Relative paths for local media in m3u playlists

[kingosticks]

With the merge of #1386, non-absolute paths for local music files are now relative to m3u/playlist_dir instead of local/media_dir. This is a change in Mopidy's behaviour and different to mpd's behaviour.

This issue is a continuation of the discussion that started here with @tkem in which he pointed out that:

According to https://en.m.wikipedia.org/wiki/M3U, which kind of serves as our reference, "local pathname relative to the M3U file location". Plus, for the file extension, you have multiple media_dirs as potential roots.

Do we want to restore the original behaviour or not? If we chose not, then I think we need to improve the documentation somewhere to make this difference clearer as it will impact those users migrating from mpd.

[tkem]

Good point! I'd also add that with plans for local to be moved out of Mopidy core into a separate extension, future versions of Mopidy may not even have a local/media_dir. And as @kingosticks already mentioned, the file extension which is to replace local in some ways provides multiple root directories, so these can't be used either.
But I see the point that this diverges from MPD behavior, so I'm open to discussion.

[jodal]

All valid points so far. I don't have any strong preference, but is maybe leaning a bit more towards following M3U conventions than MPD.

[jodal]

I think the only solution here is to keep using m3u/playlist_dir as the base directory as we don't want to depend on an extension that'll move out of core. Moving this to v2.0.1 as the docs improvement isn't a blocker for getting 2.0.0 out and into Ubuntu 16.04.

[kingosticks]

We can aim to include it the part of the docs where we outline the current shortcomings of the MPD implementation. Or maybe even as a specific "Migrating from mpd" section somewhere. It could do with being quite prominent but that's quite difficult to achieve with the docs. I just want to avoid mpd users trying Mopidy and being instantly frustrated that their collection of carefully curated playlists doesn't 'just work' (TM).

[adamcik]

Add a base directory setting?

[tkem]

@adamcik: Why didn't I think of that? ;-)

So, if m3u/base_dir is not set, use the M3U's parent directory as base directory for resolving relative paths, as in the Wikipedia "specs".

Remaining question would be what the default for base_dir should be. I think @kingosticks has a strong preference for setting this to the same as local/media_dir, which defaults to $XDG_MUSIC_DIR, which I'd be happy with. Note that if users changed local/media_dir, they would also have to update m3u/base_dir to keep existing behavior, though.

@jodal: What's the release schedule for Mopidy v2.0.0? If we can agree on adding the new base_dir config, I'd be able to post a PR within the next two to three days or so...

[kingosticks]

It seems so obvious now!

Last thing I heard it was something like 5 days in order to get into Ubuntu 16.04.

[jodal]

Ubuntu's autoimport-from-Debian freeze is Feb 18. We need to upload to Debian and sync to Ubuntu before that, so I should upload Feb 15 of 16, so I'd be quite happy if we could roll a release today, or tomorrow at latest.

[tkem]

I'll see what I can do, but Valentine's Day's duties come first ;-)

[jodal]

Landed in v2.0.0 ❤️