Gem::Package::TarWriter#add_file_signed

# File tmp/rubies/ruby-4.0.0/lib/rubygems/package/tar_writer.rb, line 170
def add_file_signed(name, mode, signer)
  digest_algorithms = [
    signer.digest_algorithm,
    Gem::Security.create_digest("SHA512"),
  ].compact.uniq

  digests = add_file_digest name, mode, digest_algorithms do |io|
    yield io
  end

  signature_digest = digests.values.compact.find do |digest|
    digest_name =
      if digest.respond_to? :name
        digest.name
      else
        digest.class.name[/::([^:]+)\z/, 1]
      end

    digest_name == signer.digest_name
  end

  raise "no #{signer.digest_name} in #{digests.values.compact}" unless signature_digest

  if signer.key
    signature = signer.sign signature_digest.digest

    add_file_simple "#{name}.sig", 0o444, signature.length do |io|
      io.write signature
    end
  end

  digests
end