fix(rtc): drop late SFU subscriber offers when peer connection is closed (#240)

`_on_subscriber_offer` unconditionally called `setRemoteDescription` on
`subscriber_pc`, so an offer that arrived after the connection was torn
down raised
`InvalidStateError: Cannot handle offer in signaling state "closed"`.
The exception propagated through the pyee error path and killed the
session.

Reproduces reliably under load: when the asyncio loop falls behind
(e.g. several concurrent agent sessions on one pod), session cleanup
and a late SFU renegotiation race, the offer arrives at a closed
peer connection, and the resulting exception drops the agent's
remaining sessions.

Guard the handler: if `subscriber_pc` is missing or `signalingState`
is `"closed"`, drop the offer and log at debug level — there is
nothing to negotiate with a closed connection.

Author: aliev

getstream/video/rtc/connection_manager.py

@@ -198,6 +198,17 @@ async def _on_ice_trickle(self, event):
     async def _on_subscriber_offer(self, event: events_pb2.SubscriberOffer):
         logger.info("Subscriber offer received")
 
+        # Offers can arrive after the subscriber peer connection has been
+        # torn down (slow asyncio loop under load, SFU sending a late
+        # renegotiation). `setRemoteDescription` would raise
+        # `InvalidStateError: Cannot handle offer in signaling state "closed"`
+        # and the exception propagates through the pyee error path, killing
+        # the session. Drop the offer instead — there is nothing to
+        # negotiate with a closed connection.
+        if self.subscriber_pc is None or self.subscriber_pc.signalingState == "closed":
+            logger.debug("Subscriber offer arrived after PC closed; dropping")
+            return
+
         with telemetry.start_as_current_span("rtc.on_subscriber_offer") as span:
             await self.subscriber_negotiation_lock.acquire()