class Gem::Package::IOSource

def initialize(io)
  @io = io
end

def path
end

def present?
  true
end

def start
  @start ||= begin
    if io.pos > 0
      raise Gem::Package::Error, "Cannot read start unless IO is at start"
    end
    value = io.read 20
    io.rewind
    value
  end
end

def with_read_io
  yield io
ensure
  io.rewind
end

def with_write_io
  yield io
ensure
  io.rewind
end