json: only use alphanumeric tags

Almost the same definition as Go identifier names.
(Leading digits are allowed.)

Fixes #1520.

R=r, r2
CC=golang-dev
https://golang.org/cl/4173061

src/pkg/json/decode.go

@@ -466,13 +466,15 @@ func (d *decodeState) object(v reflect.Value) {
 		} else {
 			var f reflect.StructField
 			var ok bool
-			// First try for field with that tag.
 			st := sv.Type().(*reflect.StructType)
-			for i := 0; i < sv.NumField(); i++ {
-				f = st.Field(i)
-				if f.Tag == key {
-					ok = true
-					break
+			// First try for field with that tag.
+			if isValidTag(key) {
+				for i := 0; i < sv.NumField(); i++ {
+					f = st.Field(i)
+					if f.Tag == key {
+						ok = true
+						break
+					}
 				}
 			}
 			if !ok {

src/pkg/json/decode_test.go

@@ -40,6 +40,11 @@ var (
 	umtrue   = unmarshaler{true}
 )
 
+type badTag struct {
+	X string
+	Y string "y"
+	Z string "@#*%(#@"
+}
 
 type unmarshalTest struct {
 	in  string
@@ -62,6 +67,9 @@ var unmarshalTests = []unmarshalTest{
 	{`{"X": [1,2,3], "Y": 4}`, new(T), T{Y: 4}, &UnmarshalTypeError{"array", reflect.Typeof("")}},
 	{`{"x": 1}`, new(tx), tx{}, &UnmarshalFieldError{"x", txType, txType.Field(0)}},
 
+	// skip invalid tags
+	{`{"X":"a", "y":"b", "Z":"c"}`, new(badTag), badTag{"a", "b", "c"}, nil},
+
 	// syntax errors
 	{`{"X": "foo", "Y"}`, nil, nil, SyntaxError("invalid character '}' after object key")},
 

src/pkg/json/encode.go

@@ -13,6 +13,7 @@ import (
 	"runtime"
 	"sort"
 	"strconv"
+	"unicode"
 	"utf8"
 )
 
@@ -35,8 +36,9 @@ import (
 //
 // Struct values encode as JSON objects.  Each struct field becomes
 // a member of the object.  By default the object's key name is the
-// struct field name.  If the struct field has a tag, that tag will 
-// be used as the name instead.  Only exported fields will be encoded.
+// struct field name.  If the struct field has a non-empty tag consisting
+// of only Unicode letters, digits, and underscores, that tag will be used
+// as the name instead.  Only exported fields will be encoded.
 //
 // Map values encode as JSON objects.
 // The map's key type must be string; the object keys are used directly
@@ -230,7 +232,7 @@ func (e *encodeState) reflectValue(v reflect.Value) {
 			} else {
 				e.WriteByte(',')
 			}
-			if f.Tag != "" {
+			if isValidTag(f.Tag) {
 				e.string(f.Tag)
 			} else {
 				e.string(f.Name)
@@ -285,6 +287,18 @@ func (e *encodeState) reflectValue(v reflect.Value) {
 	return
 }
 
+func isValidTag(s string) bool {
+	if s == "" {
+		return false
+	}
+	for _, c := range s {
+		if c != '_' && !unicode.IsLetter(c) && !unicode.IsDigit(c) {
+			return false
+		}
+	}
+	return true
+}
+
 // stringValues is a slice of reflect.Value holding *reflect.StringValue.
 // It implements the methods to sort by string.
 type stringValues []reflect.Value