lint.rb

# frozen_string_literal: true

require 'rack/utils'
require 'forwardable'

module Rack
  # Rack::Lint validates your application and the requests and
  # responses according to the Rack spec.

  class Lint
    def initialize(app)
      @app = app
      @content_length = nil
    end

    # :stopdoc:

    class LintError < RuntimeError; end
    module Assertion
      def assert(message)
        unless yield
          raise LintError, message
        end
      end
    end
    include Assertion

    ## This specification aims to formalize the Rack protocol.  You
    ## can (and should) use Rack::Lint to enforce it.
    ##
    ## When you develop middleware, be sure to add a Lint before and
    ## after to catch all mistakes.

    ## = Rack applications

    ## A Rack application is a Ruby object (not a class) that
    ## responds to +call+.
    def call(env=nil)
      dup._call(env)
    end

    def _call(env)
      ## It takes exactly one argument, the *environment*
      assert("No env given") { env }
      check_env env

      env[RACK_INPUT] = InputWrapper.new(env[RACK_INPUT])
      env[RACK_ERRORS] = ErrorWrapper.new(env[RACK_ERRORS])

      ## and returns an Array of exactly three values:
      status, headers, @body = @app.call(env)
      ## The *status*,
      check_status status
      ## the *headers*,
      check_headers headers

      check_hijack_response headers, env

      ## and the *body*.
      check_content_type status, headers
      check_content_length status, headers
      @head_request = env[REQUEST_METHOD] == HEAD
      [status, headers, self]
    end

    ## == The Environment
    def check_env(env)
      ## The environment must be an instance of Hash that includes
      ## CGI-like headers.  The application is free to modify the
      ## environment.
      assert("env #{env.inspect} is not a Hash, but #{env.class}") {
        env.kind_of? Hash
      }

      ##
      ## The environment is required to include these variables
      ## (adopted from PEP333), except when they'd be empty, but see
      ## below.

      ## <tt>REQUEST_METHOD</tt>:: The HTTP request method, such as
      ##                           "GET" or "POST". This cannot ever
      ##                           be an empty string, and so is
      ##                           always required.

      ## <tt>SCRIPT_NAME</tt>:: The initial portion of the request
      ##                        URL's "path" that corresponds to the
      ##                        application object, so that the
      ##                        application knows its virtual
      ##                        "location". This may be an empty
      ##                        string, if the application corresponds
      ##                        to the "root" of the server.

      ## <tt>PATH_INFO</tt>:: The remainder of the request URL's
      ##                      "path", designating the virtual
      ##                      "location" of the request's target
      ##                      within the application. This may be an
      ##                      empty string, if the request URL targets
      ##                      the application root and does not have a
      ##                      trailing slash. This value may be
      ##                      percent-encoded when originating from
      ##                      a URL.

      ## <tt>QUERY_STRING</tt>:: The portion of the request URL that
      ##                         follows the <tt>?</tt>, if any. May be
      ##                         empty, but is always required!

      ## <tt>SERVER_NAME</tt>, <tt>SERVER_PORT</tt>::
      ##                        When combined with <tt>SCRIPT_NAME</tt> and
      ##                        <tt>PATH_INFO</tt>, these variables can be
      ##                        used to complete the URL. Note, however,
      ##                        that <tt>HTTP_HOST</tt>, if present,
      ##                        should be used in preference to
      ##                        <tt>SERVER_NAME</tt> for reconstructing
      ##                        the request URL.
      ##                        <tt>SERVER_NAME</tt> and <tt>SERVER_PORT</tt>
      ##                        can never be empty strings, and so
      ##                        are always required.

      ## <tt>HTTP_</tt> Variables:: Variables corresponding to the
      ##                            client-supplied HTTP request
      ##                            headers (i.e., variables whose
      ##                            names begin with <tt>HTTP_</tt>). The
      ##                            presence or absence of these
      ##                            variables should correspond with
      ##                            the presence or absence of the
      ##                            appropriate HTTP header in the
      ##                            request. See
      ##                            <a href="https://tools.ietf.org/html/rfc3875#section-4.1.18">
      ##                            RFC3875 section 4.1.18</a> for
      ##                            specific behavior.

      ## In addition to this, the Rack environment must include these
      ## Rack-specific variables:

      ## <tt>rack.version</tt>:: The Array representing this version of Rack
      ##                         See Rack::VERSION, that corresponds to
      ##                         the version of this SPEC.

