import fcntl
import os
import resource
import select
import sys


try:
    MAXEVENTS = int(sys.argv[1])
except (IndexError, ValueError):
    MAXEVENTS = -1


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


# increase RLIMIT_NOFILE
try:
    soft, hard = resource.getrlimit(resource.RLIMIT_NOFILE)
    resource.setrlimit(resource.RLIMIT_NOFILE, (hard, hard))
    NUM_WRITERS = hard
except OSError:
    NUM_WRITERS = soft

# guard for already allocated FDs (stdin, stdout...)
NUM_WRITERS -= 16

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

r, w = os.pipe()

all_writers = set()
seen_writers = set()

with select.epoll() as ep:
    # register read-end
    set_nonblocking(r)
    ep.register(r, select.EPOLLIN)

    # register many write-ends
    for i in range(NUM_WRITERS):
        fd = os.dup(w)
        set_nonblocking(w)
        ep.register(fd, select.EPOLLOUT)
        all_writers.add(fd)

    fill = False

    # loop until all the writer FDs have been reported ready
    while all_writers - seen_writers:
        print("Number of missing FDs:{}".format(len(all_writers - seen_writers)))

        if fill:
            # fill the pipe every other loop, so that the next poll() call sees
            # all the writers non-ready/ready alternatively (necessary to reset
            # edge-levels)
            try:
                while True:
                    os.write(w, b'X' * 8096)
            except OSError:
                pass
            fill = False
        else:
            # drain the pipe
            try:
                while True:
                    os.read(r, 8096)
            except OSError:
                pass
            fill = True

        # find all ready writers - if `maxevent` is less than the number of
        # ready writers, some events won't be reported
        ready_writers = set(fd for fd, evt in ep.poll(-1, MAXEVENTS) if fd != r)

        # remember writers seen
        seen_writers |= ready_writers

        print("Number of ready FDs: {}".format(len(ready_writers)))