Keep original reference of slice element

- If the entered slice length is greater than zero, copy all references to the shared slice.

decode_slice.go

@@ -49,9 +49,20 @@ func newSliceDecoder(dec decoder, elemType *rtype, size uintptr, structName, fie
 	}
 }
 
-func (d *sliceDecoder) newSlice() *sliceHeader {
+func (d *sliceDecoder) newSlice(src *sliceHeader) *sliceHeader {
 	slice := d.arrayPool.Get().(*sliceHeader)
+	if src.len > 0 {
+		// copy original elem
+		if slice.cap < src.cap {
+			data := newArray(d.elemType, src.cap)
+			slice = &sliceHeader{data: data, len: src.len, cap: src.cap}
+		} else {
+			slice.len = src.len
+		}
+		copySlice(d.elemType, *slice, *src)
+	} else {
 		slice.len = 0
+	}
 	return slice
 }
 
@@ -109,7 +120,8 @@ func (d *sliceDecoder) decodeStream(s *stream, depth int64, p unsafe.Pointer) er
 				return nil
 			}
 			idx := 0
-			slice := d.newSlice()
+			slice := d.newSlice((*sliceHeader)(p))
+			srcLen := slice.len
 			capacity := slice.cap
 			data := slice.data
 			for {
@@ -121,12 +133,17 @@ func (d *sliceDecoder) decodeStream(s *stream, depth int64, p unsafe.Pointer) er
 					copySlice(d.elemType, dst, src)
 				}
 				ep := unsafe.Pointer(uintptr(data) + uintptr(idx)*d.size)
+
+				// if srcLen is greater than idx, keep the original reference
+				if srcLen <= idx {
 					if d.isElemPointerType {
 						**(**unsafe.Pointer)(unsafe.Pointer(&ep)) = nil // initialize elem pointer
 					} else {
 						// assign new element to the slice
 						typedmemmove(d.elemType, ep, unsafe_New(d.elemType))
 					}
+				}
+
 				if err := d.valueDecoder.decodeStream(s, depth, ep); err != nil {
 					return err
 				}
@@ -212,7 +229,8 @@ func (d *sliceDecoder) decode(buf []byte, cursor, depth int64, p unsafe.Pointer)
 				return cursor, nil
 			}
 			idx := 0
-			slice := d.newSlice()
+			slice := d.newSlice((*sliceHeader)(p))
+			srcLen := slice.len
 			capacity := slice.cap
 			data := slice.data
 			for {
@@ -224,12 +242,15 @@ func (d *sliceDecoder) decode(buf []byte, cursor, depth int64, p unsafe.Pointer)
 					copySlice(d.elemType, dst, src)
 				}
 				ep := unsafe.Pointer(uintptr(data) + uintptr(idx)*d.size)
+				// if srcLen is greater than idx, keep the original reference
+				if srcLen <= idx {
 					if d.isElemPointerType {
 						**(**unsafe.Pointer)(unsafe.Pointer(&ep)) = nil // initialize elem pointer
 					} else {
 						// assign new element to the slice
 						typedmemmove(d.elemType, ep, unsafe_New(d.elemType))
 					}
+				}
 				c, err := d.valueDecoder.decode(buf, cursor, depth, ep)
 				if err != nil {
 					return 0, err

decode_test.go

@@ -3052,7 +3052,7 @@ func TestMultipleDecodeWithRawMessage(t *testing.T) {
 type intUnmarshaler int
 
 func (u *intUnmarshaler) UnmarshalJSON(b []byte) error {
-	if *u != 0 {
+	if *u != 0 && *u != 10 {
 		return fmt.Errorf("failed to decode of slice with int unmarshaler")
 	}
 	*u = 10
@@ -3062,7 +3062,7 @@ func (u *intUnmarshaler) UnmarshalJSON(b []byte) error {
 type arrayUnmarshaler [5]int
 
 func (u *arrayUnmarshaler) UnmarshalJSON(b []byte) error {
-	if (*u)[0] != 0 {
+	if (*u)[0] != 0 && (*u)[0] != 10 {
 		return fmt.Errorf("failed to decode of slice with array unmarshaler")
 	}
 	(*u)[0] = 10
@@ -3072,7 +3072,7 @@ func (u *arrayUnmarshaler) UnmarshalJSON(b []byte) error {
 type mapUnmarshaler map[string]int
 
 func (u *mapUnmarshaler) UnmarshalJSON(b []byte) error {
-	if len(*u) != 0 {
+	if len(*u) != 0 && len(*u) != 1 {
 		return fmt.Errorf("failed to decode of slice with map unmarshaler")
 	}
 	*u = map[string]int{"a": 10}
@@ -3081,13 +3081,15 @@ func (u *mapUnmarshaler) UnmarshalJSON(b []byte) error {
 
 type structUnmarshaler struct {
 	A        int
+	notFirst bool
 }
 
 func (u *structUnmarshaler) UnmarshalJSON(b []byte) error {
-	if u.A != 0 {
+	if !u.notFirst && u.A != 0 {
 		return fmt.Errorf("failed to decode of slice with struct unmarshaler")
 	}
 	u.A = 10
+	u.notFirst = true
 	return nil
 }
 
@@ -3199,6 +3201,29 @@ func TestSliceElemUnmarshaler(t *testing.T) {
 	})
 }
 
+type keepRefTest struct {
+	A int
+	B string
+}
+
+func (t *keepRefTest) UnmarshalJSON(data []byte) error {
+	v := []interface{}{&t.A, &t.B}
+	return json.Unmarshal(data, &v)
+}
+
+func TestKeepReferenceSlice(t *testing.T) {
+	var v keepRefTest
+	if err := json.Unmarshal([]byte(`[54,"hello"]`), &v); err != nil {
+		t.Fatal(err)
+	}
+	if v.A != 54 {
+		t.Fatal("failed to keep reference for slice")
+	}
+	if v.B != "hello" {
+		t.Fatal("failed to keep reference for slice")
+	}
+}
+
 func TestInvalidTopLevelValue(t *testing.T) {
 	t.Run("invalid end of buffer", func(t *testing.T) {
 		var v struct{}