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

ipykernel 6.28.0 と 6.29.0 で最小の 3 セルの例を実行し、Logger.write_from_queue、Logger.log、および sys.stderr sink に注目してください。ヘルパースレッドからの出力がノートブックのセルにどのように関連付けられるかを追跡してください。完了条件は、セル 3 以降のセルからのメッセージが、logger の初期化セルではなく、それぞれのセルに表示されることです。

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.)