Exceptions from within promises

[staugaard]

It looks like the RescueMiddleware does not handle exceptions coming from within promises.

When we hit a ActiveRecord::RecordNotFound in a non-lazy resolver then the middleware takes care of it, but when we hit that same exception inside a lazy promise resolver, then the error makes it's way all the way out to the rails controller.

[rmosolgo]

Oh, interesting. I guess that's a flaw in the runtime design. Middleware only wraps resolve, not lazy_resolve.

I guess the best "fix" is to introduce an error handling plugin like

MySchema = GraphQL::Schema.define do 
  use GraphQL::Rescues
end 

which will wrap both eager- and lazy-loading with error handling. (In my opinion, the real best case would be to avoid raising errors altogether 😆 )

[ianks]

You can catch the error and return a GraphQL::Exception. This is how I handle promises in one of my resolvers. Note: al the Rails.appliction.executor.wrap stuff is to avoid autoload deadlocks in Rails 5 😞 :

    def promise(&_blk)
      Rails.application.executor.wrap do
        Concurrent::Promise
          .execute { Rails.application.executor.wrap { yield } }
          .catch { |e| handle_error(e) }
      end
    end

    def handle_error(e)
      Rails.logger.error(e)
      Rails.logger.error(e.backtrace.join("\n"))
      GraphQL::ExecutionError.new(e.message)
    end

[rmosolgo]

I just merged some new error handling for the interpreter that properly catches errors in promises: #2458

If you give it a try and have a problem, please open a new issue!