[wp-trac] [WordPress Trac] #64315: Running `_wp_cron()` on shutdown breaks `ALTERNATE_WP_CRON`

[WordPress Trac]

#64315: Running `_wp_cron()` on shutdown breaks `ALTERNATE_WP_CRON`
--------------------------+-----------------------------
 Reporter:  karl94        |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Cron API      |    Version:  trunk
 Severity:  major         |   Keywords:
  Focuses:                |
--------------------------+-----------------------------
 In #63858, [60925] moved cron spawing from the `wp_loaded` hook to
 `shutdown`.
 While this is fine for usual cron spawning via `wp_remote_post()`, the
 alternative cron behavior doesn't work properly after this change.

 `ALTERNATE_WP_CRON` is an alternative cron spawning method: it's meant to
 be used in environments where HTTP loopback request cannot be used and
 works by redirecting the client to the same URL with just an extra
 `doing_wp_cron` query param. This way, on the first request (the one that
 outputs the redirect) crons are executed "in background" while the client
 follows the redirect and loads the URL requested.

 The issue is that by moving `_wp_cron()` execution to shutdown the
 redirect can't be performed as output already started (and triggers the
 ''Cannot modify header information - headers already sent'' warning).

 https://github.com/WordPress/wordpress-
 develop/blob/217d39aa50c5e15584a921c3e58eb104f0d96eed/src/wp-
 includes/cron.php#L918

 Furthermore, `wp-cron.php` [https://github.com/WordPress/wordpress-
 develop/blob/217d39aa50c5e15584a921c3e58eb104f0d96eed/src/wp-cron.php#L205
 may call] `die()`, which would then terminate execution preventing other
 callbacks attached to the `shutdown` hook to run.

 I think that on `ALTERNATE_WP_CRON` the previous behavior should be
 restored: the  `wp_loaded` hook should be used instead of `shutdown`.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/64315>
WordPress Trac <https://core.trac.wordpress.org/>
WordPress publishing platform