Insert Rack Middleware during Tests

Posted on October 24, 2014 by Carsten Zimmermann

We recently deployed a wee internal Rails Engine that logs raw API data in order to save the original payload that was pushed over the wire. That little piece of software, however, had a bug (which is pretty uncommon for software, right?): The logged payload was always empty.

The Engine’s host application uses Rails’ ActionDispatch::XmlParamsParser middleware. It would seem that it messed up the contents of rack.input which in turn holds the POST data.

rack.input is an IO-like object and it needs to be rewound after it has been read from. XmlParamsParser reads from it, but apparently doesn’t rewind it. It seams easy enough to fix, but how to testdrive it?

I didn’t want to statically include a rack.input-modifying middleware into the Engine’s dummy app. Instead I wanted add another context that tested against a version of the dummy infected with a mock middleware and have all other tests run without it.

Here’s how I did it. Enjoy!

# spec/requests/rack_input_modifiable_spec.rb
describe 'Tracks data with rack.input modifying middleware in place' do

  # Need to bind this to a constant in order to hook it in later.
  # It does nothing except putting the IO read marker to the end.
  ReadsFromRackInput = do
    def call(env)
      env['rack.input'].read # moves to EOF

  # Dummy::Application is already loaded at this stage. We can't modify
  # the middleware stack as it's frozen, so subclass it and infect:
  def app_with_middleware
    @app_with_middleware ||= do
      config.middleware.insert_after \
        ActionDispatch::ParamsParser, ReadsFromRackInput

  # `app´ is what RSpec tests against in request specs. Think `controller´
  # for controller specs. Overwrite it with our infected app.
  def app; @app ||= app_with_middleware end

  # Tweaks to get the subclassed app to get all of its parent's behaviour.
  before do
    allow(app).to receive(:routes).and_return Dummy::Application.routes

  it_behaves_like 'tracks raw data' # i.e. the actual tests
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