      ## <tt>rack.url_scheme</tt>:: +http+ or +https+, depending on the
      ##                            request URL.

      ## <tt>rack.input</tt>:: See below, the input stream.

      ## <tt>rack.errors</tt>:: See below, the error stream.

      ## <tt>rack.multithread</tt>:: true if the application object may be
      ##                             simultaneously invoked by another thread
      ##                             in the same process, false otherwise.

      ## <tt>rack.multiprocess</tt>:: true if an equivalent application object
      ##                              may be simultaneously invoked by another
      ##                              process, false otherwise.

      ## <tt>rack.run_once</tt>:: true if the server expects
      ##                          (but does not guarantee!) that the
      ##                          application will only be invoked this one
      ##                          time during the life of its containing
      ##                          process. Normally, this will only be true
      ##                          for a server based on CGI
      ##                          (or something similar).

      ## <tt>rack.hijack?</tt>:: present and true if the server supports
      ##                         connection hijacking. See below, hijacking.

      ## <tt>rack.hijack</tt>:: an object responding to #call that must be
      ##                        called at least once before using
      ##                        rack.hijack_io.
      ##                        It is recommended #call return rack.hijack_io
      ##                        as well as setting it in env if necessary.

      ## <tt>rack.hijack_io</tt>:: if rack.hijack? is true, and rack.hijack
      ##                           has received #call, this will contain
      ##                           an object resembling an IO. See hijacking.

      ## Additional environment specifications have approved to
      ## standardized middleware APIs.  None of these are required to
      ## be implemented by the server.

      ## <tt>rack.session</tt>:: A hash like interface for storing
      ##                         request session data.
      ##                         The store must implement:
      if session = env[RACK_SESSION]
        ##                         store(key, value)         (aliased as []=);
        assert("session #{session.inspect} must respond to store and []=") {
          session.respond_to?(:store) && session.respond_to?(:[]=)
        }

        ##                         fetch(key, default = nil) (aliased as []);
        assert("session #{session.inspect} must respond to fetch and []") {
          session.respond_to?(:fetch) && session.respond_to?(:[])
        }

        ##                         delete(key);
        assert("session #{session.inspect} must respond to delete") {
          session.respond_to?(:delete)
        }

        ##                         clear;
        assert("session #{session.inspect} must respond to clear") {
          session.respond_to?(:clear)
        }
      end

      ## <tt>rack.logger</tt>:: A common object interface for logging messages.
      ##                        The object must implement:
      if logger = env[RACK_LOGGER]
        ##                         info(message, &block)
        assert("logger #{logger.inspect} must respond to info") {
          logger.respond_to?(:info)
        }

        ##                         debug(message, &block)
        assert("logger #{logger.inspect} must respond to debug") {
          logger.respond_to?(:debug)
        }

        ##                         warn(message, &block)
        assert("logger #{logger.inspect} must respond to warn") {
          logger.respond_to?(:warn)
        }

        ##                         error(message, &block)
        assert("logger #{logger.inspect} must respond to error") {
          logger.respond_to?(:error)
        }

        ##                         fatal(message, &block)
        assert("logger #{logger.inspect} must respond to fatal") {
          logger.respond_to?(:fatal)
        }
      end

      ## <tt>rack.multipart.buffer_size</tt>:: An Integer hint to the multipart parser as to what chunk size to use for reads and writes.
      if bufsize = env[RACK_MULTIPART_BUFFER_SIZE]
        assert("rack.multipart.buffer_size must be an Integer > 0 if specified") {
          bufsize.is_a?(Integer) && bufsize > 0
        }
      end

      ## <tt>rack.multipart.tempfile_factory</tt>:: An object responding to #call with two arguments, the filename and content_type given for the multipart form field, and returning an IO-like object that responds to #<< and optionally #rewind. This factory will be used to instantiate the tempfile for each multipart form file upload field, rather than the default class of Tempfile.
      if tempfile_factory = env[RACK_MULTIPART_TEMPFILE_FACTORY]
        assert("rack.multipart.tempfile_factory must respond to #call") { tempfile_factory.respond_to?(:call) }
        env[RACK_MULTIPART_TEMPFILE_FACTORY] = lambda do |filename, content_type|
          io = tempfile_factory.call(filename, content_type)
          assert("rack.multipart.tempfile_factory return value must respond to #<<") { io.respond_to?(:<<) }
          io
        end
      end

      ## The server or the application can store their own data in the
      ## environment, too.  The keys must contain at least one dot,
      ## and should be prefixed uniquely.  The prefix <tt>rack.</tt>
      ## is reserved for use with the Rack core distribution and other
      ## accepted specifications and must not be used otherwise.
      ##

