"""Cron job implementation of Zulip's incoming email gateway's helper
for forwarding emails into Zulip.

https://zulip.readthedocs.io/en/latest/production/settings.html#email-gateway

The email gateway supports two major modes of operation: An email
server (using postfix) where the email address configured in
EMAIL_GATEWAY_PATTERN delivers emails directly to Zulip, and this, a
cron job that connects to an IMAP inbox (which receives the emails)
periodically.

Run this in a cronjob every N minutes if you have configured Zulip to
poll an external IMAP mailbox for messages. The script will then
connect to your IMAP server and batch-process all messages.

We extract and validate the target stream from information in the
recipient address and retrieve, forward, and archive the message.

"""
import email
import logging
from email.message import Message
from imaplib import IMAP4_SSL
from typing import Any, Generator

from django.conf import settings
from django.core.management.base import BaseCommand, CommandError

from zerver.lib.email_mirror import logger, process_message

## Setup ##

log_format = "%(asctime)s: %(message)s"
logging.basicConfig(format=log_format)

formatter = logging.Formatter(log_format)
file_handler = logging.FileHandler(settings.EMAIL_MIRROR_LOG_PATH)
file_handler.setFormatter(formatter)
logger.setLevel(logging.DEBUG)
logger.addHandler(file_handler)


def get_imap_messages() -> Generator[Message, None, None]:
    mbox = IMAP4_SSL(settings.EMAIL_GATEWAY_IMAP_SERVER, settings.EMAIL_GATEWAY_IMAP_PORT)
    mbox.login(settings.EMAIL_GATEWAY_LOGIN, settings.EMAIL_GATEWAY_PASSWORD)
    try:
        mbox.select(settings.EMAIL_GATEWAY_IMAP_FOLDER)
        try:
            status, num_ids_data = mbox.search(None, 'ALL')
            for message_id in num_ids_data[0].split():
                status, msg_data = mbox.fetch(message_id, '(RFC822)')
                assert isinstance(msg_data[0], tuple)
                msg_as_bytes = msg_data[0][1]
                message = email.message_from_bytes(msg_as_bytes)
                yield message
                mbox.store(message_id, '+FLAGS', '\\Deleted')
            mbox.expunge()
        finally:
            mbox.close()
    finally:
        mbox.logout()


class Command(BaseCommand):
    help = __doc__

    def handle(self, *args: Any, **options: str) -> None:
        # We're probably running from cron, try to batch-process mail
        if (not settings.EMAIL_GATEWAY_BOT or not settings.EMAIL_GATEWAY_LOGIN or
            not settings.EMAIL_GATEWAY_PASSWORD or not settings.EMAIL_GATEWAY_IMAP_SERVER or
                not settings.EMAIL_GATEWAY_IMAP_PORT or not settings.EMAIL_GATEWAY_IMAP_FOLDER):
            raise CommandError("Please configure the Email Mirror Gateway in /etc/zulip/, "
                               "or specify $ORIGINAL_RECIPIENT if piping a single mail.")
        for message in get_imap_messages():
            process_message(message)