clintropolis opened a new pull request, #13608:
URL: https://github.com/apache/druid/pull/13608
### Description
This PR fixes an issue which can occur when the broker tries to emit metrics
for parallel merge on an empty result set over jdbc. There might be other ways
to trigger this as well, but this was the most reliable one.
The issue happens with an empty result set because it causes the
`Yielders.each` call to close the sequence on the main thread instead of the
query executor thread, which owns the metrics.
The fix is to create the yielder on the query executor thread as well, so
that if the sequence is consumed, any baggages that might be emitting metrics,
such as broker parallel merge metrics, will not be called in the wrong thread.
<hr>
This PR has:
- [ ] been self-reviewed.
- [ ] using the [concurrency
checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md)
(Remove this item if the PR doesn't have any relation to concurrency.)
- [ ] added documentation for new or modified features or behaviors.
- [ ] a release note entry in the PR description.
- [ ] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [ ] added or updated version, license, or notice information in
[licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md)
- [ ] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [ ] added unit tests or modified existing tests to cover new code paths,
ensuring the threshold for [code
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
is met.
- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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