Gem::Package::TarWriter#add_file

# File tmp/rubies/ruby-3.4.1/lib/rubygems/package/tar_writer.rb, line 101
def add_file(name, mode) # :yields: io
  check_closed

  name, prefix = split_name name

  init_pos = @io.pos
  @io.write Gem::Package::TarHeader::EMPTY_HEADER # placeholder for the header

  yield RestrictedStream.new(@io) if block_given?

  size = @io.pos - init_pos - 512

  remainder = (512 - (size % 512)) % 512
  @io.write "\0" * remainder

  final_pos = @io.pos
  @io.pos = init_pos

  header = Gem::Package::TarHeader.new name: name, mode: mode,
                                       size: size, prefix: prefix,
                                       mtime: Gem.source_date_epoch

  @io.write header
  @io.pos = final_pos

  self
end