Rationale behind path and method change in `ActionDispatch::ShowExceptions`

rubyonrails-talk
1

In ActionDispatch::ShowExceptions#render_exception there is code to change the request path_info and request_method.

request.path_info = "/#{status}"
request.request_method = "GET"

Inspecting the response headers, this is not visible to the end-user. Which purpose does this bit of code serve? I am setting up a custom exceptions app that changes behavior based on the path and this tripped me up.

2

I did some digging, because I had a hunch. #render_exception is called here:

if wrapper.show?(request)
        render_exception(request.dup, wrapper)
      else
        raise exception
      end

and ActionDispatch::ExceptionWrapper#show? :

def show?(request)
      # We're treating `nil` as "unset", and we want the default setting to be `:all`.
      # This logic should be extracted to `env_config` and calculated once.
      config = request.get_header("action_dispatch.show_exceptions")

      case config
      when :none
        false
      when :rescuable
        rescue_response?
      else
        true
      end
    end

And I can’t find config.action_dispatch.show_exceptions config set (in my repo at least), but I’m guessing on production it would be nil (which the comment suggests should be treated as :none),

and in my development env it says :all

(dev)> Rails.configuration.action_dispatch.show_exceptions
=> :all

So it probably has something to do with the case when, on your dev env, you submit (POST) a form and hit an exception—we show a page with the exception details.

And here I hit the limits of my understanding, and can just say it might be important for some reason in this case for the request to “think” it’s “GET.” But I don’t know how this would influence the framework behavior down the road. (An my coffee break is over, so can’t dig deeper ATM).

3

Thanks for digging, one thing that came to my mind is that maybe this is required for using the routes as exceptions_app, like config.exceptions_app = Rails.application.routes I’ve seen some examples where people have implemented explicit routes for their exceptions, f.e.

get "404", to: "errors#not_found"

I think if the path (and sometimes verb) were left unchanged, this approach would not work. So I guess it is the “intended” way to define routes for custom exceptions, since the change is hardcoded and is not based on the actual exceptions_app that is used. Otherwise it’d have to be wrapped in a conditional:

...
if @exceptions_app.is_a?(Rails::Engine::LazyRouteSet)
  request.path_info = "/#{status}"
  request.request_method = "GET"
end
response = @exceptions_app.call(request.env)
...

However, using the routes for exception pages has a smell in my book, since error pages can be accessed at will in that case and cannot be presented in the context of the actual Controller that raised.