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80b7df1b0d
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.
(cherry picked from commit 4c518c2bba)