add preconnect audio buffer API to LocalAudioTrack - #648

🟡 TOCTOU race: _preconnect_buffer checked before lock but accessed inside lock

In send_preconnect_buffer, self._preconnect_buffer is checked for None at line 121 before acquiring self._send_lock at line 124. If the lock is contended (another send_preconnect_buffer call is in-flight), the async with self._send_lock: line yields control. During this yield, stop_preconnect_buffer() (which is synchronous) can be called from another callback—setting self._preconnect_buffer = None at livekit-rtc/livekit/rtc/track.py:116. When the lock is finally acquired, self._preconnect_buffer.capture() at line 125 will raise AttributeError: 'NoneType' object has no attribute 'capture'. The fix is to move the None-check inside the lock.

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🟡 Event listener leak in _setup_preconnect_auto_send when preconnect buffer is stopped before target becomes active

In _on_participant_active, when the target identity matches but track.has_preconnect_buffer returns False, the handler returns early without calling room.off("participant_active", _on_participant_active) to unregister itself. This means the handler stays registered on the room indefinitely, holding references to track, room, and other closure variables. Worse, if start_preconnect_buffer() is called again later and the same identity fires participant_active again (e.g. after a reconnect), the stale handler will unexpectedly trigger and send the buffer.

The fix is to unregister the handler whenever the target identity matches, regardless of the buffer state — _send_buffer already checks has_preconnect_buffer internally.

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