      %w[REQUEST_METHOD SERVER_NAME SERVER_PORT
         QUERY_STRING
         rack.version rack.input rack.errors
         rack.multithread rack.multiprocess rack.run_once].each { |header|
        assert("env missing required key #{header}") { env.include? header }
      }

      ## The environment must not contain the keys
      ## <tt>HTTP_CONTENT_TYPE</tt> or <tt>HTTP_CONTENT_LENGTH</tt>
      ## (use the versions without <tt>HTTP_</tt>).
      %w[HTTP_CONTENT_TYPE HTTP_CONTENT_LENGTH].each { |header|
        assert("env contains #{header}, must use #{header[5,-1]}") {
          not env.include? header
        }
      }

      ## The CGI keys (named without a period) must have String values.
      env.each { |key, value|
        next  if key.include? "."   # Skip extensions
        assert("env variable #{key} has non-string value #{value.inspect}") {
          value.kind_of? String
        }
      }

      ## There are the following restrictions:

      ## * <tt>rack.version</tt> must be an array of Integers.
      assert("rack.version must be an Array, was #{env[RACK_VERSION].class}") {
        env[RACK_VERSION].kind_of? Array
      }
      ## * <tt>rack.url_scheme</tt> must either be +http+ or +https+.
      assert("rack.url_scheme unknown: #{env[RACK_URL_SCHEME].inspect}") {
        %w[http https].include?(env[RACK_URL_SCHEME])
      }

      ## * There must be a valid input stream in <tt>rack.input</tt>.
      check_input env[RACK_INPUT]
      ## * There must be a valid error stream in <tt>rack.errors</tt>.
      check_error env[RACK_ERRORS]
      ## * There may be a valid hijack stream in <tt>rack.hijack_io</tt>
      check_hijack env

      ## * The <tt>REQUEST_METHOD</tt> must be a valid token.
      assert("REQUEST_METHOD unknown: #{env[REQUEST_METHOD]}") {
        env[REQUEST_METHOD] =~ /\A[0-9A-Za-z!\#$%&'*+.^_`|~-]+\z/
      }

      ## * The <tt>SCRIPT_NAME</tt>, if non-empty, must start with <tt>/</tt>
      assert("SCRIPT_NAME must start with /") {
        !env.include?(SCRIPT_NAME) ||
        env[SCRIPT_NAME] == "" ||
        env[SCRIPT_NAME] =~ /\A\//
      }
      ## * The <tt>PATH_INFO</tt>, if non-empty, must start with <tt>/</tt>
      assert("PATH_INFO must start with /") {
        !env.include?(PATH_INFO) ||
        env[PATH_INFO] == "" ||
        env[PATH_INFO] =~ /\A\//
      }
      ## * The <tt>CONTENT_LENGTH</tt>, if given, must consist of digits only.
      assert("Invalid CONTENT_LENGTH: #{env["CONTENT_LENGTH"]}") {
        !env.include?("CONTENT_LENGTH") || env["CONTENT_LENGTH"] =~ /\A\d+\z/
      }

      ## * One of <tt>SCRIPT_NAME</tt> or <tt>PATH_INFO</tt> must be
      ##   set.  <tt>PATH_INFO</tt> should be <tt>/</tt> if
      ##   <tt>SCRIPT_NAME</tt> is empty.
      assert("One of SCRIPT_NAME or PATH_INFO must be set (make PATH_INFO '/' if SCRIPT_NAME is empty)") {
        env[SCRIPT_NAME] || env[PATH_INFO]
      }
      ##   <tt>SCRIPT_NAME</tt> never should be <tt>/</tt>, but instead be empty.
      assert("SCRIPT_NAME cannot be '/', make it '' and PATH_INFO '/'") {
        env[SCRIPT_NAME] != "/"
      }
    end

    ## === The Input Stream
    ##
    ## The input stream is an IO-like object which contains the raw HTTP
    ## POST data.
    def check_input(input)
      ## When applicable, its external encoding must be "ASCII-8BIT" and it
      ## must be opened in binary mode, for Ruby 1.9 compatibility.
      assert("rack.input #{input} does not have ASCII-8BIT as its external encoding") {
        input.external_encoding.name == "ASCII-8BIT"
      } if input.respond_to?(:external_encoding)
      assert("rack.input #{input} is not opened in binary mode") {
        input.binmode?
      } if input.respond_to?(:binmode?)

