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docs: record how replay gain reaches the audio sink

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The Playback section already documents the star button's dependence on
unstable Media3 APIs and what to re-check when bumping the library. The
gain path needs the same treatment: it relies on flush carrying the
timeline and period, on the sink configuring per media item, and on the
processor chain being reached at all.
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austinried
2026-10-16 13:07:07 +00:00
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docs/architecture.md
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@@ -98,6 +98,8 @@ Server writes — now-playing, scrobbles and stars — go through a `ServerActio
The media notification carries a star button for the playing track. `PlaybackService` adds a `com.subtracks.STAR` session command and a `CommandButton` to the `MediaSession`, and drives its filled/unfilled icon from the playing song's Room row. That path — `MediaSession.Callback`, `SessionCommand`, `CommandButton`, `SessionResult`/`SessionError` and `MediaSession.setMediaButtonPreferences` — is annotated `@UnstableApi` in Media3 1.11.1 and is therefore opted into explicitly: when bumping Media3, re-check those signatures and the notification's button resolution. The stable `setCustomLayout` is not a substitute here, because the notification manager only refreshes on `onMediaButtonPreferencesChanged`, so the icon would not update mid-playback.
ReplayGain is applied at the audio sink. The gain the server reports per song (`trackGain`/`albumGain` and their peaks; `baseGain` and `fallbackGain` are ignored) is synced into the song row, carried onto the media item, and applied by a `ReplayGainProcessor` in `DefaultAudioSink`'s processor chain, installed through a `RenderersFactory`. The gain is set from `AudioProcessor.flush(StreamMetadata)`, which Media3 1.11 populates with the playback `Timeline` and the current period id (`androidx/media#418`) and calls on every sink configuration — including a media-item transition at an unchanged format — so a new item's gain is in place before any of its buffers are processed. That timing is the point: `Player.Listener.onMediaItemTransition` arrives on the app thread after the playback thread has already rendered, which is too late. When bumping Media3, re-check that `flush` still carries the timeline and period, and that the chain is reached at all: float output and audio offload both bypass it, which is why float output stays off. The mode (track/album/off) and a preamp are DataStore preferences read by `PlaybackService`, and the anti-clip cap uses the peak when the server sends one.
## Data model
`sources` and `subsonic_sources` hold the configured servers. Library tables are keyed by `(sourceId, id)` and cascade from `sources`: `artists`, `albums`, `playlists`, `playlist_songs`, `songs`.