Dynamic registration of error formatters causes global overrides

[NobodysNightmare]

Hey there, we started receiving the following warning after a grape update to our application:

json is already registered with class OurSpecificFormatter::Json

This was surprising, because we only registered it once for one of two Grape root APIs that we have in our application:

class Root < ::API::RootAPI # inherits from Grape::API
  format :json

  error_representer ::Bim::Bcf::API::V2_1::Errors::ErrorRepresenter, "application/json; charset=utf-8"    
  # ...
end

Our intention was to only use this formatter in that one API, but keep the default formatter in other APIs.

After browsing the grape source code I found two surprises:

1. Just by existing, our error formatter now registers itself globally, presumably as the default formatter for JSON errors
   • We only intended to use this error formatter in a comparably narrow scope, not everywhere
2. The warning message I got was misleading, because it's the automatic registration of our formatter that caused the warning message already and jsonwas previously registered with Grape::ErrorFormatter::Json
   • The error lies in this line of code, where we should probably refer to registry[shortname] instead of klass, to say what was already registered

What's the intended way to define an error formatter that's only used in a small part of the API, but not the entire API?

References

• PR introducing dynamic registration of formatters: Dynamic registration #2516
• Our BCF API (which only is a small/additional API next to our core API): https://github.com/opf/openproject/blob/dev/modules/bim/app/controllers/bim/bcf/api/root.rb