Event loop destruction performs file read/write operations in gevent>=20.5.1

[hashbrowncipher]

• gevent version: 20.5.1 and later
• Python version: Python 3.7.7 downloaded using pyenv
• Operating System: Linux 5.4.0-47-generic, Ubuntu Focal userspace

Description:

The following program reliably hangs on gevent>=20.5.1. It works as expected on gevent 20.5.0.

from gevent.monkey import patch_all
patch_all()  # noqa: E402

import gevent
from gevent.hub import get_hub
from gevent import os
from gevent.monkey import get_original
from gevent.socket import wait_read

hang = get_original("time", "sleep")

r, w = os.pipe()


def pipe_reader():
    os.read(r, 4096)
    print(f"Read from pipe in PID: {os.getpid()}")


print(f"Parent PID: {os.getpid()}")
hub = get_hub()
os.write(w, b"test")
wait_read(r)
gevent.spawn(pipe_reader)
pid = os.fork()
if pid == 0:
    hub.destroy(destroy_loop=True)
else:
    print(f"Child PID: {pid}")
    # Briefly prevent the parent process from executing its event loop
    hang(1)
    os.waitpid(pid, 0)

The expected output looks like this, with a 1 second interval between "Child PID" and "Read from pipe in PID". "Read from pipe in PID" should match "Parent PID":

Parent PID: 2332303
Child PID: 2332306
Read from pipe in PID: 2332303

Unexpected output looks like this, and hangs indefinitely:

Parent PID: 2333255
Child PID: 2333258
Read from pipe in PID: 2333258

[jgehrcke]

This might relate to #1669.

[jamadden]

Thanks for the reports!

I don't have the prerequisites to reproduce #1669, but I do suspect that they are both the same basic issue.

I expect to have this fixed in the next gevent release.

In the meantime and for older versions, if you add hub.loop.break_() just before hub.destroy(destroy_loop=True) I expect it will fix the issue (at least it does this one).