json: fix decode bug with struct tag names with ,opts being ignored

When the encoder was updated to respect the ",omitempty"
struct tag options, the decoder half was never updated to know
about the new struct tag format. (the format is now an optional
name, followed by zero or more ",option" strings)

This only affected people who used ",omitempty" along with
a field name. In that case, the serialized JSON wouldn't
decode to the original value.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/4965049

src/pkg/json/decode.go

@@ -486,7 +486,7 @@ func (d *decodeState) object(v reflect.Value) {
 			if isValidTag(key) {
 				for i := 0; i < sv.NumField(); i++ {
 					f = st.Field(i)
-					if f.Tag.Get("json") == key {
+					if tagName(f.Tag.Get("json")) == key {
 						ok = true
 						break
 					}
@@ -918,3 +918,13 @@ func unquoteBytes(s []byte) (t []byte, ok bool) {
 	}
 	return b[0:w], true
 }
+
+// tagName extracts the field name part out of the "json" struct tag
+// value. The json struct tag format is an optional name, followed by
+// zero or more ",option" values.
+func tagName(v string) string {
+	if idx := strings.Index(v, ","); idx != -1 {
+		return v[:idx]
+	}
+	return v
+}

src/pkg/json/decode_test.go

@@ -263,6 +263,7 @@ type All struct {
 	Float64 float64
 
 	Foo  string `json:"bar"`
+	Foo2 string `json:"bar2,dummyopt"`
 
 	PBool    *bool
 	PInt     *int
@@ -331,6 +332,7 @@ var allValue = All{
 	Float32: 14.1,
 	Float64: 15.1,
 	Foo:     "foo",
+	Foo2:    "foo2",
 	String:  "16",
 	Map: map[string]Small{
 		"17": {Tag: "tag17"},
@@ -391,6 +393,7 @@ var allValueIndent = `{
 	"Float32": 14.1,
 	"Float64": 15.1,
 	"bar": "foo",
+	"bar2": "foo2",
 	"PBool": null,
 	"PInt": null,
 	"PInt8": null,
@@ -481,6 +484,7 @@ var pallValueIndent = `{
 	"Float32": 0,
 	"Float64": 0,
 	"bar": "",
+	"bar2": "",
 	"PBool": true,
 	"PInt": 2,
 	"PInt8": 3,