json: Properly handle nil slices.

Marshal nil slices as null and parse null value as a nil slice.
Fixes #2278.

R=rsc
CC=golang-dev
https://golang.org/cl/5257053

src/pkg/json/decode.go

@@ -588,7 +588,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value) {
 		switch v.Kind() {
 		default:
 			d.saveError(&UnmarshalTypeError{"null", v.Type()})
-		case reflect.Interface, reflect.Ptr, reflect.Map:
+		case reflect.Interface, reflect.Ptr, reflect.Map, reflect.Slice:
 			v.Set(reflect.Zero(v.Type()))
 		}
 

src/pkg/json/decode_test.go

@@ -456,7 +456,7 @@ var allValueIndent = `{
 	"PSlice": null,
 	"PSliceP": null,
 	"EmptySlice": [],
-	"NilSlice": [],
+	"NilSlice": null,
 	"StringSlice": [
 		"str24",
 		"str25",
@@ -528,8 +528,8 @@ var pallValueIndent = `{
 	},
 	"EmptyMap": null,
 	"NilMap": null,
-	"Slice": [],
-	"SliceP": [],
+	"Slice": null,
+	"SliceP": null,
 	"PSlice": [
 		{
 			"Tag": "tag20"
@@ -547,10 +547,10 @@ var pallValueIndent = `{
 			"Tag": "tag23"
 		}
 	],
-	"EmptySlice": [],
-	"NilSlice": [],
-	"StringSlice": [],
-	"ByteSlice": "",
+	"EmptySlice": null,
+	"NilSlice": null,
+	"StringSlice": null,
+	"ByteSlice": null,
 	"Small": {
 		"Tag": ""
 	},

src/pkg/json/encode.go

@@ -352,7 +352,15 @@ func (e *encodeState) reflectValueQuoted(v reflect.Value, quoted bool) {
 		}
 		e.WriteByte('}')
 
-	case reflect.Array, reflect.Slice:
+	case reflect.Slice:
+		if v.IsNil() {
+			e.WriteString("null")
+			break
+		}
+		// Slices can be marshalled as nil, but otherwise are handled
+		// as arrays.
+		fallthrough
+	case reflect.Array:
 		if v.Type() == byteSliceType {
 			e.WriteByte('"')
 			s := v.Interface().([]byte)

src/pkg/json/encode_test.go

@@ -28,7 +28,7 @@ type Optionals struct {
 var optionalsExpected = `{
  "sr": "",
  "omitempty": 0,
- "slr": [],
+ "slr": null,
  "mr": {}
 }`