Single threading Capybara/Selenium for using transactional fixtures

single_thread_helper.rb

require 'thread'

class MyBlockCaller
  def initialize
    @block = nil
    @lock = Mutex.new
    @result = nil

    @call_done = ConditionVariable.new
    @ready_to_call = ConditionVariable.new
    @exception = nil
  end
  def pass_block(&block)
    @lock.synchronize do
      @block=block
      @exception=nil
      @ready_to_call.broadcast
      @call_done.wait(@lock)
      result=@result
      @result=nil # garbage collect
      raise @exception if @exception
      result
    end
  end

  def call_block
    @lock.synchronize do
      looper_result=nil
      looper_exception=nil
      looper_running=true
      looper_thread = Thread.new do
        begin
          looper_result= yield
          unless looper_result
            sleep 0.05
          end
        rescue Exception => looper_exception
        ensure
          @lock.synchronize do
            looper_running=false
            @ready_to_call.broadcast
          end
        end
      end
      while looper_running
        if !@block && looper_running
          @ready_to_call.wait(@lock)
        end
        if @block
          begin
            @result=@block.call
          rescue Exception => @exception
          ensure
            @block=nil
            @call_done.signal
          end
        end
        nil
      end
      looper_thread.join
      raise looper_exception if looper_exception
      looper_result
    end
  end
end

$my_block_caller=MyBlockCaller.new

require 'action_dispatch/middleware/request_id'

ActionDispatch::RequestId.class_eval do
  def call_with_single_thread(env)
    $my_block_caller.pass_block do
      call_without_single_thread(env)
    end
  end
  alias_method_chain :call, :single_thread
end

require 'selenium/webdriver'

module Selenium
  module WebDriver
    module Remote
      class Bridge
        def execute(*args)
          $my_block_caller.call_block {raw_execute(*args)['value']}
        end
      end
    end
  end
end

require 'capybara/util/timeout'

module Capybara
  class << self

    ##
    # Provides timeout similar to standard library Timeout, but avoids threads
    #
    def timeout(seconds = 1, driver = nil, error_message = nil, &block)
      start_time = Time.now

      result = nil

      until result
        result=$my_block_caller.call_block(&block)
        return result if result

        delay = seconds - (Time.now - start_time)
        if delay <= 0
          raise TimeoutError, error_message || "timed out #{[seconds, delay, start_time].inspect}"
        end

        driver && driver.wait_until(delay)
      end
    end
  end
end