[wp-hackers] Dealing with WP cron's raciness

[Brian Hogg]

I'm guessing you're trying to press the limits of the lock with the example, but if you disable automatic wp cron (with a flag in wp-config.php, DISABLE_WP_CRON define I believe) you would be able to control how many times wp cron was fired under normal conditions, for example once per minute in a system cron job you create.  Unless for some reason you need to fire it more often?

On Sep 18, 2013, at 20:07, David Anderson <david at wordshell.net> wrote:

>> Why are you hitting the wp-cron.php file directly? WordPress doesn't
>> do that normally.
>> 
>> WordPress normally sets the doing_wp_cron transient with the current
>> time(), then calls wp-cron.php?doing_wp_cron=time. This is a locking
>> mechanism to prevent the subsequent calls after that from running.
>> 
>> This is not *perfect*, but it is pretty good and prevents most of the
>> problem you're describing. If you're calling wp-cron.php directly,
>> you're bypassing this mechanism, and the delay you've added to the
>> database will cause the locking mechanism to take longer, causing the
>> race issues.
> Hi Otto,
> 
> Thanks... I was aware of that; the reason I was hitting wp-cron.php directly in the tests I mentioned in my early mail was for two reasons: 1) On my reading of the code, wp-cron.php tries to apply the same lock (the section beginning with the comment "// Use global $doing_wp_cron lock otherwise use the GET lock. If no lock, trying grabbing a new lock."). 2) In my tests, going indirectly (hitting the site's home page) had the same effect - i.e. the action would still consistently run multiple times due to raciness. It was just slightly less racy, but I surmised that that was due to a side-effect of timing due to the extra HTTP round-trip, rather than due to any extra locking taking place (which I surmised because of 1) - I can't see any extra locking take place - only a difference as to the timing of *when* the locking takes place).
> 
> David
> 
> -- 
> WordShell - WordPress fast from the CLI - www.wordshell.net
> 
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