encoding/json: Fix panic when trying to unmarshal the empty string into an integer

Fixes #3450.

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/6035050

src/pkg/encoding/json/decode.go

@@ -593,6 +593,11 @@ func (d *decodeState) literal(v reflect.Value) {
 // produce more helpful error messages.
 func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool) {
 	// Check for unmarshaler.
+	if len(item) == 0 {
+		//Empty string given
+		d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
+		return
+	}
 	wantptr := item[0] == 'n' // null
 	unmarshaler, pv := d.indirect(v, wantptr)
 	if unmarshaler != nil {

src/pkg/encoding/json/decode_test.go

@@ -646,3 +646,22 @@ func TestAnonymous(t *testing.T) {
 		t.Fatal("Unmarshal: did set T.Y")
 	}
 }
+
+// Test that the empty string doesn't panic decoding when ,string is specified
+// Issue 3450
+func TestEmptyString(t *testing.T) {
+	type T2 struct {
+		Number1 int `json:",string"`
+		Number2 int `json:",string"`
+	}
+	data := `{"Number1":"1", "Number2":""}`
+	dec := NewDecoder(strings.NewReader(data))
+	var t2 T2
+	err := dec.Decode(&t2)
+	if err == nil {
+		t.Fatal("Decode: did not return error")
+	}
+	if t2.Number1 != 1 {
+		t.Fatal("Decode: did not set Number1")
+	}
+}