Ensure timestamp is integer when scheduling single events

[peterwilsoncc]

Validate the timestamp passed to wp_schedule_single_event to ensure it is an integer and occurs in the future.

Related: #33423

Existing in the past is also valid I believe, cron simply processes any events <= time().

I think the rule should be is_numeric( $timestamp ) && $timestamp > 0

Patch #33475

Thanks for the patch @utkarshpatel.

I am in two minds as to whether it is worth a doing_it_wrong call if the timestamp is not numeric. Reviewers, any thoughts?

@utkarshpatel for 33475.patch​ I'd probably have kept the two checks distinct from each other (valid timestamps; and already scheduled) for the simple desire of readability (Also, please always add a comment after you upload a patch, Trac doesn't send out notifications when you only upload a patch)

I don't really see a huge benefit for a _doing_it_wrong() here, developers will find out that their events are not running pretty quickly while testing (I would hope), this just prevents the cron array filling with things that won't ever be executed.

Added comment

Hey Hulse,

As you pointed out comment and new conditionI have added new patch. Please check and let me know.

33475.diff​ refreshes from project root

Seems like other functions would benefit from the same check for consistency, see 33475.2.diff​.

In 33936:

Cron: Reject events when the provided $timestamp is not a valid timestamp.

Invalid timestamps were previously accepted by the scheduling functions but would never be run due to our implementation which caused the cron option to forever contain the events.
This rejects such events which most likely only occur due to developer error.

Props utkarshpatel, wonderboymusic, SergeyBiryukov.
See #33423, Fixes #33475

In 34044:

Document @return value for wp_unschedule_event().

See #33475.