def add_file(name, mode)
check_closed
raise Gem::Package::NonSeekableIO unless @io.respond_to? :pos=
name, prefix = split_name name
init_pos = @io.pos
@io.write "\0" * 512
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
@io.write header
@io.pos = final_pos
self
end