# File lib/fpm/package/virtualenv.rb, line 54
  def input(package)
    installdir = attributes[:virtualenv_install_location]
    m = /^([^=]+)==([^=]+)$/.match(package)
    package_version = nil

    is_requirements_file = (File.basename(package) == "requirements.txt")

    if is_requirements_file
      if !File.file?(package)
        raise FPM::InvalidPackageConfiguration, "Path looks like a requirements.txt, but it doesn't exist: #{package}"
      end

      package = File.join(::Dir.pwd, package) if File.dirname(package) == "."
      package_name = File.basename(File.dirname(package))
      logger.info("No name given. Using the directory's name", :name => package_name)
      package_version = nil
    elsif m
      package_name = m[1]
      package_version = m[2]
      self.version ||= package_version
    else
      package_name = package
      package_version = nil
    end

    virtualenv_name = package_name

    self.name ||= package_name

    if self.attributes[:virtualenv_fix_name?]
      self.name = [self.attributes[:virtualenv_package_name_prefix],
                   self.name].join("-")
    end

    # prefix wins over previous virtual_install_location behaviour
    virtualenv_folder =
      if self.attributes[:prefix]
        self.attributes[:prefix]
      else
        File.join(installdir,
                  virtualenv_name)
      end

    virtualenv_build_folder = build_path(virtualenv_folder)

    ::FileUtils.mkdir_p(virtualenv_build_folder)

    if self.attributes[:virtualenv_system_site_packages?]
        logger.info("Creating virtualenv with --system-site-packages")
        safesystem("virtualenv", "--system-site-packages", virtualenv_build_folder)
    else
        safesystem("virtualenv", virtualenv_build_folder)
    end

    pip_exe = File.join(virtualenv_build_folder, "bin", "pip")
    python_exe = File.join(virtualenv_build_folder, "bin", "python")

    # Why is this hack here? It looks important, so I'll keep it in.
    safesystem(python_exe, pip_exe, "install", "-U", "-i",
               attributes[:virtualenv_pypi],
               "pip", "distribute")
    safesystem(python_exe, pip_exe, "uninstall", "-y", "distribute")

    extra_index_url_args = []
    if attributes[:virtualenv_pypi_extra_index_urls]
      attributes[:virtualenv_pypi_extra_index_urls].each do |extra_url|
        extra_index_url_args << "--extra-index-url" << extra_url
      end
    end

    find_links_url_args = []
    if attributes[:virtualenv_find_links_urls]
      attributes[:virtualenv_find_links_urls].each do |links_url|
        find_links_url_args << "--find-links" << links_url
      end
    end

    target_args = []
    if is_requirements_file
      target_args << "-r" << package
    else
      target_args << package
    end

    pip_args = [python_exe, pip_exe, "install", "-i", attributes[:virtualenv_pypi]] << extra_index_url_args << find_links_url_args << target_args
    safesystem(*pip_args.flatten)

    if attributes[:virtualenv_setup_install?]
      logger.info("Running PACKAGE setup.py")
      setup_args = [python_exe, "setup.py", "install"]
      safesystem(*setup_args.flatten)
    end

    if ! is_requirements_file && package_version.nil?
      frozen = safesystemout(python_exe, pip_exe, "freeze")
      frozen_version = frozen[/#{package}==[^=]+$/]
      package_version = frozen_version && frozen_version.split("==")[1].chomp!
      self.version ||= package_version
    end

    ::Dir[build_path + "/**/*"].each do |f|
      if ! File.readable? f
        File.lchmod(File.stat(f).mode | 444)
      end
    end

    ::Dir.chdir(virtualenv_build_folder) do
      safesystem("virtualenv-tools", "--update-path", virtualenv_folder)
    end

    if !attributes[:virtualenv_other_files_dir].nil?
      # Copy all files from other dir to build_path
      Find.find(attributes[:virtualenv_other_files_dir]) do |path|
        src = path.gsub(/^#{attributes[:virtualenv_other_files_dir]}/, '')
        dst = File.join(build_path, src)
        copy_entry(path, dst, preserve=true, remove_destination=true)
        copy_metadata(path, dst)
      end
    end

    remove_python_compiled_files virtualenv_build_folder

    # use dir to set stuff up properly, mainly so I don't have to reimplement
    # the chdir/prefix stuff special for tar.
    dir = convert(FPM::Package::Dir)
    # don't double prefix the files
    dir.attributes[:prefix] = nil
    if attributes[:chdir]
      dir.attributes[:chdir] = File.join(build_path, attributes[:chdir])
    else
      dir.attributes[:chdir] = build_path
    end

    cleanup_staging
    # Tell 'dir' to input "." and chdir/prefix will help it figure out the
    # rest.
    dir.input(".")
    @staging_path = dir.staging_path
    dir.cleanup_build

  end