Fix Twitch VOD Track audio routing in advanced + EB modes

[summeroff]

Summary

Regression introduced in 1.21's migration to the factory-API streaming path: the user-configurable Twitch VOD Track silently sent the wrong audio (or no separate VOD audio at all) to Twitch's VOD output, depending on streaming mode.

Tech support reported users on 1.21.1 seeing audio they'd muted on the VOD Track still present in their Twitch VOD. Two distinct root causes, fixed together since they share one user-visible symptom.

Root cause

1. Advanced streaming - hardcoded mixer + 0/1-based field confusion.
osn-advanced-streaming.cpp::SetupTwitchSoundtrackAudio was a fossilized copy of the legacy "Soundtrack by Twitch" plugin code:

• The VOD encoder was created with kSoundtrackArchiveTrackIdx = 5 regardless of the user's track choice, so VOD audio always came from mixer 5 (UI Track 6).
• audioTrackConfigs[streaming->twitchTrack] used raw array indexing instead of GetTrackConfig(twitchTrack), mismatching the 1-based contract honored by every other consumer in osn-audio-track.cpp.
• It also silently mutated default desktop sources' mixer masks (carry-over from the Soundtrack plugin) - actively wrong once the VOD mixer is user-chosen.

The legacy GetLegacySettings/SetLegacySettings stored the field as 0-based with -1/+1 conversions, while the new factory-API setters stored 1-based values from the desktop UI. Same field, two conventions, depending on entry point.

2. Enhanced Broadcasting advanced - VOD track silently disabled.
osn-enhanced-broadcasting-advanced-streaming.cpp::Start checked streaming->twitchVODSupported && streaming->enableTwitchVOD to decide whether to request a VOD track from the multitrack output, but never set twitchVODSupported. The field defaulted to false (from osn-streaming.hpp:44); regular AdvancedStreaming::Start computes it via isTwitchVODSupported() before SetupTwitchSoundtrackAudio, but the EB Start skipped that.

Net effect with EB on: vod_track_mixer = nullopt -> multitrack output's create_audio_encoders early-returns at the VOD branch -> no VOD encoder ever attached -> Twitch's go-live POST body went out with "vod_track_audio": false -> Twitch returned no VOD audio configuration -> VOD silently used the main stream audio.

Fix

osn-advanced-streaming.cpp

• Route the VOD encoder through GetMixerIndex(streaming->twitchTrack) (user's 1-based choice -> 0-based mixer).
• Look the bitrate config up via GetTrackConfig(streaming->twitchTrack); bail cleanly if the slot has no AudioTrack.
• Recreate the encoder when the mixer index would change between streams (mixer is fixed at obs_audio_encoder_create time).
• Drop the silent mutation of default desktop sources' mixer masks (and the corresponding restore in StopTwitchSoundtrackAudio).
• Normalize GetLegacySettings/SetLegacySettings to store twitchTrack/audioTrack as 1-based, matching the constructor defaults and the new API path - eliminates the two-conventions-in-one-field footgun.
• Dead-code removal: kSoundtrackArchiveTrackIdx, setMixer() helper.

osn-enhanced-broadcasting-advanced-streaming.cpp

• Compute streaming->twitchVODSupported = streaming->isTwitchVODSupported() when enableTwitchVOD is true, mirroring the regular path.
• Convert streaming->twitchTrack to a 0-based mixer index via GetMixerIndex before passing to StartEnhancedBroadcastingStream (which forwards to multitrack output as a raw mixer_idx).

Test plan

• Regular advanced + VOD enabled, VOD Track = 3. Log shows 'Twitch VOD Track Encoder' created with track: 3 (mixer 2). Audio muted on Track 3 via Advanced Audio Properties is excluded from VOD as expected.
• Enhanced Broadcasting + VOD enabled. Log shows 'multitrack video vod audio 0' encoder created; Go-live POST body contains "vod_track_audio": true. VOD receives the user-configured mix.
• Same scenarios with EB toggled off then on again - both modes route VOD audio correctly without restart.
• Twitch Dual Streaming (single connection, both canvases) + VOD. Same EB code path as scenario 2; covered transitively.

Out of scope (follow-ups)

• osn-simple-streaming.cpp::SetupTwitchSoundtrackAudio still has the hardcoded mixer 5 pattern, but the VOD Track UI is gated to advanced output mode in GlobalSettings.tsx so it's unreachable from a normal desktop session. Separate cleanup if simple-mode VOD is ever wired up.
• streamArchive / SetupTwitchSoundtrackAudio are misnomers now (no Soundtrack plugin involvement) and oldMixer_desktopSource1/2 on osn-streaming.hpp are unused by the advanced path. Pure rename, separate PR.

Generated with Claude Code

[Copilot]

Pull request overview

Fixes a regression where the Twitch VOD Track sent wrong (or no) audio in the new factory-API streaming path. Two distinct root causes are addressed: the advanced streaming path hardcoded the VOD mixer to track 5 (and used raw 0-based array indexing inconsistent with the rest of the codebase), and the Enhanced Broadcasting path never computed twitchVODSupported, so the VOD encoder was never requested. The legacy settings load/save is also normalized to store twitchTrack/audioTrack as 1-based, eliminating a "two conventions in one field" footgun.

Changes:

• In osn-advanced-streaming.cpp, route the VOD encoder through the user's chosen track via GetTrackConfig/GetMixerIndex, recreate the encoder when the mixer changes between streams, and drop the obsolete desktop-source mixer-mask mutation (and its restore in stop). Remove dead helpers setMixer/kSoundtrackArchiveTrackIdx.
• Normalize Get/SetLegacySettings to treat TrackIndex/VodTrackIndex in basic.ini as 1-based (matching constructor defaults and the new API path); removes the +1/-1 conversions.
• In osn-enhanced-broadcasting-advanced-streaming.cpp::Start, compute twitchVODSupported = isTwitchVODSupported() when enableTwitchVOD is true, and convert twitchTrack to a 0-based mixer index via GetMixerIndex before forwarding to StartEnhancedBroadcastingStream.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
obs-studio-server/source/osn-advanced-streaming.cpp	Routes the Twitch VOD encoder via the user-selected track (1-based) using the AudioTrack helpers, recreates the encoder when the chosen mixer changes, removes Soundtrack-plugin carryover (desktop source mixer mutation and dead constants), and normalizes legacy settings to 1-based.
obs-studio-server/source/osn-enhanced-broadcasting-advanced-streaming.cpp	Computes twitchVODSupported at Start (mirroring the regular path) and converts the 1-based twitchTrack to a 0-based mixer index before passing it to the multitrack output.

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[aleksandr-voitenko]

Could we mirror the regular advanced streaming guard here before converting the selected VOD track to a mixer index? GetMixerIndex() assumes twitchTrack is a valid 1-based track number; if settings are stale/corrupt and twitchTrack is 0 or > NUM_AUDIO_TRACKS, this can underflow or pass an invalid mixer into enhanced broadcasting.