def download(spec, source_uri, install_dir = Gem.dir)
    Gem.ensure_gem_subdirectories(install_dir) rescue nil

    if File.writable?(install_dir)
      cache_dir = File.join install_dir, "cache"
    else
      cache_dir = File.join Gem.user_dir, "cache"
    end

    gem_file_name = File.basename spec.cache_file
    local_gem_path = File.join cache_dir, gem_file_name

    FileUtils.mkdir_p cache_dir rescue nil unless File.exist? cache_dir

   # Always escape URI's to deal with potential spaces and such
    unless URI::Generic === source_uri
      source_uri = URI.parse(URI.const_defined?(:DEFAULT_PARSER) ?
                             URI::DEFAULT_PARSER.escape(source_uri.to_s) :
                             URI.escape(source_uri.to_s))
    end

    scheme = source_uri.scheme

    # URI.parse gets confused by MS Windows paths with forward slashes.
    scheme = nil if scheme =~ /^[a-z]$/i

    case scheme
    when 'http', 'https' then
      unless File.exist? local_gem_path then
        begin
          say "Downloading gem #{gem_file_name}" if
            Gem.configuration.really_verbose

          remote_gem_path = source_uri + "gems/#{gem_file_name}"

          gem = self.fetch_path remote_gem_path
        rescue Gem::RemoteFetcher::FetchError
          raise if spec.original_platform == spec.platform

          alternate_name = "#{spec.original_name}.gem"

          say "Failed, downloading gem #{alternate_name}" if
            Gem.configuration.really_verbose

          remote_gem_path = source_uri + "gems/#{alternate_name}"

          gem = self.fetch_path remote_gem_path
        end

        File.open local_gem_path, 'wb' do |fp|
          fp.write gem
        end
      end
    when 'file' then
      begin
        path = source_uri.path
        path = File.dirname(path) if File.extname(path) == '.gem'

        remote_gem_path = correct_for_windows_path(File.join(path, 'gems', gem_file_name))

        FileUtils.cp(remote_gem_path, local_gem_path)
      rescue Errno::EACCES
        local_gem_path = source_uri.to_s
      end

      say "Using local gem #{local_gem_path}" if
        Gem.configuration.really_verbose
    when nil then # TODO test for local overriding cache
      source_path = if Gem.win_platform? && source_uri.scheme &&
                       !source_uri.path.include?(':') then
                      "#{source_uri.scheme}:#{source_uri.path}"
                    else
                      source_uri.path
                    end

      source_path = unescape source_path

      begin
        FileUtils.cp source_path, local_gem_path unless
          File.identical?(source_path, local_gem_path)
      rescue Errno::EACCES
        local_gem_path = source_uri.to_s
      end

      say "Using local gem #{local_gem_path}" if
        Gem.configuration.really_verbose
    else
      raise Gem::InstallError, "unsupported URI scheme #{source_uri.scheme}"
    end

    local_gem_path
  end
#
Moves the gem +spec+ from +source_uri+ to the cache dir unless it is
already there.  If the source_uri is local the gem cache dir copy is
always replaced.