[6.8] Prevent deadlock by using separate schedulers (#48697) #48963
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Currently the BulkProcessor class uses a single scheduler to schedule flushes and retries. Functionally these are very different concerns but can result in a dead lock. Specifically, the single shared scheduler can kick off a flush task, which only finishes it's task when the bulk that is being flushed finishes. If (for what ever reason), any items in that bulk fails it will (by default) schedule a retry. However, that retry will never run it's task, since the flush task is consuming the 1 and only thread available from the shared scheduler. Since the BulkProcessor is mostly client based code, the client can provide their own scheduler. As-is the scheduler would require at minimum 2 worker threads to avoid the potential deadlock. Since the number of threads is a configuration option in the scheduler, the code can not enforce this 2 worker rule until runtime. For this reason this commit splits the single task scheduler into 2 schedulers. This eliminates the potential for the flush task to block the retry task and removes this deadlock scenario. This commit also deprecates the Java APIs that presume a single scheduler, and updates any internal code to no longer use those APIs. Fixes elastic#47599 Note - elastic#41451 fixed the general case where a bulk fails and is retried that can result in a deadlock. This fix should address that case as well as the case when a bulk failure *from the flush* needs to be retried.
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Backports the following commits to 6.8: