multiprocessing.Process.is_alive() incorrect if waitpid() was executed concurrently to it

Bug report
As per title. In my case I triggered the os.waitpid() using psutil library, but easily reproducible using os.waitpid, too.

Reproduction:

Python 3.9.5 (default, Nov 18 2021, 16:00:48) 
[GCC 10.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from multiprocessing import Process
>>> import os
>>> import time
>>> p = Process(target=time.sleep, args=(10,))
>>> p.start(); os.waitpid(p.pid, 0)
(17587, 0)
>>> p.is_alive()
True
>>> p.join() # Returns immediately
>>> 

I understand it's something of a corner case; however, join() behaves correctly here, so we definitely have enough info about whether the process is alive.

By the looks of it, if the process has been already waited on, poll() Popen method in multiprocessing returns None, which is interpreted as the process being still alive.

Your environment

• CPython versions tested on: 3.9.5, but main version seems affected
• Operating system and architecture: x64 Ubuntu Linux

Bắt đầu bằng cách lần theo multiprocessing.Process.is_alive(), luồng Popen.poll() và Process.join() trong khi tái hiện ví dụ os.waitpid() đồng thời trên Linux. So sánh cách mỗi luồng xác định rằng tiến trình con đã thoát, sau đó bổ sung phạm vi kiểm thử hồi quy cho thấy is_alive() trả về false sau khi tiến trình con đã được chờ và join() vẫn hoàn tất ngay lập tức.