d4f6b93859
This fixes two bugs: the most obvious is that there is a race where a
ScheduledEmail object could be observed in the window between creation
and when users are added; this is a momentary instance when the object
has no users, but one that will resolve itself.
The more subtle is that .save() will, if no records were found to be
updated, _re-create_ the object as it exists in memory, using an
INSERT[1]. Thus, there is a race with `deliver_scheduled_emails`
between when the users are added, and when `email.save()` runs:
1. Web request creates ScheduledEmail object
2. Web request creates ScheduledEmailUsers object
3. deliver_scheduled_emails locks the former, preventing updates.
4. deliver_scheduled_emails deletes both objects, commits, releasing lock
5. Web request calls `email.save()`; UPDATE finds no rows, so it
re-creates the ScheduledEmail object.
6. Future deliver_scheduled_emails runs find a ScheduledEmail with no
attending ScheduledEmailUsers objects
Wrapping the logical creation of both of these in a single transaction
avoids both of these races.
[1] https://docs.djangoproject.com/en/3.2/ref/models/instances/#how-django-knows-to-update-vs-insert
Zulip overview
Zulip is a powerful, open source group chat application that combines the immediacy of real-time chat with the productivity benefits of threaded conversations. Zulip is used by open source projects, Fortune 500 companies, large standards bodies, and others who need a real-time chat system that allows users to easily process hundreds or thousands of messages a day. With over 700 contributors merging over 500 commits a month, Zulip is also the largest and fastest growing open source group chat project.
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