kfaraz opened a new pull request, #18824:
URL: https://github.com/apache/druid/pull/18824
### Description
Based on discussions with @jtuglu1 in #18764 , there seems to be a race in
segment balancing between the duty run thread and the segment drop callback
thread (triggered by segment balancing).
The result of the race condition is that just when the segment on an
`HttpLoadQueuePeon` is being moved from the state `MOVING_FROM` to `DROPPING`,
a new duty run might take a snapshot of this peon, thereby assuming that this
segment actually has state `LOADED`. This leads the `StrategicSegmentAssigner`
to assume that the segment is over-replicated (loaded on both A and B) and then
trigger a drop from either A (no-op since we were going to start a drop from A
anyway) or B (segment becomes briefly unavailable).
It is possible that this fix does not fully address the issue in #18764 ,
but it would be still be useful to have this thread safety.
### Changes
- Use a synchronized block to ensure that a segment is moved from the set
`segmentsMarkedToDrop` to `segmentsToDrop` atomically
- Use `@GuardedBy` to ensure that these fields remain thread-safe
<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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