Dynacast: keep SVC tracks active while any quality is subscribed

Port the SVC special case from client-sdk-js: for SVC codecs (VP9/AV1)
all spatial layers ride in a single encoded stream and the SFU selects
layers server-side, so any enabled quality in a SubscribedQualityUpdate
must keep the whole encoding active instead of being matched per-layer.

Adds an e2e test publishing VP9 L3T3_KEY that verifies the encoding
stays active across quality requests, deactivates when the last
subscriber leaves, and reactivates on resubscribe.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: MaxHeimbrock

.changeset/dynacast_svc_any_enabled.md

@@ -0,0 +1,5 @@
+---
+livekit: patch
+---
+
+Dynacast: keep SVC (VP9/AV1) tracks active while any quality is subscribed - #1214 (@MaxHeimbrock)

livekit/src/room/mod.rs

@@ -1928,10 +1928,17 @@ impl RoomSession {
             }
         };
 
+        let video_codec = publication.publish_options().video_codec;
+        // SVC codecs carry all spatial layers in one encoded stream.
+        let is_svc = matches!(
+            video_codec,
+            crate::options::VideoCodec::VP9 | crate::options::VideoCodec::AV1
+        );
+
         let qualities: Vec<proto::SubscribedQuality> = if !update.subscribed_codecs.is_empty() {
             // This is the requested codec, which we also advertise in simulcast_codecs and use
             // for sender codec preferences, so it should match the SFU's subscribed codec key.
-            let codec = publication.publish_options().video_codec.as_str().to_lowercase();
+            let codec = video_codec.as_str().to_lowercase();
             log::info!(
                 "dynacast: SFU quality update for {}: subscribed_codecs={:?}, looking for codec '{}'",
                 track_sid,
@@ -1980,7 +1987,7 @@ impl RoomSession {
             update.subscribed_qualities.clone()
         };
 
-        if let Err(e) = video_track.set_publishing_layers(&qualities) {
+        if let Err(e) = video_track.set_publishing_layers(&qualities, is_svc) {
             log::error!("dynacast: failed to set publishing layers for {}: {}", track_sid, e);
         }
     }

livekit/src/room/track/local_video_track.rs

@@ -355,6 +355,7 @@ impl LocalVideoTrack {
     pub(crate) fn set_publishing_layers(
         &self,
         qualities: &[proto::SubscribedQuality],
+        is_svc: bool,
     ) -> RoomResult<()> {
         let transceiver = self.transceiver().ok_or_else(|| {
             RoomError::Internal("cannot set publishing layers: no transceiver".into())
@@ -368,6 +369,19 @@ impl LocalVideoTrack {
             return Ok(());
         }
 
+        // For SVC codecs all spatial layers ride in a single encoded stream
+        // and the SFU selects layers server-side, so any enabled quality
+        // keeps the whole encoding active.
+        let qualities: Vec<proto::SubscribedQuality> =
+            if is_svc && qualities.iter().any(|q| q.enabled) {
+                qualities
+                    .iter()
+                    .map(|q| proto::SubscribedQuality { enabled: true, ..q.clone() })
+                    .collect()
+            } else {
+                qualities.to_vec()
+            };
+
         let mut changed = false;
         for encoding in &mut params.encodings {
             // The SFU addresses layers by spatial index (0 = Low), so a

livekit/tests/common/e2e/video.rs

@@ -63,6 +63,15 @@ impl SolidColorTrack {
     }
 
     pub async fn publish(&mut self, codec: VideoCodec, simulcast: bool) -> RoomResult<()> {
+        self.publish_with_options(TrackPublishOptions {
+            video_codec: codec,
+            simulcast,
+            ..Default::default()
+        })
+        .await
+    }
+
+    pub async fn publish_with_options(&mut self, options: TrackPublishOptions) -> RoomResult<()> {
         let (close_tx, close_rx) = oneshot::channel();
         let track = LocalVideoTrack::create_video_track(
             "solid-color-track",
@@ -72,10 +81,7 @@ impl SolidColorTrack {
             tokio::spawn(Self::track_task(close_rx, self.rtc_source.clone(), self.params.clone()));
         self.room
             .local_participant()
-            .publish_track(
-                LocalTrack::Video(track.clone()),
-                TrackPublishOptions { video_codec: codec, simulcast, ..Default::default() },
-            )
+            .publish_track(LocalTrack::Video(track.clone()), options)
             .await?;
         let handle = TrackHandle { close_tx, track, task };
         self.handle = Some(handle);

livekit/tests/dynacast_test.rs

@@ -21,7 +21,7 @@ use {
         TestRoomOptions,
     },
     livekit::{
-        options::VideoCodec,
+        options::{TrackPublishOptions, VideoCodec},
         prelude::*,
         track::{PublishingLayerQuality, VideoQuality},
     },
@@ -428,3 +428,110 @@ async fn test_dynacast_multiple_subscribers_only_publish_requested_tracks() -> R
 
