fix(rtc_engine): hold a weak reference to the engine from its own task

[LautaroPetaccio]

The problem

engine_task took an Arc<EngineInner>:

tokio::spawn(Self::engine_task(inner.clone(), session_events, close_rx));

But the task's JoinHandle and the oneshot::Sender that stops it both live in EngineHandle (rtc_engine/mod.rs:266), which lives in running_handle inside that same EngineInner. The task therefore kept alive the only thing that could stop it — its own stop signal — and session_events' emitter is likewise reachable only through the session it pins.

An engine dropped without an explicit close() released nothing: the session, both peer connections, the Arc<LkRuntime> (and so the PeerConnectionFactory and its WebRTC threads), the signal client with its open websocket, and the task itself all stayed resident for the lifetime of the process. The SFU also kept its half of the session, since the socket was never closed.

There is no Drop on RtcEngine, EngineInner, RtcSession or SessionInner, so nothing else could recover it.

The fix

RtcSession already avoids exactly this by keeping close_tx in SessionHandle, outside the Arc'd inner, so dropping it stops its tasks. Rather than restructure EngineHandle, the task now takes a Weak and upgrades per event.

This is what AGENTS.md already requires:

If an owner transitively stores the callback/task, the callback/task must not strongly capture that owner.

A side benefit is that the task now stops on its own once the engine is gone, instead of depending solely on the close signal.

Verification

test_drop_without_close_releases_engine connects an engine, takes a drop probe, drops it without close(), and waits for the internals to be released.

• Before: fails — still alive after the full 10s window.
• After: passes, in 0.03s.

Full e2e suite against livekit-server --dev is green, including all six reconnection tests. The only delta from the baseline is the added test.

The test targets RtcEngine directly rather than going through Room, because the room layer has the same structural issue with its own close_tx and would mask an engine-level fix. test_engine() is added to the e2e harness for that purpose.

[devin-ai-integration]

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