {
 	return json.Marshal(cj)
 }
 
+// UnmarshalJSON unmarshals the bytes slice into Content.
 func (c *Content) UnmarshalJSON(data []byte) error {
 	var cj contentJSON
 	if err := json.Unmarshal(data, &cj); err != nil {

noop/noop.go

@@ -0,0 +1,78 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package noop provides a martian.RequestResponseModifier that does not
+// modify the request or response.
+package noop
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/google/martian/v3"
+	"github.com/google/martian/v3/log"
+	"github.com/google/martian/v3/parse"
+)
+
+func init() {
+	parse.Register("noop.Modifier", modifierFromJSON)
+}
+
+type noopModifier struct {
+	id string
+}
+
+// Noop returns a modifier that does not change the request or the response.
+func Noop(id string) martian.RequestResponseModifier {
+	return &noopModifier{
+		id: id,
+	}
+}
+
+// ModifyRequest logs a debug line.
+func (nm *noopModifier) ModifyRequest(*http.Request) error {
+	log.Debugf("noopModifier: %s: no request modification applied", nm.id)
+	return nil
+}
+
+// ModifyResponse logs a debug line.
+func (nm *noopModifier) ModifyResponse(*http.Response) error {
+	log.Debugf("noopModifier: %s: no response modification applied", nm.id)
+	return nil
+}
+
+type modifierJSON struct {
+	Name  string               `json:"name"`
+	Scope []parse.ModifierType `json:"scope"`
+}
+
+// modifierFromJSON takes a JSON message as a byte slice and returns
+// a headerModifier and an error.
+//
+// Example JSON configuration message:
+// {
+//  "scope": ["request", "result"],
+//  "name": "noop-name",
+// }
+func modifierFromJSON(b []byte) (*parse.Result, error) {
+	msg := &modifierJSON{}
+	if err := json.Unmarshal(b, msg); err != nil {
+		return nil, err
+	}
+
+	modifier := Noop(msg.Name)
+
+	return parse.NewResult(modifier, msg.Scope)
+}
+