Catching Invalid JSON Parse Errors with Rack Middleware

Question

I am using Rails 5 and I am trying to improve error handling for invalid JSON requests to my API.

I tried handling invalid format JSON by parsing in the controller with a rescue but realised that Rails middleware is parsing my JSON request before it hits the controller if a user adds Content Type to their request header.

I followed the below guide: https://robots.thoughtbot.com/catching-json-parse-errors-with-custom-middleware

However, I get the following error when starting the server:

.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/stack.rb:108:in `assert_index': No such middleware to insert before: ActionDispatch::ParamsParser (RuntimeError)

Now, what this means is that ActionDispatch::ParamsParser isn't running. I think that it is deprecated in Rails 5 so that rules out that option.

I also tried to use rescue_from in my API Controller:

rescue_from JSON::ParserError, with: :json_error

def json_error
  render status: 400, json: {error: "Invalid JSON format in request body"}, status: :unprocessable_entity
end

However, this also did not work. It seems to skip past it.

Or if I try this:

rescue_from JSON::ParserError, with: json_error

def json_error
  render status: 400, json: {error: "Invalid JSON format in request body"}, status: :unprocessable_entity
end

I get:

undefined local variable or method `json_error' for Api::ApiController:Class
actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb, line 46

``` ruby
   41         private
   42   
   43           def controller(req)
   44             req.controller_class
   45           rescue NameError => e
>  46             raise ActionController::RoutingError, e.message, e.backtrace
   47           end
   48   
   49           def dispatch(controller, action, req, res)
   50             controller.dispatch(action, req, res)
   51           end

Getting very lost, could use some guidance

Consequence

Did you added app/middleware/catch_json_parse_errors.rb to the project? More than that, app/middleware should be on autoload_paths of your application. – Mellifluent

Yes, I added the file. The error is related to ActionDispatch::ParamsParser, not my class – Consequence

Thanks for adding the blogpost you were referencing, it helped me find this question! – Reliable

Answer 1

Seems the guide above is outdated with Rails 5. After some investigation, it seems that the following middleware is no longer called:

config/application.rb

config.middleware.insert_before ActionDispatch::ParamsParser, "CatchJsonParseErrors"

I modified it to be:

require "./lib/middleware/catch_json_parse_errors.rb"
config.middleware.insert_before Rack::Head, CatchJsonParseErrors

This is because Rack::Head is in the middleware stack, but ActionDispatch::ParamsParser is not. Also, the use of Class names as strings is deprecated, so you need to require the file and then pass in the class.

I also modified the below class to check env['CONTENT_TYPE'] instead of env['HTTP_ACCEPT']

class CatchJsonParseErrors
  def initialize(app)
    @app = app
  end

  def call(env)
    begin
      @app.call(env)
    rescue ActionDispatch::ParamsParser::ParseError => error
      if env['CONTENT_TYPE'] =~ /application\/json/
        error_output = "There was a problem in the JSON you submitted: #{error.class}"
        return [
          400, { "Content-Type" => "application/json" },
          [ { status: 400, error: error_output }.to_json ]
        ]
      else
        raise error
      end
    end
  end
end

Consequence

Why did you change HTTP_ACCEPT to CONTENT_TYPE? The Accept-Header appears to be the correct source of information as it defines what the client expects to get back – whereas ContentType defines what comes in (which has nothing to do with the format of the response). – Tibbetts

@AndreasBaumgart that is because this middleware is suppose to check for invalid json request, not response. However i do suggest to keep the existing check on accept header. – Tameshatamez

as of today, look like it raises ActionDispatch::Http::Parameters::ParseError instead – Gemmell