test.rb

require 'bundler'
Bundler.setup(:default)

require 'rails'
require 'action_controller/railtie'

class TestApp < Rails::Application
  config.root = File.dirname(__FILE__)
  config.session_store :cookie_store, key: 'cookie_store_key'
  config.secret_token    = 'secret_token'
  config.secret_key_base = 'secret_key_base'

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  routes.draw do
    get '/with_filter' => 'class#index'
    get '/without_filter' => 'skipping_class#index'
  end
end

class ConditionalClassFilter
  def self.before(controller) controller.instance_variable_set(:"@ran_class_filter", true) end
end

class ClassController < ActionController::Base
  before_filter ConditionalClassFilter

  def index
    render text: @ran_class_filter.to_s
  end
end

class SkippingClassController < ClassController
  skip_before_filter ConditionalClassFilter
end

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

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

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

  def test_with_class_filter
    get '/with_filter'
    assert last_response.ok?
    assert_equal "true", last_response.body
  end

  def test_without_class_filter
    get '/without_filter'
    assert last_response.ok?
    assert_equal "", last_response.body
  end

  private
    def app
      Rails.application
    end
end

test.sh

#!/bin/bash

rm Gemfile.lock
bundle
ruby test.rb

git bisect start 4-0-stable master
git bisect run test.sh