      ## The input stream must respond to +gets+, +each+, +read+ and +rewind+.
      [:gets, :each, :read, :rewind].each { |method|
        assert("rack.input #{input} does not respond to ##{method}") {
          input.respond_to? method
        }
      }
    end

    class InputWrapper
      include Assertion

      def initialize(input)
        @input = input
      end

      ## * +gets+ must be called without arguments and return a string,
      ##   or +nil+ on EOF.
      def gets(*args)
        assert("rack.input#gets called with arguments") { args.size == 0 }
        v = @input.gets
        assert("rack.input#gets didn't return a String") {
          v.nil? or v.kind_of? String
        }
        v
      end

      ## * +read+ behaves like IO#read.
      ##   Its signature is <tt>read([length, [buffer]])</tt>.
      ##
      ##   If given, +length+ must be a non-negative Integer (>= 0) or +nil+,
      ##   and +buffer+ must be a String and may not be nil.
      ##
      ##   If +length+ is given and not nil, then this method reads at most
      ##   +length+ bytes from the input stream.
      ##
      ##   If +length+ is not given or nil, then this method reads
      ##   all data until EOF.
      ##
      ##   When EOF is reached, this method returns nil if +length+ is given
      ##   and not nil, or "" if +length+ is not given or is nil.
      ##
      ##   If +buffer+ is given, then the read data will be placed
      ##   into +buffer+ instead of a newly created String object.
      def read(*args)
        assert("rack.input#read called with too many arguments") {
          args.size <= 2
        }
        if args.size >= 1
          assert("rack.input#read called with non-integer and non-nil length") {
            args.first.kind_of?(Integer) || args.first.nil?
          }
          assert("rack.input#read called with a negative length") {
            args.first.nil? || args.first >= 0
          }
        end
        if args.size >= 2
          assert("rack.input#read called with non-String buffer") {
            args[1].kind_of?(String)
          }
        end

        v = @input.read(*args)

        assert("rack.input#read didn't return nil or a String") {
          v.nil? or v.kind_of? String
        }
        if args[0].nil?
          assert("rack.input#read(nil) returned nil on EOF") {
            !v.nil?
          }
        end

        v
      end

      ## * +each+ must be called without arguments and only yield Strings.
      def each(*args)
        assert("rack.input#each called with arguments") { args.size == 0 }
        @input.each { |line|
          assert("rack.input#each didn't yield a String") {
            line.kind_of? String
          }
          yield line
        }
      end

      ## * +rewind+ must be called without arguments. It rewinds the input
      ##   stream back to the beginning. It must not raise Errno::ESPIPE:
      ##   that is, it may not be a pipe or a socket. Therefore, handler
      ##   developers must buffer the input data into some rewindable object
      ##   if the underlying input stream is not rewindable.
      def rewind(*args)
        assert("rack.input#rewind called with arguments") { args.size == 0 }
        assert("rack.input#rewind raised Errno::ESPIPE") {
          begin
            @input.rewind
            true
          rescue Errno::ESPIPE
            false
          end
        }
      end

      ## * +close+ must never be called on the input stream.
      def close(*args)
        assert("rack.input#close must not be called") { false }
      end
    end

    ## === The Error Stream
    def check_error(error)
      ## The error stream must respond to +puts+, +write+ and +flush+.
      [:puts, :write, :flush].each { |method|
        assert("rack.error #{error} does not respond to ##{method}") {
          error.respond_to? method
        }
      }
    end

    class ErrorWrapper
      include Assertion

      def initialize(error)
        @error = error
      end

      ## * +puts+ must be called with a single argument that responds to +to_s+.
      def puts(str)
        @error.puts str
      end

      ## * +write+ must be called with a single argument that is a String.
      def write(str)
        assert("rack.errors#write not called with a String") { str.kind_of? String }
        @error.write str
      end

      ## * +flush+ must be called without arguments and must be called
      ##   in order to make the error appear for sure.
      def flush
        @error.flush
      end

      ## * +close+ must never be called on the error stream.
      def close(*args)
        assert("rack.errors#close must not be called") { false }
      end
    end

    class HijackWrapper
      include Assertion
      extend Forwardable

      REQUIRED_METHODS = [
        :read, :write, :read_nonblock, :write_nonblock, :flush, :close,
        :close_read, :close_write, :closed?
      ]

      def_delegators :@io, *REQUIRED_METHODS

      def initialize(io)
        @io = io
        REQUIRED_METHODS.each do |meth|
          assert("rack.hijack_io must respond to #{meth}") { io.respond_to? meth }
        end
      end
    end

