Rails test download helper [thread safe]

application_system_test_case.rb

require 'test_helper'

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  WINDOW_SIZE = [1400, 1400].freeze
  driven_by :selenium, using: :chrome, screen_size: WINDOW_SIZE do |options|
    options.add_preference(:download, prompt_for_download: false, default_directory: DownloadHelper::PATH.to_s)
    options.add_preference(:browser, set_download_behavior: { behavior: 'allow' })
  end
end

my_system_test.rb

require 'application_system_test_case'

class DownloadsTest < ApplicationSystemTestCase

  test 'should download' do
    visit download_url
    download = DownloadHelper.download { click_on 'Download' }
    file = File.open download
    # assert properties of file
  end
end

rails_download_helper.rb

# Modified version of a common download helper that adds thread safety
# Now you can initiate a mutli-threaded system test and your file download operations will still work as expected
# Before you were not guaranteed to get the download your test requested but could get one from another test
module DownloadHelper
  TIMEOUT = 1
  PATH    = Rails.root.join('tmp/downloads')
  SEMAPHORE = Mutex.new

  def self.downloads
    Dir[PATH.join('*')].select(&File.method(:file?))
                       .sort_by &File.method(:mtime)
  end

  def self.download(&block)
    SEMAPHORE.synchronize {
      wait_for_download &block
      downloads.last
    }
  end

  def self.wait_for_download(&block)
    block.call
    @current_download_count = downloads.size
    Timeout.timeout(TIMEOUT) do
      sleep 0.1 until downloaded?
    end
  end

  def self.downloaded?
    !downloading? && @current_download_count < downloads.size
  end

  def self.downloading?
    downloads.grep(/\.crdownload$/).any?
  end

  def self.clear_downloads
    FileUtils.rm_f(downloads)
  end
end