Poormanscron called multiple times simultaneously [#369752]

Simplenews users using poormanscron to send newsletters experience problems with multiple emails send to a single subscriber. This seems to be caused by poormanscron being triggered by different visitors opening a page within the time the cron job runs. Can you confirm that this situation can occur with poormanscron? In Simplenews I can reduce the damage, but my advise to simplenews users is not to use poormanscron. Is this something that can be solved in poormanscron?

Comments

Comment #1

gpk commented 12 April 2009 at 13:30

Yes if you look through this issue queue you will see that this is a known problem. cron actually uses a special flag (cron_semaphore) to try to prevent multiple simultaneous cron runs. The problem is a little more subtle than you describe: If visitor B makes a page request while a page request is running for visitor A, and if visitor A triggers the cron run (the check is made at the end of the page request processing), then although visitor A will cause the cron_semaphore flag to be set, visitor B will not see this (since the static variables are all instantiated right at the start of the page processing). Hence visitor B will also trigger the cron run.

Note that the only version of poormanscron to use the cron_semaphore flag is 6.x-1.x-dev; however similar problems occur in the other versions. I think there may also be a problem with the arithmetic in poormanscron_exit() which exacerbates the problem in certain situations.

In principle it can be fixed, if anyone has the time/inclination. Actually IIRC a fix is described in detail in one of the old issues here (probably 2 years old or more).