import os
import select
import threading
import time


p = select.poll()
rfds = []
r, w = os.pipe()


for i in range(1000):
    fd = os.dup(r)
    rfds.append(fd)
    p.register(fd, select.POLLIN)

def resize_ufds_array():
    time.sleep(0.5)

    # trigger ufds array reallocation
    for fd in rfds:
        p.unregister(fd)
    p.register(w, select.POLLOUT)
    p.poll()

    # and make the call to poll() from the main thread return
    os.write(w, b'hello')

t = threading.Thread(target=resize_ufds_array)
t.start()
p.poll()