[wp-trac] [WordPress Trac] #49693: Drop duplicate recurring cron events

[WordPress Trac]

#49693: Drop duplicate recurring cron events
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 Reporter:  aidvu        |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  Cron API     |    Version:  trunk
 Severity:  normal       |   Keywords:  has-patch has-unit-tests
  Focuses:               |
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 It's easy to break a site by mistakenly scheduling millions of events
 because of a typo or a bad check.

 Here's one example: https://wordpress.org/support/topic/error-in-
 addcustomcronschedule-degrades-site-performance/

 In the past, I've also seen plugins use `get_option` and all kinds of
 different checks, instead of `wp_next_scheduled`.

 Fixing the site afterwards can be a bit of a PITA, specially for high
 traffic ones. DB replication is often an issue when the `cron` option
 grows in size due to the bug.

 It's probably a good idea to prevent scheduling recurring events that have
 the same hook, schedule and args.

 Additional thoughts:
 - This might make the `wp_next_scheduled` check obsolete when queuing an
 event?
 - Changing the arguments of `wp_schedule_event` isn't something I wanted
 to do, but considering the current code for reschedule/unschedule in `wp-
 cron.php`, it seemed the best route (case of missing schedule, and trying
 to get interval from the event that we're rescheduling).
 - We're currently doing this with a `schedule_event` filter. Another route
 could be to add it to `default-filters`, but I think it should be in the
 Cron API itself.

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Ticket URL: <https://core.trac.wordpress.org/ticket/49693>
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