Add status codes and hooks to wp-cron.php

[westonruter]

I've been debugging a site problem which turned out to be one of the cron actions resulting in a memory limit breach, and so PHP did a fatal error in the middle of the cron loop. (It's a high-traffic site so it has DISABLE_WP_CRON with system cron pinging wp-cron.php every minute.) There is currently very little debugging hooks provided in wp-cron.php to help diagnose why and where a scheduled event hook is failing.

Some ideas for how wp-cron.php could be improved to facilitate debugging (and also to help with general logging):

• Return a status message pinpointing at which point wp-cron.php exited
• Return relevant HTTP status codes
• Allow die behavior to be overridden, similarly to how wp_die() is used elsewhere
• Add hooks before before and after each level in the triple-nested crons loop.

[westonruter]

Echo status codes from wp-cron.php. Use _ajax_wp_die_handler for DOING_CRON too. Introduce filterable wp_cron_exit_handler with HTTP status codes. Add hooks for cron loops. Commits:https://github.com/x-team/wordpress-develop/compare/master...d6450f9 PR: ​https://github.com/x-team/wordpress-develop/pull/21

[westonruter]

Fix order of operations in request validity check

[jrtashjian]

The patch has been refreshed against the latest trunk. I've also went through and documented the various parts that had @todo docs in the comments.

I recently submitted a new patch for #18471 but I believe this same ticket is related and accomplishes the task better.

[peterwilsoncc]

This is worth some investigation so I'm reopening this and putting it down for a future release. The Cron component has a bit of a backlog, so I won't give a timeframe for now as it would set a false expectation.