[BUG] [Go] File download returns nil

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?
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Description

Have an OpenAPI v3 endpoint definition to download a file, when you create the client, and try to execute a download, you get a nil file pointer back.

openapi-generator version

master

OpenAPI declaration file content or url

https://gist.github.com/acranbury/0b2358548c7e45ab5cc2f857b964cd28

Generation Details

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g go -i apicurio_test.yaml -o /var/tmp/apicurio_test

Related issues/PRs

This seems related to https://github.com/OpenAPITools/openapi-generator/issues/8392 where the file upload fails

Suggest a fix

The issue is that client.decode expects a **os.File to be passed in, but the return type of the file download endpoint is already **os.File. The endpoint passes &localVarReturnValue which ends up as a ***os.File, so the file write never happens and nil is returned.

It seems like the **os.File return type of the endpoint is unnecessary, and could be reduced to just *os.File, or decode could expect a ***os.File, although a triple pointer makes me shudder just a little.

Reproduce the generated Go client from the linked OpenAPI declaration using the provided openapi-generator command. Start at the generated client's client.decode and the file-download endpoint, checking how the return value is passed through. Done means the download writes the file and returns a non-nil file pointer instead of nil.