rails--rails/actionpack/lib/action_dispatch/middleware/static.rb

require 'rack/utils'

module ActionDispatch
  class Static
    FILE_METHODS = %w(GET HEAD).freeze

    def initialize(app, root)
      @app = app
      @file_server = ::Rack::File.new(root)
    end

    def call(env)
      path   = env['PATH_INFO'].chomp('/')
      method = env['REQUEST_METHOD']

      if FILE_METHODS.include?(method)
        if file_exist?(path)
          return @file_server.call(env)
        else
          cached_path = directory_exist?(path) ? "#{path}/index" : path
          cached_path += ::ActionController::Base.page_cache_extension

          if file_exist?(cached_path)
            env['PATH_INFO'] = cached_path
            return @file_server.call(env)
          end
        end
      end

      @app.call(env)
    end

    private
      def file_exist?(path)
        full_path = File.join(@file_server.root, ::Rack::Utils.unescape(path))
        File.file?(full_path) && File.readable?(full_path)
      end

      def directory_exist?(path)
        full_path = File.join(@file_server.root, ::Rack::Utils.unescape(path))
        File.directory?(full_path) && File.readable?(full_path)
      end
  end
end