Fix marshaling invalid chars

Signed-off-by: John Howard <howardjohn@google.com>

v2/jsonpatch.go

@@ -1,7 +1,6 @@
 package jsonpatch
 
 import (
-	"bytes"
 	"encoding/json"
 	"fmt"
 	"reflect"
@@ -24,21 +23,28 @@ func (j *Operation) Json() string {
 }
 
 func (j *Operation) MarshalJSON() ([]byte, error) {
-	var b bytes.Buffer
-	b.WriteString("{")
-	b.WriteString(fmt.Sprintf(`"op":"%s"`, j.Operation))
-	b.WriteString(fmt.Sprintf(`,"path":"%s"`, j.Path))
-	// Consider omitting Value for non-nullable operations.
-	if j.Value != nil || j.Operation == "replace" || j.Operation == "add" {
-		v, err := json.Marshal(j.Value)
-		if err != nil {
-			return nil, err
-		}
-		b.WriteString(`,"value":`)
-		b.Write(v)
+	// Ensure for add and replace we emit `value: null`
+	if j.Value == nil && (j.Operation == "replace" || j.Operation == "add") {
+		return json.Marshal(struct {
+			Operation string      `json:"op"`
+			Path      string      `json:"path"`
+			Value     interface{} `json:"value"`
+		}{
+			Operation: j.Operation,
+			Path:      j.Path,
+		})
 	}
-	b.WriteString("}")
-	return b.Bytes(), nil
+	// otherwise just marshal normally. We cannot literally do json.Marshal(j) as it would be recursively
+	// calling this function.
+	return json.Marshal(struct {
+		Operation string      `json:"op"`
+		Path      string      `json:"path"`
+		Value     interface{} `json:"value,omitempty"`
+	}{
+		Operation: j.Operation,
+		Path:      j.Path,
+		Value:     j.Value,
+	})
 }
 
 type ByPath []Operation

v2/jsonpatch_test.go

@@ -841,6 +841,10 @@ var (
 	emptyKeyB = `{"":[]}`
 )
 
+var (
+	specialChars = string([]byte{123, 34, 92, 98, 34, 58, 91, 93, 125})
+)
+
 func TestCreatePatch(t *testing.T) {
 	cases := []struct {
 		name string
@@ -888,6 +892,8 @@ func TestCreatePatch(t *testing.T) {
 		{"Null Key uses replace operation", nullKeyA, nullKeyB},
 		// empty key
 		{"Empty key", emptyKeyA, emptyKeyB},
+		// special chars
+		{"Special chars", empty, specialChars},
 	}
 
 	for _, c := range cases {