GODRIVER-3574 Align BSON interface slice decoding with json package. (#2075)

Author: qingyang-hu

bson/bsoncodec.go

@@ -69,6 +69,9 @@ func (vde ValueDecoderError) Error() string {
 	if vde.Received.IsValid() {
 		received = vde.Received.Type().String()
 	}
+	if !vde.Received.CanSet() {
+		received = "unsettable " + received
+	}
 	return fmt.Sprintf("%s can only decode valid and settable %s, but got %s", vde.Name, strings.Join(typeKinds, ", "), received)
 }
 

bson/decoder_test.go

@@ -172,6 +172,29 @@ func TestDecodingInterfaces(t *testing.T) {
 					assert.Equal(t, testStruct{[3]interface{}{&str, &i, nil}}, receiver)
 				}
 
+				return data, &receiver, check
+			},
+		},
+		{
+			name: "overwriting prepopulated slice",
+			stub: func() ([]byte, interface{}, func(*testing.T)) {
+				type testStruct struct {
+					Values []interface{}
+				}
+
+				data := docToBytes(struct {
+					Values []interface{}
+				}{
+					Values: []interface{}{1, 2, 3},
+				})
+
+				receiver := testStruct{[]interface{}{7, 8}}
+
+				check := func(t *testing.T) {
+					t.Helper()
+					assert.Equal(t, testStruct{[]interface{}{1, 2, int32(3)}}, receiver)
+				}
+
 				return data, &receiver, check
 			},
 		},

bson/default_value_decoders.go

@@ -1332,8 +1332,12 @@ func decodeDefault(dc DecodeContext, vr ValueReader, val reflect.Value) ([]refle
 
 	eType := val.Type().Elem()
 
+	isInterfaceSlice := eType.Kind() == reflect.Interface && val.Len() > 0
+
+	// If this is not an interface slice with pre-populated elements, we can look up
+	// the decoder for eType once.
 	var vDecoder ValueDecoder
-	if !(eType.Kind() == reflect.Interface && val.Len() > 0) {
+	if !isInterfaceSlice {
 		vDecoder, err = dc.LookupDecoder(eType)
 		if err != nil {
 			return nil, err
@@ -1351,14 +1355,22 @@ func decodeDefault(dc DecodeContext, vr ValueReader, val reflect.Value) ([]refle
 		}
 
 		var elem reflect.Value
-		if vDecoder == nil {
+		if isInterfaceSlice && idx < val.Len() {
+			// Decode into an existing interface{} slot.
+
 			elem = val.Index(idx).Elem()
 			switch {
 			case elem.Kind() != reflect.Ptr || elem.IsNil():
 				valueDecoder, err := dc.LookupDecoder(elem.Type())
 				if err != nil {
 					return nil, err
 				}
+
+				// If an element is allocated and unsettable, it must be overwritten.
+				if !elem.CanSet() {
+					elem = reflect.New(elem.Type()).Elem()
+				}
+
 				err = valueDecoder.DecodeValue(dc, vr, elem)
 				if err != nil {
 					return nil, newDecodeError(strconv.Itoa(idx), err)
@@ -1380,6 +1392,15 @@ func decodeDefault(dc DecodeContext, vr ValueReader, val reflect.Value) ([]refle
 				}
 			}
 		} else {
+			// For non-interface slices, or if we've exhausted the pre-populated
+			// slots, we create a fresh value.
+
+			if vDecoder == nil {
+				vDecoder, err = dc.LookupDecoder(eType)
+				if err != nil {
+					return nil, err
+				}
+			}
 			elem, err = decodeTypeOrValueWithInfo(vDecoder, dc, vr, eType)
 			if err != nil {
 				return nil, newDecodeError(strconv.Itoa(idx), err)