[SPARK-51596][SS] Fix concurrent StateStoreProvider maintenance and closing #50391
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What changes were proposed in this pull request?
Moves the unload operation away from task thread into the maintenance thread. To ensure unloading still occurs ASAP (rather than potentially waiting for the maintenance interval) as was introduced by https://issues.apache.org/jira/browse/SPARK-33827, we immediately trigger a maintenance thread to do the unload.
This gives us an extra benefit that unloading other providers doesn't block the task thread. To capitalize on this, unload() should not hold the loadedProviders lock the entire time (which will block other task threads), but instead release it once it has deleted the unloading providers from the map and close the providers without the lock held.
Why are the changes needed?
Currently, both the task thread and maintenance thread can call unload() on a provider. This leads to a race condition where the maintenance could be conducting maintenance while the task thread is closing the provider, leading to unexpected behavior.
Does this PR introduce any user-facing change?
No
How was this patch tested?
Added unit test
Was this patch authored or co-authored using generative AI tooling?
No