Transaction rollbacks can cause mail loss

[guettli]

I want to know if loosing mail can happen.

Maybe like this.

1. Cron job calls python manage.py getmail
2. DB transaction starts
3. Mail gets fetched from pop3 server
4. Mail gets stored in DB
5. Mails gets handled by custom signal handler
6. Mail gets deleted on pop3 server
7. DB commit, but BANG: One DB constraint complaints and the whole transaction gets rolled back.

This would result in message lost, since the pop3 server has deleted the mail, but the mail was never entered into the DB since the transaction was rolled back.

[ad-m]

There is no transaction for each message, unfortunately (see #66), so message was deleted before transaction of message was commited, so data loss is possible IMO. I'm not have the time to work on such a feature, but feel free to submit a PR.

[guettli]

Django 1.9 has a nice signal: on_commit():
https://docs.djangoproject.com/en/1.10/topics/db/transactions/#django.db.transaction.on_commit

Sometimes you need to perform an action related to the current database transaction, but only if the transaction successfully commits.

I think the delete on the pop3/imap server should happen in this signal.

If things go wrong you import the message twice. That's better than message loss.

[ad-m]

I know how works transaction, but there is some issues with implementations and different backend compatibility to use django.db.transaction.on_commit. See email downloading https://github.com/coddingtonbear/django-mailbox/blob/master/django_mailbox/models.py#L383 and transport https://github.com/coddingtonbear/django-mailbox/blob/master/django_mailbox/transports/pop3.py#L46 https://github.com/coddingtonbear/django-mailbox/blob/master/django_mailbox/transports/generic.py#L29 https://github.com/coddingtonbear/django-mailbox/blob/master/django_mailbox/transports/imap.py#L135 . It works on different layer.

[ad-m]

Pull requests are welcome to provide callback from ``Mailbox.get_new_mail` to transports about successful mail save.

[guettli]

I won't use django-mailbox in the next weeks. I won't provide a pull request. Good bye.