In the PyCon 2015 talk, the following code was presented for a co-routine based Fibonacci server. future_monitor() was added to make the server working (otherwise it would stuck). But why is future_monitor() needed? why is future_done() callback is not enough?

from socket import *
from collections import deque
from concurrent.futures import ProcessPoolExecutor as Pool
from select import select

pool = Pool(4)

def fib(n):
    if n <= 2:
        return 1
    else:
        return fib(n-1) + fib(n-2)

def fib_server(address):
    sock = socket(AF_INET, SOCK_STREAM)
    sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
    sock.bind(address)
    sock.listen(5)
    while True:
        yield 'recv', sock
        conn, addr = sock.accept() # blocking
        print("connection", addr)
        tasks.append(fib_handler(conn))

def fib_handler(conn):
    while True:
        yield 'recv', conn
        req = conn.recv(100)  # blocking
        if not req:
            break
        n = int(req)
        future = pool.submit(fib, n)
        yield 'future', future 
        result = future.result()  # blocking
        resp = str(result).encode('ascii') + b'n'
        yield 'send', conn
        conn.send(resp)  # blocking
    print('closed')

tasks = deque()
recv_wait = {}
send_wait = {}
future_wait = {}

future_notify, future_event = socketpair()

def future_done(future):
    tasks.append(future_wait.pop(future))
    future_notify.send(b'x')

def future_monitor():
    while True:
        yield 'recv', future_event
        future_event.recv(100)

tasks.append(future_monitor())

def run():
    while any([tasks, recv_wait, send_wait]):
        while not tasks:
            # no active task to run wait for IO
            can_recv, can_send, _ = select(recv_wait, send_wait, [])
            for s in can_recv:
                tasks.append(recv_wait.pop(s))
            for s in can_send:
                tasks.append(send_wait.pop(s))
        task = tasks.popleft()
        try:
            why, what = next(task)
            if why == 'recv':
                recv_wait[what] = task
            elif why == 'send':
                send_wait[what] = task
            elif why == 'future':
                future_wait[what] = task
                what.add_done_callback(future_done)
            else:
                raise RuntimeError("We don't know what to do with :", why)
        except StopIteration:
            print('task done')

if __name__ == "__main__":
    tasks.append(fib_server(('localhost', 25000)))
    run()

[1] Talk’s video: https://youtu.be/MCs5OvhV9S4?t=0

Tried removing future_monitor, and the server became unresponsive. Looking for insight as to why.