[Tiered Caching] [Bug Fix] Use concurrentMap instead of HashMap to fix Concurrent Modification Exception

[kiranprakash154]

Description

#14032 reported msearches failing with a Concurrent Modification Exception, though they were not able to repro it consistently they were able to attach a debugger and find out the line throwing the exception, which was

OpenSearch/server/src/main/java/org/opensearch/indices/IndicesRequestCache.java

Line 571 in 5b93f2e

cleanupKeyToCountMap.computeIfAbsent(shardId, k -> new HashMap<>()).merge(cleanupKey.readerCacheKeyId, 1, Integer::sum);

We are using a HashMap (not thread safe) for the inner map of the cleanupKeyToCountMap and hence it throws a Concurrent Modification Exception when the map is getting updated by multiple threads concurrently.

The fix is to use a thread safe Concurrent Map instead.

Related Issues

Resolves #14032

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 5a4484d: 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 a3b7625: SUCCESS

[andrross]

Approving as we can follow up on my comments later if there's a rush to merge this. Letting @sohami have a final look.

[sohami]

@kiranprakash154 Discussed with @andrross. Merging the change for 2.15, but we will need follow-up to address the comments here

[kiranprakash154]

@kiranprakash154 Discussed with @andrross. Merging the change for 2.15, but we will need follow-up to address the comments here

Yep, I'd love that. Thanks for helping this get through