Log messages written from a helper thread appear in the wrong notebook cell (regression) #1491

I recently found that, with some configurations, log messages emitted by Loguru would appear in the wrong Jupyter notebook cell: namely, the cell that initialized the logger, rather than the cell that was responsible for the log message. I opened Delgan/loguru#1427 to address this, but have since closed it, since the issue appears to be a regression in ipykernel, not in Loguru itself.

The problem appears for all versions of Loguru, but only for ipykernel >= 6.29. It is confined to the case where the logger is configured to write messages using a separate helper thread, which is useful when logging from multiple threads simultaneously. A minimal working example that illustrates the issue without the use of Loguru is the following:

(cell 1)

import sys
import multiprocessing
import threading

class Logger:
    def __init__(self, sink):
        self.sink = sink
        self.queue = multiprocessing.SimpleQueue()
        self.lock = threading.Lock()
        self.thread = threading.Thread(target=self.write_from_queue, daemon=True)
        self.thread.start()

    def write_from_queue(self):
        while True:
            message = self.queue.get()
            with self.lock:
                self.sink.write(message)

    def log(self, message):
        self.queue.put(message)

(cell 2)

logger = Logger(sink=sys.stderr)

(cell 3)

logger.log("a message")

With ipykernel 6.28.0, the log message appears under cell 3, as intended, but with 6.29.0, it appears under cell 2 instead, and subsequent messages logged using the same logger will always appear under cell 2.

(Note: I tested with Jupyter Notebook 7.5.0 and Python 3.12.11 on Ubuntu 24.04.)