Fix cron double-enqueue because delay close to 0.01 and possibly clock-drift

[ccouton]

Fixes one problem in #1536

Case of clock drift and delay calculation result close to 0.001 so task is executed immediately which causes an unnecessary duplicate key value violates unique constraint-error even on single-process installations.

[ccouton]

@bensheldon thanks for your help.
If we have 0.02 it will be the same no? it will execute immediately after ?

[bensheldon]

@ccouton doublecheck my math, but I believe this line should prevent Concurrent Ruby from ever immediately running the task:

https://github.com/bensheldon/good_job/pull/1543/files#diff-66b4fe9e81a6b387c63fb9fbea11ffd29405422be569fee3e4e125e647b820e1R92-R94

[ccouton]

Yes but it will be scheludled 0.02s after so a risk to be before the next at and so at the same of the previous at

[bensheldon]

oh right! we do want to run it immediately if it is in fact in the past. I'll tweak that.

[ccouton]

Sorry i'm not clear enough, we have the case when clock drift cause an execution just before the real next scheduled at. Here i think this issue is not fixed. And maybe not easy to fix. We have ti execute it immeditaly if it's the first execution but if it's an other if cron_at is leather than current for me there is no reason to execute it?

[bensheldon]

@ccouton this should (theoretically, assuming I did it right) prevent the duplicate key value violates unique constraint error you shared in the previous example. The duplicate key value/column uses the scheduled/intended time (so on 60.0) rather than on the actual time (maybe 60.0000023). So if it is scheduled to run at 60.0 but it actually runs at 61.000035 and the next run is at 61.0 you will not get a duplicate key error.

I don't have a strategy for preventing the case where some delay causes a previously scheduled time to run at about the same time as the next scheduled time. I would say that if your schedule is at such a cadence that you frequently run into that (like every second), cron is maybe the wrong tool for that and it would be better to manage your own sleep loop.