OpenApi onErrorStatusConflict: OneOf transforms errors into a tagged union

[RossWilliams]

Setting openApi configuration parameter onErrorStatusConflict to oneOf transforms the error shapes under the same http status code to a tagged union set. However, server SDKs do not generate tagged unions for errors, leading to a mismatch between client and server. Additionally, Smithy Errors are not tagged unions. They are designed as an array of possible outputs, with the shape name providing the discriminating tag. I believe the problem occurs because the code generation relies on the UnionShape, which are a distinct concept from the errors array.

namespace example

 use aws.protocols#restJson1
 use smithy.framework#ValidationException

 @restJson1
 service Example {
     version: "2006-03-01",
     operations: [Op],
     errors: [
         Error1,
         Error2
     ]
 }

@http(uri: "/op", method: "GET")
 operation Op {
     input: String,
     output: String
 }

 @httpError(418)
 structure Error1 {
     TeapotName: String
 }

 @httpError(418)
 structure Error2 {
     IsHot: Boolean
 }

{
   "openapi": "3.0.2",
   "info": {
     "title": "Example",
     "version": "2006-03-01"
   },
   "paths": {
     "/op": {
       "get": {
         "operationId": "Op",
         "responses": {
           "200": {
             "description": "GetOp 200 response",
             "content": {
               "application/json": {
                 "schema": {
                   "$ref": "#/components/schemas/GetOpResponseContent"
                 }
               }
             }
           },
           "418": {
             "description": "GetOp418Error 418 response",
             "content": {
               "application/json": {
                 "schema": {
                   "$ref": "#/components/schemas/GetOp418ErrorResponseContent"
                 }
               }
             }
           }
         }
       }
     }
   },
   "components": {
     "schemas": {
       "Error1": {
         "type": "object",
         "properties": {
           "TeapotName": {
             "type": "string"
           }
         }
       },
       "Error2": {
         "type": "object",
         "properties": {
           "IsHot": {
             "type": "boolean"
           }
       },
       "GetOp418ErrorResponseContent": {
         "oneOf": [
           {
             "type": "object",
             "title": "Error1",
             "properties": {
               "Error1": {
                 "$ref": "#/components/schemas/Error1"
               }
             },
             "required": [
               "Error1"
             ]
           },
           {
             "type": "object",
             "title": "Error2",
             "properties": {
               "Error2": {
                 "$ref": "#/components/schemas/Error2"
               }
             },
             "required": [
               "Error2"
             ]
           }
         ]
       },
       "GetOpResponseContent": {
         "type": "string"
       }
     }
   }
 }

Issue introduced in #1995 . Creating a tagged union of errors is not necessary for OpenApi per this comment.

The correct output shape for the 418 component is:

"GetOp418ErrorResponseContent": {
   "oneOf": [
     {
         "$ref": "#/components/schemas/Error1"
     },
     {
         "$ref": "#/components/schemas/Error2"
     }
   ]
 }

[nated0g]

I think this may be the relevant code?

smithy/smithy-openapi/src/main/java/software/amazon/smithy/openapi/fromsmithy/protocols/AwsRestJson1Protocol.java

Lines 130 to 143 in 996fd94

	if (context.getConfig().getOnErrorStatusConflict() != null
	&& context.getConfig().getOnErrorStatusConflict().equals(ONE_OF)
	&& targetsSyntheticError(cleanedShape, context)) {
	UnionShape.Builder asUnion = UnionShape.builder().id(cleanedShape.getId());
	UnionShape targetUnion = context.getModel()
	.expectShape(
	cleanedShape.getAllMembers().values().stream().findFirst().get().getTarget(),
	UnionShape.class);
	for (MemberShape member : targetUnion.getAllMembers().values()) {
	String name = member.getMemberName();
	asUnion.addMember(member.toBuilder().id(cleanedShape.getId().withMember(name)).build());
	}
	return context.getJsonSchemaConverter().convertShape(asUnion.build()).getRootSchema();
	}

[JordonPhillips]

In times like these I really wish we had some way to see how people were using features like this in a detailed way. I can make the change no problem, but I'm wondering if I should make it a oneOfUntagged or something like that to not break anybody relying on the current behavior. My inclination is that nobody is, because how could they be, but I'm not sure. I'll put up a pr anyway as a place for discussion with the team