feat: add FLAC lossless format support

[akbad]

Summary

• Adds FLAC lossless audio format support to librespot-python based on format codes in librespot upstream
• Refactors quality picker classes for DRYness while maintaining backward compatibility

Details

• Add FLAC_FLAC (16) and FLAC_FLAC_24BIT (22) format codes to metadata.proto

  • Added upstream by Update protobufs librespot-org/librespot#796 and Update protobuf files librespot-org/librespot#1424
  • Regenerated Metadata_pb2.py with protoc 3.20.1 (as in requirements)
  • Removed obsolete AAC_24_NORM as done upstream (format that previously occupied code 16)

• Add enum values for FLAC:

  • SuperAudioFormat.FLAC maps FLAC formats to FLAC container type
  • AudioQuality.LOSSLESS maps FLAC formats to lossless quality tier

• Refactor to generic DRY FormatOnlyAudioQuality base class, while maintaining 100% backward compatibility via:

  • Maintaining existing VorbisOnlyAudioQuality as a wrapper
  • Adding new LosslessOnlyAudioQuality wrapper for FLAC file selection

• Also fixes minor bug within unknown-format error in decoders.py

[akbad]

Please note the failed DeepSource Python check stems from me reusing the existing way of formatting strings for the logger within my changes. Let me know if this is something I should update.

[kokarare1212]

Thanks for the PR!
It looks great!

[Googolplexed0]

@akbad I've implemented this lossless format in a downstream project, but I'm having no luck getting FLAC files (even with songs that the mainline app claims to have lossless versions of). All track.files returned by __session.api().get_metadata_4_track(track_id_or_track) contain only formats up to OGG_VORBIS_320, as it did before I pulled this.

Did you confirm it worked properly for your setup, and if so, can you provide a few songs (preferably links or uuids) that you saw success with so that I can try to replicate it?

[akbad]

@Googolplexed0 I tried the same thing and sadly it didn't work properly. I looked into it and my conclusion is that everything in this PR is indeed needed to support FLAC, and it's simply the Spotify endpoints used that aren't serving FLAC files in response to our requests.

My guess is that there's some secret sauce in the Spotify desktop client's requests that we need to replicate somehow here in order to get it to serve us FLAC files. I tried tracing how Spotify's desktop client does it (i.e. using mitmproxy) but didn't have enough time to completely figure it out (and won't for a little while sadly). Whenever this is done, it would probably provide the missing piece to the puzzle.

[Googolplexed0]

My guess is that there's some secret sauce

I would assume you are correct, given how long it took them to roll out this particular feature.

but didn't have enough time to completely figure it out (and won't for a little while sadly)

Bummer. If you have some advice on how I could help with the snooping efforts, let me know. I would really like to gain this functionality if possible.

[yeralin]

Created an issue to track this #320

[ljrk0]

I tried starting to trace as well but didn't conclude anything productive. FWIW, I found the following setup quite nice to trace (to circumvent certificate pinning etc.), although also a bit over-engineered.

1. Use Android Studio to fire up a VM. VM should support have "play enabled"
2. Emulator: Install Google services/OpenGapps (optional?)
3. Emulator: Install Spotify APK from: https://www.apkmirror.com/apk/spotify-ab/spotify-music-podcasts/spotify-music-and-podcasts-9-0-96-648-release/

Then, use HTTPToolkit to Intercept from Android Emulator using Frida.
Optionally, set-up Burp in combination with HTTPTk:

1. Export Burp Proxy Cert.
2. Configure Burp 127.0.0.1:8080 as upstream proxy in HTTPToolkit and import the cert

You can also use e.g. blackboxprotobuf to decode some protobufs. But I'm too tired for that now.