SAI-5884: hook lucene Unloader into Solr container lifecycle

[magibney]

Note

Integrates Lucene Unloader across CoreContainer and Directory lifecycle with a dedicated executor, metrics, coordination points, and adds test/gradle options to control unloading.

• Core/Container:
  • Add ScheduledExecutorService (unloaderExecutor) in CoreContainer with lifecycle management and accessor.
  • New org.apache.solr.core.UnloadHelper supplying Lucene Unloader.UnloadHelper, tracking metrics, and reacting to cluster props.
• Directory/Storage:
  • CachingDirectoryFactory: wire UnloaderCoordinationPoint.setUnloadHelperSupplier(...); cache CoreContainer weak ref; initialize unload helper from object cache.
  • TeeDirectory: implement UnloaderCoordinationPoint; propagate unload helper to access and persistent directories.
  • TeeDirectoryFactory: tolerate null CoreContainer; keep weak ref volatile; initialize node-level state via ObjectCache.
• Metrics:
  • Extend MetricUtils to support custom histogram/timer snapshot writing via SnapshotWriter.
• Build/Tests:
  • Add Gradle test options: lucene.unload.ttl, lucene.unload.initial, lucene.unload.disable, lucene.unload.trackReloads.
  • CI: pass unload params in Solr test workflow.

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Bug: Volatile field read twice causing potential race condition

The code reads the volatile field dynamicMinSegSize into a local variable dynamic but then reads dynamicMinSegSize again instead of using the local variable. Since dynamicMinSegSize is volatile and can be modified by another thread, this creates a race condition where the threshold check could use a different value than was tested in the conditional. The assignment should be threshold = dynamic to use the cached value.

 

[cursor]

Bug: Cluster properties listener incorrectly removed after first call

The second registerClusterPropertiesListener for unloadMinSegSize returns true at line 148, but according to the ClusterPropertiesListener interface contract, returning true means "the listener should be removed". This causes the listener to unregister itself after the first cluster properties change notification, preventing it from responding to future changes to unloadMinSegSize. The first listener for minUnloadTime correctly returns false to remain active.