Fix flaky PruneFileCacheIT.testPruneCacheWithRealData

[andrross]

I found a deterministic repro with seed B289DF27E8BA9F8. The issue is that all data for the index was still being referenced by the cache so nothing was prunable and the test failed. The behavior of the cache is that cloned IndexInputs can remained referenced until they are GC'd. The fix is to attempt to trigger GC in the polling loop until data is prunable.

Related Issues

Resolves #19724

Check List

• Functionality includes testing.

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📝 Walkthrough

Walkthrough

A test file was refactored to stabilize flakiness by introducing a CacheUsage data carrier to separately track total used and active (prunable) cache bytes. All cache usage assertions and validations were updated to use the new composite structure, with added GC-aware checks to verify used and active state relationships.

Changes

Cohort / File(s)	Summary
File Cache Pruning Test Refactor server/src/internalClusterTest/java/org/opensearch/action/admin/cluster/filecache/PruneFileCacheIT.java	Replaced single-value cache usage tracking with composite CacheUsage object (used, active fields). Updated assertions and validations to distinguish between total used bytes and prunable active bytes. Added GC-aware busy-wait checks verifying used > 0 and active < used. Switched to Hamcrest matchers (lessThan, greaterThan) and adjusted test logging terminology.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested labels

flaky-test, >test-failure, autocut

🚥 Pre-merge checks | ✅ 4 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Merge Conflict Detection	⚠️ Warning	❌ Merge conflicts detected (13 files): ⚔️ client/rest/build.gradle (content) ⚔️ plugins/arrow-flight-rpc/src/main/java/org/opensearch/arrow/flight/bootstrap/ServerConfig.java (content) ⚔️ plugins/arrow-flight-rpc/src/main/java/org/opensearch/arrow/flight/transport/FlightStreamPlugin.java (content) ⚔️ plugins/arrow-flight-rpc/src/main/java/org/opensearch/arrow/flight/transport/FlightTransport.java (content) ⚔️ plugins/arrow-flight-rpc/src/test/java/org/opensearch/arrow/flight/bootstrap/ServerConfigTests.java (content) ⚔️ plugins/repository-hdfs/build.gradle (content) ⚔️ plugins/repository-s3/build.gradle (content) ⚔️ qa/wildfly/build.gradle (content) ⚔️ server/src/internalClusterTest/java/org/opensearch/action/admin/cluster/filecache/PruneFileCacheIT.java (content) ⚔️ server/src/main/java/org/opensearch/common/settings/FeatureFlagSettings.java (content) ⚔️ server/src/main/java/org/opensearch/common/util/FeatureFlags.java (content) ⚔️ server/src/main/java/org/opensearch/index/shard/IndexShard.java (content) ⚔️ server/src/main/java/org/opensearch/node/Node.java (content) These conflicts must be resolved before merging into main.	Resolve conflicts locally and push changes to this branch.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically identifies the test being fixed (PruneFileCacheIT.testPruneCacheWithRealData) and indicates the nature of the change (fixing flakiness).
Description check	✅ Passed	The description includes root cause analysis, the specific seed that reproduced the issue, the fix approach, and the linked issue number, though some template sections could be more detailed.
Linked Issues check	✅ Passed	The PR addresses issue #19724 by implementing a stabilization fix: introducing GC triggering in the polling loop to ensure data becomes prunable, directly resolving the root cause of the test flakiness.
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing the flaky test through cache usage tracking refactoring and GC triggering logic, with no unrelated alterations present.

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[github-actions]

❌ Gradle check result for 701a87f: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❕ Gradle check result for 986543b: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.29%. Comparing base (d7eb5e7) to head (986543b).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #20617      +/-   ##
============================================
+ Coverage     73.22%   73.29%   +0.06%     
- Complexity    72129    72172      +43     
============================================
  Files          5798     5798              
  Lines        329832   329832              
  Branches      47545    47545              
============================================
+ Hits         241522   241750     +228     
+ Misses        68998    68750     -248     
- Partials      19312    19332      +20     

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