Fix toBuilder method in EngineConfig to include mergedSegmentTransfer…

[udabhas]

Description

Recently a change was introduced to Include MergedSegmentTransferTracker in EngineConfig, but the builder method was not updated, leading to consumers of builder method returning null when getMergedSegmentTransferTracker() is invoked. This is leading to _cat/indices api not able to fill details properly.

Currently the builder method is used by opensearch-storage-encryption plugin.

Error earlier in _cat/indices call -

2025-11-26T09:05:39,618][DEBUG][o.o.a.a.i.s.TransportIndicesStatsAction][58cb8e866462db649c30b67b616dc2f5] #[java.lang.NullPointerException]#[indices:monitor/stats] failed to execute operation for shard [[enc][0], node[ggkokXB2TOewTaL-6gu37w], [P], s[STARTED], a[id=Z_jU9sA9RwSmskOJY4_aOA]]
java.lang.NullPointerException: Cannot invoke "org.opensearch.index.merge.MergedSegmentTransferTracker.stats()" because "this.mergedSegmentTransferTracker" is null
        at org.opensearch.index.engine.OpenSearchConcurrentMergeScheduler.stats(OpenSearchConcurrentMergeScheduler.java:220)
        at org.opensearch.index.engine.InternalEngine.getMergeStats(InternalEngine.java:2635)
        at org.opensearch.index.shard.IndexShard.mergeStats(IndexShard.java:1660)
        at org.opensearch.action.admin.indices.stats.CommonStats.<init>(CommonStats.java:209)        at org.opensearch.action.admin.indices.stats.TransportIndicesStatsAction.shardOperation(TransportIndicesStatsAction.java:141)        at org.opensearch.action.admin.indices.stats.TransportIndicesStatsAction.shardOperation(TransportIndicesStatsAction.java:67)        at org.opensearch.action.support.broadcast.node.TransportBroadcastByNodeAction$BroadcastByNodeTransportRequestHandler.onShardOperation(TransportBroadcastByNodeAction.java:504)        at org.opensearch.action.support.broadcast.node.TransportBroadcastByNodeAction$BroadcastByNodeTransportRequestHandler.messageReceived(TransportBroadcastByNodeAction.java:476)        at org.opensearch.action.support.broadcast.node.TransportBroadcastByNodeAction$BroadcastByNodeTransportRequestHandler.messageReceived(TransportBroadcastByNodeAction.java:463)        at com.amazonaws.elasticsearch.ccs.CrossClusterRequestInterceptor$CrossClusterRequestHandler.messageReceived(CrossClusterRequestInterceptor.java:155)
        at org.opensearch.indexmanagement.rollup.interceptor.RollupInterceptor$interceptHandler$1.messageReceived(RollupInterceptor.kt:118)
        at org.opensearch.performanceanalyzer.transport.PerformanceAnalyzerTransportRequestHandler.messageReceived(PerformanceAnalyzerTransportRequestHandler.java:44)
        at org.opensearch.performanceanalyzer.transport.RTFPerformanceAnalyzerTransportRequestHandler.messageReceived(RTFPerformanceAnalyzerTransportRequestHandler.java:92)
        at com.amazonaws.elasticsearch.iam.IamTransportRequestHandler.messageReceived(IamTransportRequestHandler.java:69)
        at org.opensearch.wlm.WorkloadManagementTransportInterceptor$RequestHandler.messageReceived(WorkloadManagementTransportInterceptor.java:63)
        at org.opensearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:108)
        at org.opensearch.transport.TransportService$7.doRun(TransportService.java:1140)
        at org.opensearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:984)
        at org.opensearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:52)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
        at java.base/java.lang.Thread.run(Thread.java:1583)

Response _cat/indices

% curl localhost:9200/_cat/indices?v
health status index                          uuid                   pri rep docs.count docs.deleted store.size pri.store.size
green  open   .plugins-ml-config             ZLIVDoVOSZmyf3AFz5ONOQ   1   0          1            0        4kb            4kb
green  open   .plugins-ml-jobs               zAzD0FCCRlGEvTorgIldiA   1   0          1            0     12.3kb         12.3kb
green  open   enc2                           aQVN34HfQBOE_0jBkchvOw   1   0
green  open   .opendistro-job-scheduler-lock RQf2uCR1Toa4DvdH_cosog   1   0          1            0       12kb           12kb
green  open   enc                            aRRST582RVOdcRbURp891A   1   0

After adding missing MergedSegmentTransferTracker, _cat/indices working as expected

% curl localhost:9200/_cat/indices?v
health status index                          uuid                   pri rep docs.count docs.deleted store.size pri.store.size
green  open   .plugins-ml-config             ZLIVDoVOSZmyf3AFz5ONOQ   1   0          1            0        4kb            4kb
green  open   .plugins-ml-jobs               zAzD0FCCRlGEvTorgIldiA   1   0          1            0     12.3kb         12.3kb
green  open   enc2                           aQVN34HfQBOE_0jBkchvOw   1   0          0            0       208b           208b
green  open   .opendistro-job-scheduler-lock RQf2uCR1Toa4DvdH_cosog   1   0          1            0      4.9kb          4.9kb
green  open   enc                            aRRST582RVOdcRbURp891A   1   0          0            0       208b           208b

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

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

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Summary by CodeRabbit

• Bug Fixes
  • Fixed an issue where engine configuration objects were not properly preserving all internal state when cloned, ensuring complete configuration transfers.

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[coderabbitai]

Walkthrough

The pull request adds support for the mergedSegmentTransferTracker field to EngineConfig.Builder by introducing a new builder method and updating the toBuilder() method to propagate this configuration when cloning an EngineConfig instance. The changelog documents this fix.

Changes

Cohort / File(s)	Summary
Documentation CHANGELOG.md	Added changelog entry under Fixed (Unreleased 3.x) documenting that EngineConfig.toBuilder() now includes mergedSegmentTransferTracker
Builder Enhancement server/src/main/java/org/opensearch/index/engine/EngineConfig.java	Added public builder method mergedSegmentTransferTracker(MergedSegmentTransferTracker) to EngineConfig.Builder; updated toBuilder() to propagate the mergedSegmentTransferTracker field in the builder chain

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

• The changes follow an established builder pattern already present in the codebase
• Single new method addition with straightforward field propagation
• Verify that mergedSegmentTransferTracker is properly initialized and handled throughout the builder lifecycle
• Ensure changelog entry accurately reflects the fix and matches code changes

Poem

🐰 A tracker field, once lost in the clone,
Now finds its way through the builder's tone.
From old config to new it shall flow,
With toBuilder's grace, the transfer will go! 🎉

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing the toBuilder method in EngineConfig to include mergedSegmentTransferTracker.
Description check	✅ Passed	The description provides comprehensive context, including the root cause from PR #18929, the bug impact, stack traces, before/after evidence, and affected plugins.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 1

📜 Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 468f944 and 65327b2.

📒 Files selected for processing (2)

• CHANGELOG.md (1 hunks)
• server/src/main/java/org/opensearch/index/engine/EngineConfig.java (1 hunks)

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🔇 Additional comments (2)

server/src/main/java/org/opensearch/index/engine/EngineConfig.java (2)

357-358: Excellent fix for the toBuilder() completeness.

The addition of both clusterApplierService and mergedSegmentTransferTracker to the toBuilder() method correctly addresses the NPE issue described in the PR. Both fields were missing from the builder chain, which would cause getMergedSegmentTransferTracker() and getClusterApplierService() to return null on configs created via toBuilder().

The fix ensures that plugins like opensearch-storage-encryption that rely on the builder pattern will receive fully populated configuration objects.

---

826-829: Builder method follows the established pattern.

The mergedSegmentTransferTracker() builder method correctly follows the standard builder pattern used throughout this class, enabling method chaining and proper field assignment.

[github-actions]

✅ Gradle check result for 65327b2: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.33%. Comparing base (97d3864) to head (65327b2).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...java/org/opensearch/index/engine/EngineConfig.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #20105      +/-   ##
============================================
- Coverage     73.33%   73.33%   -0.01%     
- Complexity    71679    71708      +29     
============================================
  Files          5790     5790              
  Lines        327549   327666     +117     
  Branches      47181    47205      +24     
============================================
+ Hits         240217   240279      +62     
- Misses        68080    68123      +43     
- Partials      19252    19264      +12     

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