MsgCallbackSubscriber: class-level default Event() is shared between instances

[majkelx]

Problem

In serverish/messenger/msg_callback_sub.py the stop event is declared as a param with a mutable default created at class definition time:

_stop_event = param.ClassSelector(default=Event(), class_=Event, doc="Event to stop reading messages")

The single Event() instance is evaluated once and becomes the shared default for all MsgCallbackSubscriber instances (and subclasses). Consequences:

• Calling stop() (or close(), which calls stop()) on one subscriber sets the shared event, which silently terminates the _task_body loop of every other subscriber in the process (if cont is False or self._stop_event.is_set(): break).
• Since the event is never cleared, any subscriber created after one has been stopped exits its loop on the first received message.
• Additionally, the Event is bound to whatever event loop first uses it, which can misbehave across loop lifecycles (e.g. in tests).

Fix

Create the event per instance in __init__ instead of as a class-level param default:

def __init__(self, **kwargs) -> None:
    self._stop_event = Event()
    super().__init__(**kwargs)

The new MsgKvSubscriber (KV buckets, feature/kv-buckets branch) already does it this way — MsgCallbackSubscriber should be aligned.

Notes

Worth a quick audit for other mutable param defaults of this kind in the codebase.