import fcntl
import os
import select

NUM_WRITERS = 20
MAXEVENTS = 5

def set_nonblocking(fd):
    flags = fcntl.fcntl(fd, fcntl.F_GETFL)
    fcntl.fcntl(fd, fcntl.F_SETFL, os.O_NONBLOCK|flags)

print("Working with {} FDs, {} maxevents".format(NUM_WRITERS, MAXEVENTS))

all_writers = set()
seen_writers = set()

ep = select.epoll()
try:
    for i in range(NUM_WRITERS):
        r, w = os.pipe()
        all_writers.add(w)
        set_nonblocking(r)
        set_nonblocking(w)
        ep.register(w, select.EPOLLOUT)

    while all_writers - seen_writers:
        print(sorted(all_writers - seen_writers))
        for (fd, event) in ep.poll(-1, MAXEVENTS):
            if event & select.POLLOUT:
                seen_writers.add(fd)
                try:
                    while True:
                        os.write(fd, b'X' * 512)
                except OSError:
                    pass

finally:
    ep.close()

assert seen_writers == all_writers