[New-bugs-announce] [issue44138] multiprocessing documentation should note behavior when process with Lock dies

[Kushal Kumaran]

New submission from Kushal Kumaran <Kushal.Kumaran at veritas.com>:

If a process holding a multiprocessing.Lock dies, other waiters on the lock will be stuck.  This is mostly the same as with threads, but threads-users can usually avoid this with careful coding (dealing with errors, etc.), and significant crashes will normally take down the entire process anyway.  With multiprocessing, a process with lock held could get SIGKILLed with no recourse.

A simple program demonstrating the problem:
```
import multiprocessing
import os
import signal

lk = multiprocessing.Lock()

def f():
    my_pid = os.getpid()
    print("PID {} going to wait".format(my_pid))
    with lk:
        print("PID {} got the lock".format(my_pid))
        os.kill(my_pid, signal.SIGKILL)

if __name__ == '__main__':
    for i in range(5):
        multiprocessing.Process(target=f).start()
```

Running this will have one of the processes acquiring the lock and dying; the other processes will wait forever.

The reason behind this behavior is obvious from the implementation that uses POSIX semaphores (I don't know how the win32 implementation behaves).

I don't think the behavior can be changed, since releasing the lock on process crash could leave other processes having to deal with unexpected state. A note in the documentation for the multiprocessing module is all I could think of. I don't see a way to use multiprocessing.Lock with safety against process crashes. If someone has a scenario where they can guarantee their data consistency in the face of process crash, they should use some alternative mechanism such as file-based locking.

----------
components: Library (Lib)
messages: 393695
nosy: kushal-kumaran
priority: normal
severity: normal
status: open
title: multiprocessing documentation should note behavior when process with Lock dies
type: enhancement
versions: Python 3.10, Python 3.11, Python 3.6, Python 3.7, Python 3.8, Python 3.9

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44138>
_______________________________________