Daily (and other) scheduled jobs slip forward each day until they're eventually missed
If this subject line looks familiar, there's a reason.
This issue was originally raised and fixed in April 2015. However, a code simplification in December 2015 removed the fix.
Many of us are familiar with this issue. Cron runs at 10:00:00, scheduled jobs are executed in order, and one job might not fire until 10:00:03. If the job fires hourly, we would expect the job to fire at 11:00:00, but since an hour hasn't elapsed since 10:00:00, the job doesn't fire until the next cron, maybe at 11:15:00. This has repercussions for a number of scheduled job use cases.
After some thought, I realized the correct solution is to record the time the cron job fires, not the time the job actually is started. I believe that when the last_run field was created, there was no sense that these times might be different, hence the bug.
I wrote a patch (I'm leaving it in production for a few days before submitting a PR) that does two things:
-
last_runnow refers to the time the cron job started, not the scheduled job. - To avoid race-y conditions on very slow shared hosts, I force the
last_runtime to always have "00" as its seconds. This is a belt-and-suspenders (or belt-and-braces in non-'Merican) approach that reflects that a) shared hosting could be slow enough that this would otherwise be a problem, b) cron doesn't allow scheduled at the seconds level, so this shouldn't impact anyone's scheduling.