refact(subtypes): Remove return from transformations

This return value is just a reference to the input, which
is being mutated. We may as well make that clear via
the signature, which will also simplify the new custom
transformation function registration logic.

internal/types/subtypes/interceptor.go

@@ -93,7 +93,7 @@ func messageDomain(m proto.Message) string {
 //
 // Also note that for any of the id based lookups to function, the file that contains
 // the proto.Message definition must set the "domain" custom option.
-func transformRequestAttributes(req proto.Message) (proto.Message, error) {
+func transformRequestAttributes(req proto.Message) error {
 	domain := messageDomain(req)
 
 	r := req.ProtoReflect()
@@ -115,7 +115,7 @@ func transformRequestAttributes(req proto.Message) (proto.Message, error) {
 	case itemField != nil:
 		itemR := itemField.Message()
 		if itemR == nil {
-			return req, nil
+			return nil
 		}
 		id := fieldValue(r, idField)
 
@@ -136,19 +136,15 @@ func transformRequestAttributes(req proto.Message) (proto.Message, error) {
 		case typeField != nil && t != "":
 			st = Subtype(t)
 		default: // need either type or id
-			return req, nil
-		}
-		if err := convertAttributesToSubtype(item, st); err != nil {
-			return req, err
+			return nil
 		}
+		return convertAttributesToSubtype(item, st)
 	case idField != nil && attributesField != nil:
 		id := r.Get(idField).String()
 		st = SubtypeFromId(domain, id)
-		if err := convertAttributesToSubtype(req, st); err != nil {
-			return req, err
-		}
+		return convertAttributesToSubtype(req, st)
 	}
-	return req, nil
+	return nil
 }
 
 func transformResponseItemAttributes(item proto.Message) error {
@@ -206,7 +202,7 @@ func transformResponseItemAttributes(item proto.Message) error {
 //        // other subtype attributes types
 //      }
 //    }
-func transformResponseAttributes(res proto.Message) (proto.Message, error) {
+func transformResponseAttributes(res proto.Message) error {
 	r := res.ProtoReflect()
 	fields := r.Descriptor().Fields()
 
@@ -215,27 +211,25 @@ func transformResponseAttributes(res proto.Message) (proto.Message, error) {
 	switch {
 	case itemField != nil:
 		if itemR := itemField.Message(); itemR == nil {
-			return res, nil
+			return nil
 		}
 
 		item := r.Get(itemField).Message().Interface()
-		if err := transformResponseItemAttributes(item); err != nil {
-			return res, err
-		}
+		return transformResponseItemAttributes(item)
 	case itemsField != nil:
 		if !itemsField.IsList() {
-			return res, nil
+			return nil
 		}
 		items := r.Get(itemsField).List()
 
 		for i := 0; i < items.Len(); i++ {
 			item := items.Get(i).Message().Interface()
 			if err := transformResponseItemAttributes(item); err != nil {
-				return res, err
+				return err
 			}
 		}
 	}
-	return res, nil
+	return nil
 }
 
 // AttributeTransformerInterceptor is a grpc server interceptor that will
@@ -285,10 +279,8 @@ func transformResponseAttributes(res proto.Message) (proto.Message, error) {
 func AttributeTransformerInterceptor(_ context.Context) grpc.UnaryServerInterceptor {
 	const op = "subtypes.AttributeTransformInterceptor"
 	return func(interceptorCtx context.Context, req interface{}, _ *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
-		var err error
 		if reqMsg, ok := req.(proto.Message); ok {
-			req, err = transformRequestAttributes(reqMsg)
-			if err != nil {
+			if err := transformRequestAttributes(reqMsg); err != nil {
 				return nil, handlers.InvalidArgumentErrorf("Error in provided request.",
 					map[string]string{"attributes": "Attribute fields do not match the expected format."})
 			}
@@ -297,8 +289,7 @@ func AttributeTransformerInterceptor(_ context.Context) grpc.UnaryServerIntercep
 		res, handlerErr := handler(interceptorCtx, req)
 
 		if res, ok := res.(proto.Message); ok {
-			res, err = transformResponseAttributes(res)
-			if err != nil {
+			if err := transformResponseAttributes(res); err != nil {
 				return nil, handlers.ApiErrorWithCodeAndMessage(codes.Internal, "failed building attribute struct: %v", err)
 			}
 		}

internal/types/subtypes/interceptor_test.go

@@ -298,9 +298,9 @@ func TestTransformRequestAttributes(t *testing.T) {
 
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
-			got, err := transformRequestAttributes(tc.req)
+			err := transformRequest(tc.req)
 			require.NoError(t, err)
-			assert.Empty(t, cmp.Diff(got, tc.expected, protocmp.Transform()))
+			assert.Empty(t, cmp.Diff(tc.req, tc.expected, protocmp.Transform()))
 		})
 	}
 }
@@ -582,9 +582,9 @@ func TestTransformResponseAttributes(t *testing.T) {
 
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
-			got, err := transformResponseAttributes(tc.resp)
+			err := transformResponse(tc.resp)
 			require.NoError(t, err)
-			assert.Empty(t, cmp.Diff(got, tc.expected, protocmp.Transform()))
+			assert.Empty(t, cmp.Diff(tc.resp, tc.expected, protocmp.Transform()))
 		})
 	}
 }