    ## === Hijacking
    #
    # AUTHORS: n.b. The trailing whitespace between paragraphs is important and
    # should not be removed. The whitespace creates paragraphs in the RDoc
    # output.
    #
    ## ==== Request (before status)
    def check_hijack(env)
      if env[RACK_IS_HIJACK]
        ## If rack.hijack? is true then rack.hijack must respond to #call.
        original_hijack = env[RACK_HIJACK]
        assert("rack.hijack must respond to call") { original_hijack.respond_to?(:call) }
        env[RACK_HIJACK] = proc do
          ## rack.hijack must return the io that will also be assigned (or is
          ## already present, in rack.hijack_io.
          io = original_hijack.call
          HijackWrapper.new(io)
          ##
          ## rack.hijack_io must respond to:
          ## <tt>read, write, read_nonblock, write_nonblock, flush, close,
          ## close_read, close_write, closed?</tt>
          ##
          ## The semantics of these IO methods must be a best effort match to
          ## those of a normal ruby IO or Socket object, using standard
          ## arguments and raising standard exceptions. Servers are encouraged
          ## to simply pass on real IO objects, although it is recognized that
          ## this approach is not directly compatible with SPDY and HTTP 2.0.
          ##
          ## IO provided in rack.hijack_io should preference the
          ## IO::WaitReadable and IO::WaitWritable APIs wherever supported.
          ##
          ## There is a deliberate lack of full specification around
          ## rack.hijack_io, as semantics will change from server to server.
          ## Users are encouraged to utilize this API with a knowledge of their
          ## server choice, and servers may extend the functionality of
          ## hijack_io to provide additional features to users. The purpose of
          ## rack.hijack is for Rack to "get out of the way", as such, Rack only
          ## provides the minimum of specification and support.
          env[RACK_HIJACK_IO] = HijackWrapper.new(env[RACK_HIJACK_IO])
          io
        end
      else
        ##
        ## If rack.hijack? is false, then rack.hijack should not be set.
        assert("rack.hijack? is false, but rack.hijack is present") { env[RACK_HIJACK].nil? }
        ##
        ## If rack.hijack? is false, then rack.hijack_io should not be set.
        assert("rack.hijack? is false, but rack.hijack_io is present") { env[RACK_HIJACK_IO].nil? }
      end
    end

    ## ==== Response (after headers)
    ## It is also possible to hijack a response after the status and headers
    ## have been sent.
    def check_hijack_response(headers, env)

      # this check uses headers like a hash, but the spec only requires
      # headers respond to #each
      headers = Rack::Utils::HeaderHash.new(headers)

      ## In order to do this, an application may set the special header
      ## <tt>rack.hijack</tt> to an object that responds to <tt>call</tt>
      ## accepting an argument that conforms to the <tt>rack.hijack_io</tt>
      ## protocol.
      ##
      ## After the headers have been sent, and this hijack callback has been
      ## called, the application is now responsible for the remaining lifecycle
      ## of the IO. The application is also responsible for maintaining HTTP
      ## semantics. Of specific note, in almost all cases in the current SPEC,
      ## applications will have wanted to specify the header Connection:close in
      ## HTTP/1.1, and not Connection:keep-alive, as there is no protocol for
      ## returning hijacked sockets to the web server. For that purpose, use the
      ## body streaming API instead (progressively yielding strings via each).
      ##
      ## Servers must ignore the <tt>body</tt> part of the response tuple when
      ## the <tt>rack.hijack</tt> response API is in use.

      if env[RACK_IS_HIJACK] && headers[RACK_HIJACK]
        assert('rack.hijack header must respond to #call') {
          headers[RACK_HIJACK].respond_to? :call
        }
        original_hijack = headers[RACK_HIJACK]
        headers[RACK_HIJACK] = proc do |io|
          original_hijack.call HijackWrapper.new(io)
        end
      else
        ##
        ## The special response header <tt>rack.hijack</tt> must only be set
        ## if the request env has <tt>rack.hijack?</tt> <tt>true</tt>.
        assert('rack.hijack header must not be present if server does not support hijacking') {
          headers[RACK_HIJACK].nil?
        }
      end
    end
    ## ==== Conventions
    ## * Middleware should not use hijack unless it is handling the whole
    ##   response.
    ## * Middleware may wrap the IO object for the response pattern.
    ## * Middleware should not wrap the IO object for the request pattern. The
    ##   request pattern is intended to provide the hijacker with "raw tcp".

