encoding/json: escape output from Marshalers.

Fixes #3127.

R=rsc, r
CC=golang-dev
https://golang.org/cl/5707054

src/pkg/encoding/json/encode.go

@@ -260,7 +260,7 @@ func (e *encodeState) reflectValueQuoted(v reflect.Value, quoted bool) {
 		b, err := m.MarshalJSON()
 		if err == nil {
 			// copy JSON into buffer, checking validity.
-			err = Compact(&e.Buffer, b)
+			err = compact(&e.Buffer, b, true)
 		}
 		if err != nil {
 			e.error(&MarshalerError{v.Type(), err})

src/pkg/encoding/json/encode_test.go

@@ -167,3 +167,22 @@ func TestRefValMarshal(t *testing.T) {
 		t.Errorf("got %q, want %q", got, want)
 	}
 }
+
+// C implements Marshaler and returns unescaped JSON.
+type C int
+
+func (C) MarshalJSON() ([]byte, error) {
+	return []byte(`"<&>"`), nil
+}
+
+func TestMarshalerEscaping(t *testing.T) {
+	var c C
+	const want = `"\u003c\u0026\u003e"`
+	b, err := Marshal(c)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if got := string(b); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}

src/pkg/encoding/json/indent.go

@@ -9,11 +9,24 @@ import "bytes"
 // Compact appends to dst the JSON-encoded src with
 // insignificant space characters elided.
 func Compact(dst *bytes.Buffer, src []byte) error {
+	return compact(dst, src, false)
+}
+
+func compact(dst *bytes.Buffer, src []byte, escape bool) error {
 	origLen := dst.Len()
 	var scan scanner
 	scan.reset()
 	start := 0
 	for i, c := range src {
+		if escape && (c == '<' || c == '>' || c == '&') {
+			if start < i {
+				dst.Write(src[start:i])
+			}
+			dst.WriteString(`\u00`)
+			dst.WriteByte(hex[c>>4])
+			dst.WriteByte(hex[c&0xF])
+			start = i + 1
+		}
 		v := scan.step(&scan, int(c))
 		if v >= scanSkipSpace {
 			if v == scanError {