AmatyaAvadhanula opened a new pull request, #14995: URL: https://github.com/apache/druid/pull/14995
<!-- Thanks for trying to help us make Apache Druid be the best it can be! Please fill out as much of the following information as is possible (where relevant, and remove it when irrelevant) to help make the intention and scope of this PR clear in order to ease review. --> <!-- Please read the doc for contribution (https://github.com/apache/druid/blob/master/CONTRIBUTING.md) before making this PR. Also, once you open a PR, please _avoid using force pushes and rebasing_ since these make it difficult for reviewers to see what you've changed in response to their reviews. See [the 'If your pull request shows conflicts with master' section](https://github.com/apache/druid/blob/master/CONTRIBUTING.md#if-your-pull-request-shows-conflicts-with-master) for more details. --> ### Description Fixes an issue where a race can cause a second attempt to publish segments for the same sequence. During the second publish, the segments have already been published and are no longer associated with the sequence. This causes the tasks to fail with an error like: `org.apache.druid.java.util.common.ISE: Attempting to publish with empty segment set, but total row count was not 0: [165263].` ### Observations The following sequence of events can cause the error 1. Submit a publish request for all segments associated with sequence id x 2. Iterate over the same sequence x in `maybePersistAndPublishSequences` 3. The publish request in step 1 succeeds, and its callback to clear segments and other states associataed with sequence x. 4. Since sequence x is no longer in the set of publishing segments, it is considered for publish again. 5. No segments are associated with sequence x, and new data has been appended for sequence x + 1. 6. Step 4 fails as the number of segment is 0, but the appenderator has non-zero rows, causing the above error. ### Fix When iterating over the sequences to check if they need to published, maintain a snapshot of the publishing sequences before iterating, so that a removal on successful publish doesn't cause the race to occur. Tested on a local cluster by setting thread level breakpoints. <!-- In each section, please describe design decisions made, including: - Choice of algorithms - Behavioral aspects. What configuration values are acceptable? 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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) - [x] 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. - [x] been tested in a test Druid cluster. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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