[Bug]: SubscriptionManager._message_subs leaks empty sets on message unsubscription and connection teardown

[bukeeastrey]

Duplicate check

• I searched the existing open and closed issues and this is not a duplicate.

AgentOS version or commit

v2026.8.29

Area

Gateway

Reproduction steps

1. Open a WebSocket connection and subscribe to session messages for several session keys (sessions.messages.subscribe).
2. Unsubscribe from the sessions via sessions.messages.unsubscribe or disconnect the WebSocket client.
3. Inspect subscription_manager._message_subs.

Expected behavior

Empty sets for sessions with no remaining subscribers are deleted from _message_subs.

Actual behavior

Stale session_key keys containing empty set() objects linger in _message_subs indefinitely, accumulating memory over time on a long-running gateway.

Environment

Windows 11, Python 3.12, uv

[andreapn]

Accepted, p3. Confirmed at src/agentos/gateway/websocket.py:549 — the leak is visible as an asymmetry inside SubscriptionManager itself:

def unsubscribe_messages(self, conn_id: str, session_key: str) -> None:
    if session_key in self._message_subs:
        self._message_subs[session_key].discard(conn_id)      # no cleanup

def unsubscribe_topic(self, conn_id: str, topic: str) -> None:
    if topic in self._topic_subs:
        self._topic_subs[topic].discard(conn_id)
        if not self._topic_subs[topic]:
            del self._topic_subs[topic]                        # cleanup

remove_connection() has the same split: it collects empty_topics and deletes them, but the _message_subs loop only calls subs.discard(conn_id).

Growth is bounded by the number of distinct session keys a gateway ever serves rather than by connection count, so this is p3 rather than urgent — but the topic path already shows the intended pattern and the message path should match it.

Three PRs target this (#648, #637, #610) — we will pick one.