class Rack::Builder

def self.load_file(path, **options)

run App
require './app.rb'
use Rack::ContentLength

$ cat config.ru

Example config.ru file:

use of +__END__+ will not result in a syntax error.
Ignores content in the file after +__END__+, so that

contents as if specified inside a Rack::Builder block.
Load the given file as a rackup file, treating the 
def self.load_file(path, **options)
  config = ::File.read(path)
  config.slice!(/\A#{UTF_8_BOM}/) if config.encoding == Encoding::UTF_8
  if config[/^#\\(.*)/]
    fail "Parsing options from the first comment line is no longer supported: #{path}"
  end
  config.sub!(/^__END__\n.*\Z/m, '')
  return new_from_string(config, path, **options)
end