Fix unique scheduling issues #619
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When deciding whether or not to discard a retryable job due to a unique conflict, don't consider the job that we're trying to schedule. It obviously cannot conflict with itself, only other rows.
If two unique jobs happen to be getting scheduled in the same batch, only one of them can successfully be moved to `available`—the other must be discarded. Pick the one that's first in line.
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PR #590 introduced a more flexible unique job mechanism that allowed for the unique states to be customized, all while still benefiting from the performance improvements of using a unique index rather than an advisory lock. The
retryablestate is included in the default list of states, but can be removed if the user doesn't want to prevent an erroring job from blocking duplicate inserts.However this created an issue: when trying to schedule a
retryablejob (move it toavailable) it could potentially have a conflict with a duplicate unique job. To handle this, special logic was added to try to deal with this scenario for unique jobs, moving the conflicting row todiscardedrather thanavailable. Unfortunately this logic had issues and was insufficiently tested. There were a couple specific scenarios that caused issues:scheduledor had errored and becomeretryablewould actually be considered a conflict with itself because the query didn't properly exclude the row being scheduled.retryableunique jobs at the same time would lead to a unique conflict because there was no mechanism to prevent this.The query changes in this PR address both of the above cases along with test coverage.
The increased complexity is unfortunate, and we're probably nearing the limit of what should be dealt with in a single SQL query. If this still isn't complete I'm more inclined to fix the issues by catching these conflicts at the application level, explicitly moving the conflicting row(s) to
discarded, and trying again. This can be looped with a backoff or recursed to ensure that progress keeps being made as individual conflicts get resolved. But hopefully that won't be necessary.Fixes #618.