clintropolis opened a new pull request, #14130:
URL: https://github.com/apache/druid/pull/14130
### Description
Through a bit of luck, stumbled into a possible bug with broker parallel
merging on certain java versions which can result in all worker threads
becoming completely blocked effectively blocking queries.
I have adjusted the `ParallelMergeCombiningSequenceTest` to check that the
pool becomes idle after a short period whenever an exception occurs. The adjust
tests fail pretty consistently for me with java 11 and 17, but can repeat
indefinitely on java 8 and java 20 on my laptop. Some of these timeout tests
are still ever so slightly racy since the exception message can vary slightly
depending on which part in the code triggers the timeout exception, but the
true important part is that previously missing a check that the pool becomes
idle.
I still haven't determined why this happens, perhaps a java bug or at least
some issue with how we are trying to configure/use the pool on these versions.
#### Release note
Broker parallel merge is now disabled for Java versions greater than 8 and
less than 20 due to a bug with how this mechanism operates on these versions
that can result in all pool threads becoming blocked effectively halting the
ability to process most queries. Java 8 and Java 20 do not appear to exhibit
these symptoms and so still have parallel result merging enabled by default.
<hr>
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- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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