Self-remove event subscriber on WebSocketClosedError

[Zsailer]

SubscribeWebsocket.event_listener writes to the subscriber's WebSocket without guarding against WebSocketClosedError, and only cleans up through on_close, which isn't reliably fired when the peer disappears without a clean close handshake (network drop, browser tab killed, stream reset).

When that happens, the stale bound event_listener stays in event_logger._modified_listeners. Since it's a bound method, it keeps a strong reference to its SubscribeWebsocket instance, and therefore to the tornado WebSocket handler, the request, and any state captured during the handshake. Nothing ever removes it, so it lives until the process exits. This is a memory leak that grows linearly with the number of unclean disconnects over the server's lifetime.

The fix guards write_message with a WebSocketClosedError handler and removes the listener from the logger when the write fails. EventLogger.remove_listener uses set.discard, so double-remove is a no-op.

[krassowski]

Makes sense to me, just the docstring looks not like something I would want to read in docs. Something shorter documenting behaviour rather than imperative code comment would be better there IMO.

[Zsailer]

In full transparency, I patched this code a while ago internally (human written), then realized I never opened a PR to fix upstream.

I used AI to port my changes over (hench, why it went hard on the docstring 😆 ). Acknowledging here that I used AI.