`multiprocessing.Queue` methods have asymmetric behavior

Bug report

Bug description:

Currently, the qsize() and full() methods rely on the semaphore value (they change their result immediately after the put operation is performed). However, the empty() method relies on the underlying pipe's readiness, which violates user expectations (since similar queue.Queue methods have symmetric behavior) and leads to the following:

#!/usr/bin/env python3

import sys
import time

from multiprocessing import Process, Queue, set_start_method

CONSUMERS = 100  # any sufficiently large number
ITEMS = CONSUMERS * 100  # should result in exceeding the underlying pipe
DELAY = 1  # "infinitesimal", but it can actually be as large as you like


def consume(queue):
    for _ in range(ITEMS // CONSUMERS):
        if queue.empty():  # it should never be printed, but it will be
            print("EMPTY!")

        queue.get()


def main():
    queue = Queue()

    for i in range(ITEMS):
        queue.put(i)

    time.sleep(DELAY)
    assert queue.qsize() == ITEMS

    consumers = [
        Process(target=consume, args=[queue], daemon=True)
        for _ in range(CONSUMERS)
    ]

    for consumer in consumers:
        consumer.start()

    for consumer in consumers:
        consumer.join()


if __name__ == "__main__":
    set_start_method("fork")
    sys.exit(main())

The problem was initially noticed in one Stack Overflow question, when the queue was considered empty despite having a sufficiently large number of items. I have only cited one case, but in fact the problem will always occur when buffer flushing is too slow (especially if any complex objects are serialized).

I decided to mark this as a bug, as this behavior can be fixed as a result of solving #87302. Otherwise, I think clarifying this point in the documentation may also be sufficient.

Related (implicitly used in the above code to reproduce): #128186.

CPython versions tested on:

3.9, 3.10, 3.11, 3.12, 3.13, 3.14

Operating systems tested on:

Linux

Linked PRs

• gh-144832

Bắt đầu với tài liệu của multiprocessing.Queue về qsize(), full() và empty(), sau đó xem lại bản tái hiện trong issue cùng các issue liên quan #87302 và #128186. Kiểm tra PR được liên kết #144832 và xác định xem hành vi đó đã được giải quyết ở đó chưa; được xem là hoàn tất khi các phương thức của queue hoạt động nhất quán hoặc hành vi được ghi trong tài liệu giải quyết rõ ràng trường hợp được báo cáo.