rack-tracer.rb

require 'rack'

# Track changes in rack requests and responses made by middleware.
#
# SomeMiddleware
#   request: ...
#   response: ...
# AnotherMiddleware
#   request: ...
#   response: ...
# Endpoint
#   request: ...
#   response: ...
#
# Endpoints are not aware of the middlewares above them. Sinatra has
# middlewares, but then we miss the general ones from config.ru
module Rack::Tracer
  def self.included(base)
    trace = Trace.new
    base.instance_variable_set :@_rack_trace, trace

  end

  def call(env)
    response = super
    response.define_singleton_method(:rack_trace) do
      @_rack_trace
    end
    response
  end

  class Trace
    def initialize(middlewares=[])
      @middlewares = middlewares
      @chain = []
    end
  end
end

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

  def call(env)
    env['X-From'] = 'SomeMiddleware'
    @app.call(env)
  end
end

class SomeEndpoint
  include Rack::Tracer

  def call(env)
    [200, {'X-From' => 'SomeEndpoint'}, ["SomeEndpoint body"]]
  end
end

require 'minitest/autorun'
require 'rack/test'

class RackTracerTest < Minitest::Test
  include Rack::Test::Methods

  def app
    Rack::Builder.app do
      use SomeMiddleware
      run SomeEndpoint.new
    end
  end

  # Look into TracePoint API http://ruby-doc.org/core-2.0.0/TracePoint.html to
  # record calls to `call` method. Not good enough because we can't record what
  # was sent in / returned
  def test_tracepoint_on_call_methods
    trace = TracePoint.new(:call) do |tp|
      if [SomeMiddleware, SomeEndpoint].include?(tp.defined_class)
        p [tp.lineno, tp.defined_class, tp.method_id, tp.event]
      end
    end
    trace.enable
    get "/"
    trace.disable
  end

  def xtest_traces
    get "/"
    trace = last_response.rack_trace
    assert_kind_of Rack::Tracer::Trace, trace
    assert_same_elements [SomeMiddleware, SomeEndpoint], trace.chain.map(&:middleware)
  end

  def xtest_trace_chain_inspect
    get "/"
    trace = last_response.rack_trace
  end
end