encoding/json: don't marshal special float values

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5500084

src/pkg/encoding/json/encode.go

@@ -12,6 +12,7 @@ package json
 import (
 	"bytes"
 	"encoding/base64"
+	"math"
 	"reflect"
 	"runtime"
 	"sort"
@@ -170,6 +171,15 @@ func (e *UnsupportedTypeError) Error() string {
 	return "json: unsupported type: " + e.Type.String()
 }
 
+type UnsupportedValueError struct {
+	Value reflect.Value
+	Str   string
+}
+
+func (e *UnsupportedValueError) Error() string {
+	return "json: unsupported value: " + e.Str
+}
+
 type InvalidUTF8Error struct {
 	S string
 }
@@ -290,7 +300,11 @@ func (e *encodeState) reflectValueQuoted(v reflect.Value, quoted bool) {
 			e.Write(b)
 		}
 	case reflect.Float32, reflect.Float64:
-		b := strconv.AppendFloat(e.scratch[:0], v.Float(), 'g', -1, v.Type().Bits())
+		f := v.Float()
+		if math.IsInf(f, 0) || math.IsNaN(f) {
+			e.error(&UnsupportedValueError{v, strconv.FormatFloat(f, 'g', -1, v.Type().Bits())})
+		}
+		b := strconv.AppendFloat(e.scratch[:0], f, 'g', -1, v.Type().Bits())
 		if quoted {
 			writeString(e, string(b))
 		} else {

src/pkg/encoding/json/encode_test.go

@@ -6,6 +6,7 @@ package json
 
 import (
 	"bytes"
+	"math"
 	"reflect"
 	"testing"
 )
@@ -107,3 +108,21 @@ func TestEncodeRenamedByteSlice(t *testing.T) {
 		t.Errorf(" got %s want %s", result, expect)
 	}
 }
+
+var unsupportedValues = []interface{}{
+	math.NaN(),
+	math.Inf(-1),
+	math.Inf(1),
+}
+
+func TestUnsupportedValues(t *testing.T) {
+	for _, v := range unsupportedValues {
+		if _, err := Marshal(v); err != nil {
+			if _, ok := err.(*UnsupportedValueError); !ok {
+				t.Errorf("for %v, got %T want UnsupportedValueError", v, err)
+			}
+		} else {
+			t.Errorf("for %v, expected error", v)
+		}
+	}
+}