[9.0] Prevent work starvation bug if using scaling EsThreadPoolExecutor with core pool size = 0 (#124732) #124965
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…h core pool size = 0 (elastic#124732) When `ExecutorScalingQueue` rejects work to make the worker pool scale up while already being at max pool size (and a new worker consequently cannot be added), available workers might timeout just about at the same time as the task is then force queued by `ForceQueuePolicy`. This has caused starvation of work as observed for `masterService#updateTask` in elastic#124667 where max pool size 1 is used. This configuration is most likely to expose the bug. This PR changes `EsExecutors.newScaling` to not use `ExecutorScalingQueue` if max pool size is 1 (and core pool size is 0). A regular `LinkedTransferQueue` works perfectly fine in this case. If max pool size > 1, a probing approach is used to ensure the worker pool is adequately scaled to at least 1 worker after force queueing work in `ForceQueuePolicy`. Fixes elastic#124667 Relates to elastic#18613
mosche
added
>bug
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Backports the following commits to 9.0: