[wp-hackers] WP cron and "multi-threading"

[Ryan Boren]

Owen Winkler wrote:
> I've used wp_schedule_event() to register a cron event during the 
> activation of my plugin, and I have a couple of issues.
> 
> #1: The cron event seems to leave remnants of non-existent args in the 
> serialized options array.  Every time it runs it adds an array wrapper 
> to the existing zero-length array of arguments.  I don't know if this 
> patch (http://trac.wordpress.org/ticket/3169) will fix it.  I'm not sure 
> if it's causing any issues with the plugin itself since I'm not using 
> the parameters, but I don't think it will be a problem, at least until 
> the size of the data exceeds the field length.

I committed 3169.

> #2: I need the cron to run *exactly once* at each interval.  Assuming 
> that traffic on the site is buzzing along, the current cron 
> implementation could allow events to be processed more than once, 
> depending on how long it takes for the hooked function to run.  In my 
> case, the computation in the cron hook usually takes 2 seconds or more, 
> leaving plenty of room for another incoming hit to trigger the same event.
> 
> I've implemented a flock() check in my own code, something I'd rather 
> not do.  It might be beneficial to rearrange the cron code so that it 
> unschedules the event before it executes it.
> 
> Any other clever suggestions as to how to keep the cron from executing 
> more than once on a single registered instance would be helpful.

I have nothing clever to offer. Even unscheduling beforehand leaves a 
window of contention, although the window is narrowed.

Ryan