failure_catcher.rb

class CustomFormatter
  RSpec::Core::Formatters.register self, :dump_failures, :retry_command

  def initialize(output)
    @output = output
  end

  # create files called rspec_#.failures with a list of failed examples
  def dump_failures(notification)
    return if notification.failure_notifications.empty?
    f = File.new("rspec#{ENV['TEST_ENV_NUMBER']}.failures", "w+")
    notification.failure_notifications.each do |example|
      f.puts retry_command(example)
    end
    f.close
  end

  def retry_command(example)
    example_name = example.description.gsub("\"", "\\\"")
    "-e \"#{example_name}\""
  end
end

Rakefile

def gather_failures
  opts = ""
  files = Dir.glob("*.failures")
  files.each { |file| opts << File.read(file).gsub(/\n/, " ")}
  all_failures = "./all.failures"
  File.write(all_failures, opts.rstrip)
  return File.read all_failures
end

def cleanup(files = "")
  system("rm #{files}") unless Dir.glob("#{files}").empty?
end

def launch(params = {})
  if params[:test_options].include? "-e"
    count = params[:test_options].split(/failed/).count - 1
  end
  system("parallel_rspec -n #{params[:processes] || 5} \
        --test-options '#{params[:test_options]}' critical_path_specs/"
  )
end

def run(processes = 5)
  launch(
    processes: processes,
    test_options: "--require ./failure_catcher.rb --format CustomFormatter"
  )
end

def rerun(processes = 5)
  launch_critical_paths(processes: processes, test_options: gather_failures)
end

desc "parallel test execution with failure retries"
task :run_tests, :number_of_processes do |t, args|
  run args[:number_of_processes]
  unless $?.success?
    rerun args[:number_of_processes]
    cleanup "*.failures"
  end
end