SOLR-17720: Fix deadlock possibility in CollectionPropertiesZkStateReader

[aparnasuresh85]

https://issues.apache.org/jira/browse/SOLR-17720

Description

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Fix deadlock possibility in CollectionPropertiesZkStateReader

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The deadlock occurs due to conflicting lock acquisition orders between removeCollectionPropsWatcher() and refreshAndWatch():

removeCollectionPropsWatcher() first locks collectionPropsObservers via compute(collection, ), then waits on getCollectionLock(collection).

refreshAndWatch() first locks getCollectionLock(coll), then waits on collectionPropsObservers.remove(coll).

To resolve this, the fix moves the synchronized (getCollectionLock(collection)) block outside the compute() call in removeCollectionPropsWatcher(), ensuring a consistent lock order and preventing deadlock. While the overall locking logic could be simplified further, this change specifically addresses the deadlock scenario.

Tests

No new tests introduced. Ensure existing tests pass.

Checklist

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• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[dsmiley]

@aparnasuresh85 , looks like you accidentally added a temp file.

Otherwise, this looks good! Straight-forward fix/PR. Just needs a CHANGES.txt file in bug fix section for 9.9

[dsmiley]

Once those things are done, I'll merge!