Conditionally call Rack middleware (e.g. ContentLength) depending on path

application.rb

# config/application.rb
config.middleware.use 'ConditionalContentLength', ['/foo', '/bar']

conditional_content_length.rb

# lib/conditional_content_length.rb
class ConditionalContentLength
  def initialize(app, paths)
    @app = app
    @paths = paths
  end

  def call(env)
    path_start_matches = @paths.inject(false) do |matched, path|
      matched or env['PATH_INFO'].start_with? path
    end

    if path_start_matches
      Rack::ContentLength.new(@app).call(env)
    else
      @app.call(env)
    end
  end
end