HTTP 406 Not Acceptable on ExternalTaskApi.getExternalTaskErrorDetails()

[mattiasgreen]

Requests to ExternalTaskApi.getExternalTaskErrorDetails(externalTaskId) fail with HTTP 406 Not Acceptable due to accept header value incorrectly listing only "application/json". Correct value is "application/json, text/plain".

• The getExternalTaskErrorDetails operation produces content types application/json for HTTP 500 and text/plain for HTTP 200. See https://artifacts.camunda.com/ui/native/camunda-bpm/org/camunda/bpm/camunda-engine-rest-openapi/7.19.0/camunda-engine-rest-openapi-7.19.0.jar
• OpenAPI Generator produces client code which does not honor this contract, resulting in requests which accept only application/json. This is due to a special case which, if multiple content types are supported as accept header values, and application/json is one of them, it will strip out all other mime types and produce accept header with only application/json. https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ApiClientTest.java#L47-L63
• This results in HTTP 406 Not Acceptable from Camunda Engine REST.

[mattiasgreen]

Snippet from generated selectHeaderAccept()

    public String selectHeaderAccept(String[] accepts) {
        if (accepts.length == 0) {
            return null;
        }
        for (String accept : accepts) {
            if (isJsonMime(accept)) {
                return accept;
            }
        }
        return StringUtil.join(accepts, ",");
    }

Snippet from generated getExternalTaskErrorDetailsCall()

        final String[] localVarAccepts = {
            "text/plain", "application/json"
        };
        final String localVarAccept = localVarApiClient.selectHeaderAccept(localVarAccepts);

Note that this results in localVarAccept variable value "application/json", instead of "text/plain, application/json"

[mattiasgreen]

Temporary local workaround is to patch this generated file, either by editing or overriding this method.

      final String[] localVarAccepts = {
            "text/plain", "application/json"
      };
      final String localVarAccept = StringUtil.join(localVarAccepts, ",");

[mattiasgreen]

Alternatives for permanent fix, top of my head:

1. Raise issue and PR for OpenAPI Generator for this "special case" with accept headers and "application/json". In my opinion, this is clearly a bug. However, there may be many clients and solutions out there by now relying on this behavior, so it is unlikely this will be fixed upstream.
2. Figure out how to automatically "patch" this behavior when generating the client. Perhaps by adding/injecting the workaround after generation. Seems hackish, but perhaps doable.
3. Change Camunda REST API to only return "application/json" for both status 200 and 500. That is, wrap the error details in simple and flat JSON response.
4. Figure out if Engine REST can disable the accept header check to avoid HTTP 406 and return. Seems like the worst of options, and I'm not clear if OpenAPI-generated client would even accept the response.

[mattiasgreen]

Found this open issue which is spot on.
"[BUG][Go] Content type selection breaks when multiple content types present"
OpenAPITools/openapi-generator#10340