[BUG] golang client mishandles return type of string with content-type = application/json

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

API responses of JSON-encoded simple string value with content-type = application/json are mishandled by the golang client and are returned to the caller without JSON-decoding.

openapi-generator version

latest (5.1.1)

OpenAPI declaration file content or url

https://gist.github.com/jpriebe/3bfc34a63b586616bc22190b84a5440e

Generation Details

docker run \
    --rm \
    --volume "$PWD:/workdir" \
    openapitools/openapi-generator-cli:latest \
    generate \
    --input-spec /workdir/openapi.yaml \
    --generator-name go \
    --package-name "client" \
    --output /workdir

Steps to reproduce

• build an openapi spec for an API that produces application/json and has an endpoint that returns a string
• implement a server based on the spec
• generate a golang client with the spec
• use the client to make a call to the endpoint in question
• assuming the server returned a JSON-encoded string (as it should), the client will return JSON-encoded string instead of the raw string of the response

Suggest a fix

I think the problem is here:

https://github.com/OpenAPITools/openapi-generator/blob/60dcf8613f6b75b606757c4a991fe018ad10ee99/modules/openapi-generator/src/main/resources/go/client.mustache#L418

When the caller expects a string, it will pass in a pointer to a string to the decode() function. The login on line 418 will pop the HTTP response body directly into the string and return.

I think that the decode() function should be modified in one of the following ways:

• add a check for content-type = text/plain to the logic block at line 418
• move the logic block at line 418 to line 442

Bottom line: if the server's content-type is application/json, the decode() function should be decoding JSON no matter what the expected go type is.

Start with modules/openapi-generator/src/main/resources/go/client.mustache at the referenced line around 418, then review the surrounding decode() logic. Reproduce the generated Go client's handling of an application/json response containing a string, and confirm that JSON content is decoded to the raw string rather than returned encoded.