Update Bento.MetainfoError.transform to adapt to malformed torrent

[folz]

Hey @ericls, mind dropping a comment explaining the two changes you made? Happy to merge your changes into Bento, but was curious about the to_existing_atom -> to_atom change especially since you already define the atoms in the function, and the motivation behind the change in the first place. If there's a bug I missed, I'd love to hear about it!

[ericls]

The motivation of this is that I've found some torrents may have additional fields names, but they are still "usable" torrents.

Changing from to_existing_atom to to_atom is actually bad. Dynamically generating atoms is not a good practice, since atoms are not garbage collected and there's a limit on how much atoms you can have.

So I made another change to find the values of the fields represented by the pre-defined atoms from the map. This change also solves the problem where some torrents may have field names that are not even "atomizable". According to: http://elixir-lang.org/docs/stable/elixir/String.html#to_atom/1

Currently Elixir does not support the conversion of strings that contain Unicode codepoints greater than 0xFF.

(Actually, I found that most if not all codepoints greater than 128 are not "atomizable")

I have to admit that I'm pretty new to elixir and in general new to functional programming. Please point it out if there's something wrong with my code and comment.

[folz]

Ah, makes sense. Could you create a test case that fails on the current version of transform() but passes with your changes? Once you have that, push to your master and I can get it merged in.

[mogeko]

Ah, makes sense. Could you create a test case that fails on the current version of transform() but passes with your changes? Once you have that, push to your master and I can get it merged in.

I found one.

If we try to use Bento.torrent! Go to encode huck_finn_librivox_archive.torrent and we will get an error.

iex(1)> File.read!("test/_data/huck_finn_librivox_archive.torrent") |> Bento.torrent!
** (ArgumentError) errors were found at the given arguments:

  * 1st argument: not an already existing atom

    :erlang.binary_to_existing_atom("collections", :utf8)
    (bento 1.0.0-dev) lib/bento/metainfo.ex:87: anonymous fn/1 in Bento.Metainfo.transform/1
    (elixir 1.14.3) lib/enum.ex:1658: Enum."-map/2-lists^map/1-0-"/2
    (bento 1.0.0-dev) lib/bento/metainfo.ex:67: Bento.Metainfo.info/1
    (bento 1.0.0-dev) lib/bento/metainfo.ex:80: Bento.Metainfo.info!/1
    (bento 1.0.0-dev) lib/bento.ex:105: Bento.torrent!/1
    iex:2: (file)

Because there is a field "collections" in this torent file, it is out of our %Bento.Metainfo.MultiFile{}. Of course, BEAM could not find :collections in memory, which caused an error.

But it works for @ericls 's version (it skipped any field out of %Bento.Metainfo.MultiFile{}):

iex(1)> File.read!("test/_data/huck_finn_librivox_archive.torrent") |> Bento.torrent!
%Bento.Metainfo.Torrent{
  info: %Bento.Metainfo.MultiFile{
    "piece length": 1048576,
    pieces: <<245, 194, 107, 181, 217, 135, 191, 175, 128, 28, 49, 218, 242, 60,
      247, 168, 52, 83, 77, 117, 197, 98, 168, 210, 236, 252, 53, 170, 157, 63,
      183, 127, 167, 74, 217, 199, 32, 189, 231, 236, 27, 183, 167, 20, 42, 243,
      112, ...>>,
    private: nil,
    name: "huck_finn_librivox",
    files: [...]
  },
  announce: "http://bt1.archive.org:6969/announce",
  "announce-list": [
    ["http://bt1.archive.org:6969/announce"],
    ["http://bt2.archive.org:6969/announce"]
  ],
  "creation date": 1451670299,
  comment: "This content hosted at the Internet Archive at https://archive.org/details/huck_finn_librivox\nFiles may have changed, which prevents torrents from downloading correctly or completely; please check for an updated torrent at https://archive.org/download/huck_finn_librivox/huck_finn_librivox_archive.torrent\nNote: retrieval usually requires a client that supports webseeding (GetRight style).\nNote: many Internet Archive torrents contain a 'pad file' directory. This directory and the files within it may be erased once retrieval completes.\nNote: the file huck_finn_librivox_meta.xml contains metadata about this torrent's contents.",
  "created by": "ia_make_torrent",
  encoding: nil
}

[mogeko]

I believe we can use Bento.Decoder.transform/2 instead of transform/1. #24