"""The listening loop of the web service and of its bootstrap server.

Only the Python standard library is used, so the bootstrap server, which must run whatever else
is broken, can use it.
"""

import threading
import time
from http.server import ThreadingHTTPServer


def serve(wanted, handler, log, interval):
    """Work out where to listen every interval seconds with wanted, and run an HTTP server of handler at each place.

    wanted answers ([(address, port), ...], '') or ([], the reason not to listen). A server whose
    place is no longer wanted is stopped, and a new one listens at each new place; a place that
    cannot be bound is tried again the next round. Every new message is logged with log. Does
    not return.
    """
    servers, reported = {}, None
    while True:
        targets, reason = wanted()
        for target in [target for target in servers if target not in targets]:
            server, thread = servers.pop(target)
            server.shutdown()
            server.server_close()
            thread.join()
        messages = [reason] if reason else []
        for target in targets:
            if target in servers:
                continue
            try:
                server = ThreadingHTTPServer(target, handler)
            except OSError as exc:
                messages.append(f'cannot listen on {target[0]}:{target[1]}: {exc}')
                continue
            thread = threading.Thread(target=server.serve_forever, name=f'server {target[0]}', daemon=True)
            thread.start()
            servers[target] = (server, thread)
        if servers:
            messages.append('serving on ' + ', '.join(f'http://{address}:{port}' for address, port in servers))
        message = '; '.join(messages)
        if message and message != reported:
            log(message)
            reported = message
        time.sleep(interval)