Plug: retries for temporary filename often fail on Windows OS

epailty · May 29, 2017, 2:52am

There is a problem with the random filename logic in Plug when run on Windows, because :os.timestamp does not give microsecond resolution - at this point in the code:

github.com

elixir-plug/plug/blob/master/lib/plug/upload.ex#L102


  path = path(prefix, tmp)

  case :file.write_file(path, "", [:write, :raw, :exclusive, :binary]) do
    :ok ->
      :ets.update_element(@table, self(), {3, [path | paths]})
      {:ok, path}

    {:error, reason} when reason in [:eexist, :eacces] ->
      open_random_file(prefix, tmp, attempts + 1, paths)
  end
end

defp open_random_file(_prefix, tmp, attempts, _paths) do
  {:too_many_attempts, tmp, attempts}
end

defp path(prefix, tmp) do
  sec = :os.system_time(:second)
  rand = :rand.uniform(999_999_999_999_999)
  scheduler_id = :erlang.system_info(:scheduler_id)
  tmp <> "/" <> prefix <> "-" <> i(sec) <> "-" <> i(rand) <> "-" <> i(scheduler_id)

Typically:

iex(1)> IO.inspect :os.timestamp; IO.inspect :os.timestamp
{1496, 22882, 547000}
{1496, 22882, 547000}

And as there are only 10 reties, it sometimes results in:

    ** (Plug.UploadError) tried 10 times to create an uploaded file at C:\Windows\TEMP/plug-1495 but failed. Set PLUG_TMPDIR to a directory with write permission

See pull request:

Azolo · May 29, 2017, 9:54pm

Look at Forking a Repo. You have to push the change to your own fork of the repo to do a PR.

epailty · May 30, 2017, 5:55am

Thanks for that, see https://github.com/elixir-lang/plug/pull/560

Azolo · May 30, 2017, 7:08am

Thanks for contributing!