Bug report

When using a ProcessPoolExecutor with forked child processes, if one of the child processes suddenly dies (segmentation fault, not a Python exception) and if simultaneously data is being sent into the call queue, then the parent process hangs forever.

Reproduction

import ctypes
from concurrent.futures import ProcessPoolExecutor


def segfault():
    ctypes.string_at(0)


def func(i, data):
    print(f"Start {i}.")
    if i == 1:
        segfault()
    print(f"Done {i}.")
    return i


data = list(range(100_000_000))
count = 10
with ProcessPoolExecutor(2) as pool:
    list(pool.map(func, range(count), [data] * count))
print(f"OK")

In Python 3.8.10 it raises a BrokenProcessPool exception whereas in 3.9.13 and 3.10.5 it hangs.

Analysis

When a crash happens in a child process, all workers are terminated and they stop reading in communication pipes. However if data is being send in the call queue, the call queue thread which writes data from buffer to pipe (multiprocessing.queues.Queue._feed) can get stuck in send_bytes(obj) when the unix pipe it's writing to is full. _ExecutorManagerThread is blocked in self.join_executor_internals() on line

Proposed solution

Drain call queue buffer either in terminate_broken method before calling join_executor_internals or in queue close method.
I will create a pull request with a possible implementation.

Your environment

• CPython versions tested on: reproduced in 3.10.5 and 3.9.13 (works well in 3.8.10: BrokenProcessPool exception)
• Operating system and architecture: Linux, x86_64

Linked PRs

• gh-94777: Fix deadlock in ProcessPoolExecutor #94784
• [3.11] gh-94777: Fix deadlock in ProcessPoolExecutor (GH-94784) #106607
• [3.12] gh-94777: Fix deadlock in ProcessPoolExecutor (GH-94784) #106609