My goal is to emit an interface to listen on a socket forever … until someone up the decision chain decides it’s enough.

This is my implementation, it does not work. Mixing threads, sockets, object lifetime, default params and a language I do not speak too well is confusing.

I tested individually different aspects of this code and everything was as expected except the line containing the comment BUG where I attempt to force the main thread to block until the server hears the child screaming or a timeout passes but instead recv() simply doesn’t see the change in alive.

#!/usr/bin/env python3


import socket
import threading
import time


MAX_MSG_BYTES=1024
TEST_PORT=42668


def recv( s: socket.socket, alive: bool=True ) -> bytes:
    '''
    Accepts packets on a socket until terminated.
    '''
    s.settimeout(1)  # 1 second
    while alive:
        print("'alive' is still", alive)
        try:
            data = s.recv(MAX_MSG_BYTES)
            assert data  # Empty packets were a problem.
            yield data
        except TimeoutError:
            pass  # expected error, any other is propagated up


def test_nonblocking_recv() -> None:
    # Create 3 sockets - sever administrative, server content and client content.
    # Bind the latter and forget about the former.
    server_s = socket.create_server(('', TEST_PORT))
    server_s.listen()
    client_s = socket.create_connection(('localhost', TEST_PORT))
    content_s = next(iter(server_s.accept()))  # Accept 1 connection.

    # client_s.sendall('If this is commented out, the server hangs.'.encode('utf8'))

    alive = True
    def read_one_message():
        data = recv(content_s, alive)
        print(next(iter(data)))  # BUG this causes outside alive to not be seen

    content_th = threading.Thread(target=read_one_message)
    content_th.start()
    time.sleep(3)
    alive = False
    print("But main thread 'alive' is", alive)

    content_th.join()
    assert threading.active_count() == 1


if __name__ == '__main__':
    test_nonblocking_recv()