Gem::RemoteFetcher#download

# File tmp/rubies/ruby-3.0.5/lib/rubygems/remote_fetcher.rb, line 112
def download(spec, source_uri, install_dir = Gem.dir)
  install_cache_dir = File.join install_dir, "cache"
  cache_dir =
    if Dir.pwd == install_dir # see fetch_command
      install_dir
    elsif File.writable?(install_cache_dir) || (File.writable?(install_dir) && (not File.exist?(install_cache_dir)))
      install_cache_dir
    else
      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

  require "fileutils"
  FileUtils.mkdir_p cache_dir rescue nil unless File.exist? cache_dir

  source_uri = Gem::Uri.new(source_uri)

  scheme = source_uri.scheme

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

  # REFACTOR: split this up and dispatch on scheme (eg download_http)
  # REFACTOR: be sure to clean up fake fetcher when you do this... cleaner
  case scheme
  when 'http', 'https', 's3' then
    unless File.exist? local_gem_path
      begin
        verbose "Downloading gem #{gem_file_name}"

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

        self.cache_update_path remote_gem_path, local_gem_path
      rescue FetchError
        raise if spec.original_platform == spec.platform

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

        verbose "Failed, downloading gem #{alternate_name}"

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

        self.cache_update_path remote_gem_path, local_gem_path
      end
    end
  when 'file' then
    begin
      path = source_uri.path
      path = File.dirname(path) if File.extname(path) == '.gem'

      remote_gem_path = Gem::Util.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

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

    source_path = Gem::UriFormatter.new(source_path).unescape

    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

    verbose "Using local gem #{local_gem_path}"
  else
    raise ArgumentError, "unsupported URI scheme #{source_uri.scheme}"
  end

  local_gem_path
end