fix(datastore): Handle loading nil values (#8544)

* fix(datastore): Handle loading nil values

datastore/integration_test.go

@@ -342,6 +342,34 @@ func TestIntegration_ListValues(t *testing.T) {
 	}
 }
 
+func TestIntegration_PutGetUntypedNil(t *testing.T) {
+	ctx := context.Background()
+	client := newTestClient(ctx, t)
+	defer client.Close()
+
+	type X struct {
+		I interface{} `json:"i"`
+	}
+
+	putX := &X{
+		I: nil,
+	}
+	key, err := client.Put(ctx, IncompleteKey("X", nil), putX)
+	if err != nil {
+		t.Fatalf("client.Put got: %v, want: nil", err)
+	}
+
+	getX := &X{}
+	err = client.Get(ctx, key, getX)
+	if err != nil {
+		t.Fatalf("client.Get got: %v, want: nil", err)
+	}
+
+	if !reflect.DeepEqual(getX, putX) {
+		t.Fatalf("got: %v, want: %v", getX, putX)
+	}
+}
+
 func TestIntegration_GetMulti(t *testing.T) {
 	ctx := context.Background()
 	client := newTestClient(ctx, t)

datastore/load.go

@@ -302,11 +302,17 @@ func setVal(v reflect.Value, p Property) (s string) {
 			return fmt.Sprintf("%v is unsettable", v.Type())
 		}
 
-		rpValue := reflect.ValueOf(pValue)
-		if !rpValue.Type().AssignableTo(v.Type()) {
-			return fmt.Sprintf("%q is not assignable to %q", rpValue.Type(), v.Type())
+		// When retrieved property value is untyped nil, its type cannot be determined
+		// So, set v to zero value of its datatype
+		if pValue == nil {
+			v.Set(reflect.Zero(v.Type()))
+		} else {
+			rpValue := reflect.ValueOf(pValue)
+			if !rpValue.Type().AssignableTo(v.Type()) {
+				return fmt.Sprintf("%q is not assignable to %q", rpValue.Type(), v.Type())
+			}
+			v.Set(rpValue)
 		}
-		v.Set(rpValue)
 
 	case reflect.Ptr:
 		// v must be a pointer to either a Key, an Entity, or one of the supported basic types.

datastore/load_test.go

@@ -1290,6 +1290,27 @@ func TestLoadNull(t *testing.T) {
 	}
 }
 
+func TestLoadNilInterface(t *testing.T) {
+	type WithAny struct {
+		AnyField interface{}
+	}
+
+	withAny1 := &WithAny{}
+	err := loadEntityProto(withAny1, &pb.Entity{
+		Key: keyToProto(testKey0),
+		Properties: map[string]*pb.Value{
+			"AnyField": {ValueType: &pb.Value_NullValue{}},
+		},
+	})
+	if err != nil {
+		t.Fatalf("loadEntityProto got: %v, want: nil", err)
+	}
+
+	if withAny1.AnyField != nil {
+		t.Fatalf("got: %v, want: nil", withAny1.AnyField)
+	}
+}
+
 type KeyLoaderEnt struct {
 	A int
 	K *Key