You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: sendfile.rb
    path of file: ./repos/rack/lib/rack
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

require_relative 'constants'
require_relative 'utils'
require_relative 'body_proxy'

module Rack

  # = Sendfile
  #
  # The Sendfile middleware intercepts responses whose body is being
  # served from a file and replaces it with a server specific x-sendfile
  # header. The web server is then responsible for writing the file contents
  # to the client. This can dramatically reduce the amount of work required
  # by the Ruby backend and takes advantage of the web server's optimized file
  # delivery code.
  #
  # In order to take 
    
    def call(env)
      _, headers, body = response = @app.call(env)

      if body.respond_to?(:to_path)
        case type = variation(env)
        when /x-accel-redirect/i
          path = ::File.expand_path(body.to_path)
          if url = map_accel_path(env, path)
            headers[CONTENT_LENGTH] = '0'
            # '?' must be percent-encoded because it is not query string but a part of path
            headers[type.downcase] = ::Rack::Utils.escape_path(url).gsub('?', '%3F')
            obody = body
            response[2] = Rack::BodyProxy.new([]) do
              obody.close if obody.respond_to?(:close)
            end
          else
            env[RACK_ERRORS].puts "x-accel-mapping header missing"
          end