feat(session): subscribe only to the slot's own camera and mic

Every plugin source connected with auto_subscribe on, so each one pulled
every camera in the room and discarded all but one. In the 2026-10-04
old-gods-of-appalachia show (8 sources, 7 cameras) each source carried
4-7 Mbps and all 8 hit congestion in the same instant: they share the
director's single downlink, so the SFU starved the cameras on screen.

The session now connects with auto_subscribe off and calls
setSubscribed(true) on exactly the wanted publications: at connect, on
onTrackPublished (re-sweeping the participant, since SDK 1.10.1 delivers
that event's publication null), and after a reconnect. Any subscription
it did not ask for is reported and handed back. Audio-only sources no
longer subscribe-then-disable video; they never subscribe to it.

The integration test gains a bystander camera that must never be
subscribed (fails with 3 unwanted subscriptions before this change) and
a full publisher leave/rejoin as a new participant session.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

core/tests/test_integration_livekit.cpp

@@ -18,6 +18,11 @@ You may obtain a copy of the License at
 // (unpublish, republish) that motivated this whole plugin, and asserts the
 // wrapper recovers instead of going stale or erroring out.
 //
+// It also proves selective subscription: a second "bystander" camera
+// publishes into the same room, and the session must never subscribe to
+// it. In a real show every OBS source used to pull the whole room, so the
+// director's downlink carried every camera once per source.
+//
 // It needs a reachable LiveKit server, so it SKIPS (exit 0) unless these are
 // set -- CI on the three build runners has no server, and this must not turn
 // into a red build there:
@@ -26,8 +31,11 @@ You may obtain a copy of the License at
 //   STPLUGIN_IT_PUBLISH_TOKEN      JWT: roomJoin + canPublish for the room
 //   STPLUGIN_IT_SUBSCRIBE_TOKEN    JWT: roomJoin + canSubscribe for the room
 //   STPLUGIN_IT_PUBLISHER_IDENTITY the identity in the publish token
+//   STPLUGIN_IT_BYSTANDER_TOKEN    optional: JWT for a second publisher; the
+//                                  selective-subscription check is skipped
+//                                  without it
 //
-// scripts/livekit-dev-room.py mints all four against a `livekit-server --dev`.
+// scripts/livekit-dev-room.py mints all of them against a `livekit-server --dev`.
 
 #include <atomic>
 #include <chrono>
@@ -221,6 +229,7 @@ int main()
     const std::string publish_token = envOrEmpty("STPLUGIN_IT_PUBLISH_TOKEN");
     const std::string subscribe_token = envOrEmpty("STPLUGIN_IT_SUBSCRIBE_TOKEN");
     const std::string publisher_identity = envOrEmpty("STPLUGIN_IT_PUBLISHER_IDENTITY");
+    const std::string bystander_token = envOrEmpty("STPLUGIN_IT_BYSTANDER_TOKEN");
 
     if (url.empty() || publish_token.empty() || subscribe_token.empty() || publisher_identity.empty()) {
         std::printf("integration_livekit: SKIPPED (STPLUGIN_IT_* not set)\n");
@@ -239,6 +248,20 @@ int main()
     ST_ASSERT(publisher.publishAudio());
     publisher.startPump();
 
+    // A bystander already publishing when the session connects -- the
+    // "source added mid-show" case, where the room is full of cameras.
+    std::unique_ptr<Publisher> bystander;
+    if (!bystander_token.empty()) {
+        bystander = std::make_unique<Publisher>();
+        step("bystander connect + publish");
+        ST_ASSERT(bystander->connect(url, bystander_token));
+        ST_ASSERT(bystander->publishVideo());
+        ST_ASSERT(bystander->publishAudio());
+        bystander->startPump();
+    } else {
+        std::printf("  selective-subscription check SKIPPED (STPLUGIN_IT_BYSTANDER_TOKEN not set)\n");
+    }
+
     // --- subscribe through the wrapper under test --------------------------
 
     std::atomic<int> video_frames{0};
@@ -287,6 +310,17 @@ int main()
     std::atomic<int> state_changes{0};
     session.setStateHandler([&](SessionState, const std::string &) { state_changes.fetch_add(1); });
 
+    // The session reports every subscription it did not ask for (and drops
+    // it). Zero is the assertion: the SFU must never have started sending
+    // the bystander's tracks to this subscriber at all.
+    std::atomic<int> unwanted_subscriptions{0};
+    session.setDiagnosticHandler([&](DiagnosticLevel, const std::string &message) {
+        std::printf("  diagnostic: %s\n", message.c_str());
+        std::fflush(stdout);
+        if (message.find("unwanted track") != std::string::npos)
+            unwanted_subscriptions.fetch_add(1);
+    });
+
     SessionConfig config;
     config.ws_url = url;
     config.token = subscribe_token;
@@ -315,6 +349,21 @@ int main()
     ST_ASSERT_EQ(last_sample_rate.load(), 48000);
     ST_ASSERT(last_channels.load() >= 1);
 
