clear track notifier observers on subscription teardown (#4413)

When a subscriber disconnects, observer closures registered on the
publisher's TrackChangedNotifier and TrackRemovedNotifier were never
removed. These closures capture the SubscriptionManager, which holds
the ParticipantImpl, preventing the entire participant object graph
(PCTransport, SDPs, RTP stats, DownTracks) from being garbage collected.

In rooms with many participants that disconnect and reconnect frequently,
this causes unbounded memory growth proportional to the number of
disconnect events. The leaked memory is not recoverable while the room
remains open.

Clear notifiers in both handleSubscribedTrackClose (individual
subscription teardown) and SubscriptionManager.Close (full participant
teardown), matching the existing cleanup in handleSourceTrackRemoved.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/rtc/subscriptionmanager.go

@@ -19,12 +19,13 @@ package rtc
 import (
 	"context"
 	"errors"
+	"maps"
+	"slices"
 	"sync"
 	"time"
 
 	"github.com/pion/webrtc/v4/pkg/rtcerr"
 	"go.uber.org/atomic"
-	"golang.org/x/exp/maps"
 
 	"github.com/livekit/livekit-server/pkg/rtc/types"
 	"github.com/livekit/livekit-server/pkg/sfu"
@@ -114,6 +115,16 @@ func (m *SubscriptionManager) Close(isExpectedToResume bool) {
 
 	prometheus.RecordTrackSubscribeCancels(int32(m.getNumCancellations()))
 
+	// Remove observer closures from track change/remove notifiers to allow
+	// this participant and its transports to be garbage collected.
+	m.lock.Lock()
+	subs := maps.Clone(m.subscriptions)
+	m.lock.Unlock()
+	for _, sub := range subs {
+		sub.setChangedNotifier(nil)
+		sub.setRemovedNotifier(nil)
+	}
+
 	subTracks := m.GetSubscribedTracks()
 	downTracksToClose := make([]*sfu.DownTrack, 0, len(subTracks))
 	for _, st := range subTracks {
@@ -332,7 +343,7 @@ func (m *SubscriptionManager) GetSubscribedParticipants() []livekit.ParticipantI
 	m.lock.RLock()
 	defer m.lock.RUnlock()
 
-	return maps.Keys(m.subscribedTo)
+	return slices.Collect(maps.Keys(m.subscribedTo))
 }
 
 func (m *SubscriptionManager) IsSubscribedTo(participantID livekit.ParticipantID) bool {
@@ -945,6 +956,8 @@ func (m *SubscriptionManager) handleSubscribedTrackClose(s *mediaTrackSubscripti
 		return
 	}
 	s.setSubscribedTrack(nil)
+	s.setChangedNotifier(nil)
+	s.setRemovedNotifier(nil)
 
 	var relieveFromLimits bool
 	switch subTrack.MediaTrack().Kind() {