kfaraz opened a new pull request, #18162:
URL: https://github.com/apache/druid/pull/18162
### Description
Addresses the race condition identified in #18161
Kudos to @jtuglu-netflix for identifying it!
### Race condition
- Segment is moving from server A to server B
- If it has just finished its MOVE_TO operation on B but the MOVE_FROM
operation on A is still pending,
the segment may be chosen to drop from server B causing under-replication
- If it is chosen to drop from server A, that operation would be ignored
since the segment already has an ongoing
operation on server A
### Fix
- Use the delta of MOVE_FROM - MOVE_TO to track pending drop callbacks
- Add a simulation test to verify the change. This test fails before the fix.
<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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