     Ok(())
 }
+
+/// Verifies dynacast behavior for an SVC track (VP9, L3T3_KEY).
+///
+/// SVC tracks carry all spatial layers in a single encoded stream and the SFU
+/// selects layers server-side, so:
+/// 1. Subscriber quality requests must never deactivate the encoding while at
+///    least one quality is subscribed (any-enabled rule).
+/// 2. Unsubscribing the last subscriber deactivates the encoding.
+/// 3. Resubscribing reactivates it.
+#[cfg(feature = "__lk-e2e-test")]
+#[test_log::test(tokio::test)]
+async fn test_dynacast_svc() -> Result<()> {
+    let mut pub_room_opts = RoomOptions::default();
+    pub_room_opts.dynacast = true;
+    let pub_options = TestRoomOptions { room: pub_room_opts, ..Default::default() };
+    let sub_options = TestRoomOptions::default();
+
+    let mut rooms = test_rooms_with_options([pub_options, sub_options]).await?;
+    let (pub_room, _pub_events) = rooms.remove(0);
+    let (_sub_room, mut sub_events) = rooms.remove(0);
+
+    let pub_room = Arc::new(pub_room);
+    let solid_params = SolidColorParams { width: 1280, height: 720, luma: 128 };
+    let mut solid_track = SolidColorTrack::new(pub_room.clone(), solid_params);
+    solid_track
+        .publish_with_options(TrackPublishOptions {
+            video_codec: VideoCodec::VP9,
+            simulcast: false,
+            scalability_mode: Some("L3T3_KEY".to_string()),
+            ..Default::default()
+        })
+        .await?;
+
+    let sub_publication: RemoteTrackPublication = timeout(Duration::from_secs(15), async {
+        loop {
+            let Some(event) = sub_events.recv().await else {
+                return Err(anyhow!("Event channel closed before TrackSubscribed"));
+            };
+            if let RoomEvent::TrackSubscribed { publication, .. } = event {
+                return Ok(publication);
+            }
+        }
+    })
+    .await??;
+
+    let pub_video_track = publisher_video_track(&pub_room)?;
+
+    // --- Baseline: the single SVC encoding is active ---
+    let layers =
+        wait_for_layers(&pub_video_track, "svc baseline", Duration::from_secs(15), |layers| {
+            layers.len() == 1 && layers[0].active
+        })
+        .await?;
+    log::info!("dynacast svc baseline layers: {:?}", layers);
+
+    // --- Request LOW quality: the SVC encoding must stay active ---
+    log::info!("dynacast svc test: requesting LOW quality");
+    sub_publication.set_video_quality(VideoQuality::Low);
+
+    // The resulting quality update arrives asynchronously; poll to make sure
+    // the encoding never gets deactivated by it.
+    let deadline = tokio::time::Instant::now() + Duration::from_secs(10);
+    while tokio::time::Instant::now() < deadline {
+        let layers = pub_video_track.publishing_layers();
+        assert!(
+            !layers.is_empty() && layers.iter().all(|layer| layer.active),
+            "SVC encoding must stay active after LOW request, got {:?}",
+            layers
+        );
+        time::sleep(Duration::from_millis(250)).await;
+    }
+
+    // --- Request HIGH quality again: still active ---
+    log::info!("dynacast svc test: requesting HIGH quality");
+    sub_publication.set_video_quality(VideoQuality::High);
+
+    let deadline = tokio::time::Instant::now() + Duration::from_secs(10);
+    while tokio::time::Instant::now() < deadline {
+        let layers = pub_video_track.publishing_layers();
+        assert!(
+            !layers.is_empty() && layers.iter().all(|layer| layer.active),
+            "SVC encoding must stay active after HIGH request, got {:?}",
+            layers
+        );
+        time::sleep(Duration::from_millis(250)).await;
+    }
+
+    // --- Unsubscribe: with no subscribers left the encoding is deactivated ---
+    log::info!("dynacast svc test: unsubscribing");
+    sub_publication.set_subscribed(false);
+
+    wait_for_layers(&pub_video_track, "svc unsubscribed", Duration::from_secs(30), |layers| {
+        !layers.is_empty() && layers.iter().all(|layer| !layer.active)
+    })
+    .await?;
+
+    // --- Resubscribe: the encoding comes back ---
+    log::info!("dynacast svc test: resubscribing");
+    sub_publication.set_subscribed(true);
+
+    wait_for_layers(&pub_video_track, "svc resubscribed", Duration::from_secs(30), |layers| {
+        !layers.is_empty() && layers.iter().all(|layer| layer.active)
+    })
+    .await?;
+
+    Ok(())
+}