feat: add support for DJ mode

[hkcryos]

Spotify's AI DJ mode uses a special context type (lexicon_set_type: your_dj) that doesn't work with the standard context resolution. The current implementation fails because:

1. DJ tracks have an empty uri field in ContextTrack - the actual URI is in metadata["canonical_track_uri"]
2. The context URL points to a session-specific lexicon endpoint that returns 404 from other sessions
3. No transfer commands are sent for track advancement in DJ mode

I've got this working locally and am enjoying it now. My implementation does it like this:

• Adding ContextTrackUri() helper that falls back to canonical_track_uri metadata
• Creating a single-track context from TransferState.Playback.CurrentTrack for DJ transfers
• Caching NextTracks from ClusterUpdate messages (the service broadcasts the full queue before transfer)
• Using the cached tracks to advance playback when a DJ track ends
• Skipping autoplay resolution for DJ contexts (that gives a 404)

I haven't looked at the TTS DJ-speak between tracks (I don't like that!) or the cross-fading between tracks (again, I dislike it, so didn't implement it).

Happy to commit a branch if there's interest in supporting this.

[fegauthier]

Would be great to support that!

Thanks!

[devgianlu]

Hi @hkcryos and thank you for the interest in the project! Unfortunately DJ X is not available in my country so I woudn't be able to test and merge your code straightaway without the help of someone else.

Additionally, getting the tracks from ClusterUpdate messages doesn't seem like the best strategy, the client should be able to pull the track list from somewhere if it's not in the context transfer already.

From the related librespot issue I can see that lexicon_context_url is provided in the context metadata. The URL starts with hm://, but that usually means that it can be transformed in a HTTP API by stripping the prefix. We already do that here.

If you think you can get this working I think it'll be a pretty nice addition, even without TTS or crossfade.

[hkcryos]

Thanks @devgianlu - as you say, that's really unfortunate.

I entirely agree about lexicon_context_url. I should've said - I tried that approach initially but when you follow it, you get a 404.

Looking at the behaviour from the web player (which seems to be the same for the Windows client):

The app never calls the lexicon URL - it appears in Pathfinder response metadata but no HTTP request is ever made to it
Pathfinder returns 0 tracks for the DJ playlist (content.items: [], totalCount: 0)
Track list comes from dealer WebSocket - ClusterUpdate messages contain NextTracks with the full queue
The ClusterUpdate caching approach appears to be how the apps work too, which feels odd to me given we have the lexicon - but that is what I saw.

Happy to dig further if it would help? Or we could see if others could confirm it isn't just me?

[devgianlu]

@hkcryos I would try seeing what the desktop client does, you can intercept HTTPS requests with something like mitmproxy.

[JaragonCR]

@devgianlu I would be able to test DJ, for this. Let me know what I can do. I do use balenaSound so some log extraction might be tricky, but we made it work before ;)

[JaragonCR]

I have a POC... man was that trip. Interactive mode works totally different than Zeroconf I could not understand why it worked in test but worked in prod. I'm going to do more testing when I have a chance, I could not figure the DJ speaking, so it should just skip that part BUT mix it up and other important parts do work.

https://github.com/JaragonCR/go-librespot

[devgianlu]

@JaragonCR I looked briefly at the changes in your fork. They look ok, but it's a quite a bit of stuff for a single commit.

Generally, I don't like the Mercury stuff, I high doubt the client uses it and I really don't want more of it. If that's for narration only, we can skip it for now.

Feel free to make a PR if you can split it up into smaller chunks and keep only what's strictly needed. It doesn't need to be perfect from the get-go :)

[JaragonCR]

@devgianlu that was some quick service ;). Balena likes all the commits squashed so I did that :( sorry. I think I needed mercury for the switch it up button, but let me go back to the notes and circle back. All narration is off and IMHO will stay off I spent hours trying to make it work and the law of diminishing returns came to mind.

I will try to go back to several commits, 1 makes interactive work, 1 makes zeroconf work, clean mercury. Can't promise I'll have more time this week, but will continue to look into this.