gevent gets stuck when gevent.threadpool is used inside a fork hook

[sumerc]

Description

subprocess.Popen gets stuck inside the fork hook if gevent.threadpool is used. The weird thing is if the print(gevent.threadpool) line is removed, issue does not happen.

Reproducer

import gevent.monkey
gevent.monkey.patch_all(
        subprocess=True,
    )

import os, gevent, subprocess, time

def hook():
    print("inside fork hook")
    gevent.get_hub().threadpool.apply(lambda: print("run with threadpool!"))
    

os.register_at_fork(after_in_child=hook)
print(gevent.get_hub().threadpool, " >>> HUB.threadpool before hook") # --> if you remove this line, this works!!
r = subprocess.Popen("pwd", close_fds=False)

time.sleep(1.0)
print("ended!", r)

• Issue only happens when the print(gevent.get_hub().threadpool, " >>> HUB.threadpool before hook") is there. Somehow, reading threadpool before the fork hook causes the problem.
• Issue happens both on Linux and MacOsx.
• Issue only happens with Python 3.11 and above. 3.10 works just fine.

[ddorian]

I can see the same thing with tqdm too. For me it happens on python 3.11.8, gevent 24.2.1, tqdm 4.66.2. Disabling thread monkey patching fixes it.

import gevent.monkey

gevent.monkey.patch_all(thread=True)


def tttqqqdddmmm():
    import tqdm

    def do_it():
        for i in tqdm.tqdm(range(5)):
            print(i)

    global_threadpool = gevent.get_hub().threadpool
    global_threadpool.spawn(do_it).get()
    exit("tqdm_exit")


tttqqqdddmmm()

Output:

tqdm_exit
0
1
2
3
4
# here it's stuck