+    // A bystander that starts publishing while the session is up -- the
+    // other half: a player (re)joining mid-show.
+    if (bystander) {
+        step("bystander republish mid-session");
+        bystander->stopPump();
+        ST_ASSERT(bystander->unpublishVideo());
+        ST_ASSERT(bystander->publishVideo());
+        bystander->startPump();
+        // Long enough for an auto-subscription to have landed if one were
+        // coming; the wanted feed keeps flowing meanwhile.
+        const int before = video_frames.load();
+        ST_ASSERT(waitFor([&] { return video_frames.load() >= before + 30; }, 20000));
+        ST_ASSERT_EQ(unwanted_subscriptions.load(), 0);
+    }
+
     // --- publisher swap: the bug this plugin exists to make impossible -----
 
     const int before_swap = video_frames.load();
@@ -344,10 +393,38 @@ int main()
     ST_ASSERT(session.state() == SessionState::Connected);
     ST_ASSERT_EQ(bad_frames.load(), 0);
 
+    // --- full rejoin: the player leaves and comes back as a new session ----
+    //
+    // What a browser reload does, and what a player did twice on 2026-10-04
+    // to unstick their camera. A new participant session publishes brand-new
+    // tracks, and with auto_subscribe off nothing arrives unless the session
+    // asks for them again.
+
+    step("publisher leaves the room");
+    publisher_holder.reset();
+    ST_ASSERT(waitFor([&] { return !session.hasVideo(); }, 15000));
+    ST_ASSERT(session.state() == SessionState::Connected);
+
+    step("publisher rejoins and republishes");
+    publisher_holder = std::make_unique<Publisher>();
+    Publisher &rejoined = *publisher_holder;
+    ST_ASSERT(rejoined.connect(url, publish_token));
+    ST_ASSERT(rejoined.publishVideo());
+    ST_ASSERT(rejoined.publishAudio());
+    rejoined.startPump();
+    const int before_rejoin = video_frames.load();
+    const int audio_before_rejoin = audio_frames.load();
+    ST_ASSERT(waitFor([&] { return video_frames.load() >= before_rejoin + 15; }, 25000));
+    ST_ASSERT(waitFor([&] { return audio_frames.load() >= audio_before_rejoin + 10; }, 20000));
+    ST_ASSERT(session.hasVideo());
+    ST_ASSERT_EQ(bad_frames.load(), 0);
+
     // --- teardown ----------------------------------------------------------
 
     step("teardown");
-    publisher.stopPump();
+    if (bystander)
+        ST_ASSERT_EQ(unwanted_subscriptions.load(), 0);
+    publisher_holder->stopPump();
     session.disconnect();
     ST_ASSERT(session.state() == SessionState::Disconnected);
     ST_ASSERT(!session.hasVideo());
@@ -355,6 +432,7 @@ int main()
     // The publisher's Room must be torn down while the SDK is still
     // initialized, or its FFI disconnect fails on the way out.
     publisher_holder.reset();
+    bystander.reset();
     LiveKitSession::globalShutdown();
 
     std::printf("integration_livekit: %d video frames, %d audio frames, %d state changes\n", video_frames.load(),

core/tests/test_session.cpp

@@ -92,6 +92,34 @@ void testTrackSelection()
     ST_ASSERT(!isWantedAudioTrack(want, "other", MediaKind::Audio, MediaSource::Microphone));
 }
 
+void testSubscriptionSelection()
+{
+    const std::string want = "cam1";
+
+    // A camera source subscribes to exactly its slot's camera and mic.
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Camera, true, true));
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::Microphone, true, true));
+
+    // Every other participant's tracks are the bandwidth this exists to save:
+    // with N sources in one OBS, each of them used to pull the whole room.
+    ST_ASSERT(!shouldSubscribe(want, "cam2", MediaKind::Video, MediaSource::Camera, true, true));
+    ST_ASSERT(!shouldSubscribe(want, "cam2", MediaKind::Audio, MediaSource::Microphone, true, true));
+    // ...and so is the slot's own screenshare.
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Screenshare, true, true));
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::ScreenshareAudio, true, true));
+
+    // An audio-only source (the soundboard) never subscribes to video at all,
+    // rather than subscribing and then disabling it.
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Camera, false, true));
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::Microphone, false, true));
+    // A video-only source does not pull the mic.
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::Microphone, true, false));
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Camera, true, false));
+
+    // No selection subscribes to nothing.
+    ST_ASSERT(!shouldSubscribe("", "cam1", MediaKind::Video, MediaSource::Camera, true, true));
+}
+
 void testStateMachineHappyPath()
 {
     SessionStateMachine m;
@@ -527,6 +555,7 @@ int main()
 {
     testFrameGeometry();
     testTrackSelection();
+    testSubscriptionSelection();
     testStateMachineHappyPath();
     testPublisherSwapIsNotAnError();
     testReconnect();