    ## == The Response

    ## === The Status
    def check_status(status)
      ## This is an HTTP status. When parsed as integer (+to_i+), it must be
      ## greater than or equal to 100.
      assert("Status must be >=100 seen as integer") { status.to_i >= 100 }
    end

    ## === The Headers
    def check_headers(header)
      ## The header must respond to +each+, and yield values of key and value.
      assert("headers object should respond to #each, but doesn't (got #{header.class} as headers)") {
         header.respond_to? :each
      }
      header.each { |key, value|
        ## Special headers starting "rack." are for communicating with the
        ## server, and must not be sent back to the client.
        next if key =~ /^rack\..+$/

        ## The header keys must be Strings.
        assert("header key must be a string, was #{key.class}") {
          key.kind_of? String
        }
        ## The header must not contain a +Status+ key.
        assert("header must not contain Status") { key.downcase != "status" }
        ## The header must conform to RFC7230 token specification, i.e. cannot
        ## contain non-printable ASCII, DQUOTE or "(),/:;<=>?@[\]{}".
        assert("invalid header name: #{key}") { key !~ /[\(\),\/:;<=>\?@\[\\\]{}[:cntrl:]]/ }

        ## The values of the header must be Strings,
        assert("a header value must be a String, but the value of " +
          "'#{key}' is a #{value.class}") { value.kind_of? String }
        ## consisting of lines (for multiple header values, e.g. multiple
        ## <tt>Set-Cookie</tt> values) separated by "\\n".
        value.split("\n").each { |item|
          ## The lines must not contain characters below 037.
          assert("invalid header value #{key}: #{item.inspect}") {
            item !~ /[\000-\037]/
          }
        }
      }
    end

    ## === The Content-Type
    def check_content_type(status, headers)
      headers.each { |key, value|
        ## There must not be a <tt>Content-Type</tt>, when the +Status+ is 1xx,
        ## 204 or 304.
        if key.downcase == "content-type"
          assert("Content-Type header found in #{status} response, not allowed") {
            not Rack::Utils::STATUS_WITH_NO_ENTITY_BODY.include? status.to_i
          }
          return
        end
      }
    end

    ## === The Content-Length
    def check_content_length(status, headers)
      headers.each { |key, value|
        if key.downcase == 'content-length'
          ## There must not be a <tt>Content-Length</tt> header when the
          ## +Status+ is 1xx, 204 or 304.
          assert("Content-Length header found in #{status} response, not allowed") {
            not Rack::Utils::STATUS_WITH_NO_ENTITY_BODY.include? status.to_i
          }
          @content_length = value
        end
      }
    end

    def verify_content_length(bytes)
      if @head_request
        assert("Response body was given for HEAD request, but should be empty") {
          bytes == 0
        }
      elsif @content_length
        assert("Content-Length header was #{@content_length}, but should be #{bytes}") {
          @content_length == bytes.to_s
        }
      end
    end

    ## === The Body
    def each
      @closed = false
      bytes = 0

      ## The Body must respond to +each+
      assert("Response body must respond to each") do
        @body.respond_to?(:each)
      end

      @body.each { |part|
        ## and must only yield String values.
        assert("Body yielded non-string value #{part.inspect}") {
          part.kind_of? String
        }
        bytes += part.bytesize
        yield part
      }
      verify_content_length(bytes)

      ##
      ## The Body itself should not be an instance of String, as this will
      ## break in Ruby 1.9.
      ##
      ## If the Body responds to +close+, it will be called after iteration. If
      ## the body is replaced by a middleware after action, the original body
      ## must be closed first, if it responds to close.
      # XXX howto: assert("Body has not been closed") { @closed }


      ##
      ## If the Body responds to +to_path+, it must return a String
      ## identifying the location of a file whose contents are identical
      ## to that produced by calling +each+; this may be used by the
      ## server as an alternative, possibly more efficient way to
      ## transport the response.

      if @body.respond_to?(:to_path)
        assert("The file identified by body.to_path does not exist") {
          ::File.exist? @body.to_path
        }
      end

      ##
      ## The Body commonly is an Array of Strings, the application
      ## instance itself, or a File-like object.
    end

    def close
      @closed = true
      @body.close  if @body.respond_to?(:close)
    end

    # :startdoc:

  end
end

## == Thanks
## Some parts of this specification are adopted from PEP333: Python
## Web Server Gateway Interface
## v1.0 (http://www.python.org/dev/peps/pep-0333/). I'd like to thank
## everyone involved in that effort.