Only clear_scheduled_emails previously took a lock on the users before
removing them; make deliver_scheduled_emails do so as well, by using
prefetch_related to ensure that the table appears in the SELECT. This
is not necessary for correctness, since all accesses of
ScheduledEmailUser first access the ScheduledEmail and lock it; it is
merely for consistency.
Since SELECT ... FOR UPDATE takes an UPDATE lock on all tables
mentioned in the SELECT, merely doing the prefetch is sufficient to
lock both tables; no `on=(...)` is needed to `select_for_update`.
This also does not address the pre-existing potential deadlock from
these two use cases, where both try to lock the same ScheduledEmail
rows in opposite orders.
`deliver_scheduled_emails` and `deliver_scheduled_messages` use their
respective tables like a queue, but do not have guarantees that there
was only one consumer (besides the EMAIL_DELIVERER_DISABLED setting),
and could send duplicate messages if multiple consumers raced in
reading rows.
Use database locking to ensure that the database only feeds a given
ScheduledMessage or ScheduledEmail row to a single consumer. A second
consumer, if it exists, will block until the first consumer commits
the transaction.
This makes it parallel with deliver_scheduled_messages, and clarifies
that it is not used for simply sending outgoing emails (e.g. the
`email_senders` queue).
This also renames the supervisor job to match.
Alex Vandiver