After being kicked again and again (which hurts, the guy is a black belt in karate) by Dodji who wanted me to blog, here is a first post.
Album Artist support
These last days, I've been trying to get back to Rhythmbox development to scratch a few itches of my own.
First, I've looked at how Rhythmbox could handle compilations, ie albums containing tracks by different artists. Currently, if the album has 12 different artists, these 12 artists will appear separately in the artist list which can quickly create a big mess. I wrote a basic patch to make it possible to set an "album artist" for such albums and to use that instead of the multiple different artists in the artist list. I had to experiment a bit with various approaches, but in the end, the patch is surprisingly small.
Then, I wanted Rhythmbox to be able to provide UIDs for the songs in its database. What I call an UID is some kind of identifier that is unique to a song and that can be generated by only looking at the song data. This can be useful for various things : iPod (or whatever your portable media player of choice is) synchronization, associating user data (rating, play count, ...) to a song which persists even if the user does a mv of the song from the shell, ... I learnt after doing that work that Charlotte had been looking for such a feature in Rhythmbox for her nice Rhythmbox SOC which was good news :)
To generate that UID, I chose to hash the song title, artist, album (read from the tags of the song) with the first 8kB of data of the file (actually, this hashing scheme was heavily inspired by what Amarok does). I'm not sure yet if this is the best way to uniquely identify a song, but we'll only know after people try to use it. Before you ask, I thought about using musicbrainz/musicDNS acoustic fingerprints but as far as I know, none of those fingerprints can be generated using free software end to end, there's always some closed source webservice that must be queried to get a fingerprint from a few parameters that were generated by analyzing the song audio data.
Once again, this feature was straightforward to implement.
The main issue I had was to debug the UID generation. Indeed, metadata reading (where I chose to add the UID generation) is done by a separate process which communicates with Rhythmbox through dbus. Reading metadata is basically equivalent to feeding random data to the tag reading library, so it's really hard to guarantee the library won't crash or hang in some corner cases. Using that external process allows Rhythmbox not to crash or hang if such an event should occur during metadata reading.
But this external process also makes debugging harder: it's short lived, spawned on-demand and run in the background (ie it's not possible to print stuff to stderr or stdout). So moch's help was really welcome since he explained me how to be able to run that metadata helper process by hand and to tell rhythmbox to use it. It's really simple, all you have to do is to (optionally) increase ATTENTION_SPAN in metadata/rb-metadata-dbus-service.c so that the helper stays alive longer (by default, it dies after 30 seconds of inactivity).
Then, you can run rhythmbox-metadata in nemiver (or in your favourite debugger), this will output a line like :
If you now set the RB_DBUS_METADATA_ADDRESS environment variable to that value before running Rhythmbox, then Rhythmbox will use the metadata helper you just launched in your debugger. Now all that is left to do is debugging!
The result of this work can be found in the uid branch of my Rhythmbox git tree. It still needs some polishing, but the basics should already be working (including automatically updating your database to add UIDs when you first run it).