[Segment Replication] Fix bug where replica shows stale doc count during engine reset.

[mch2]

Description

This change fixes an issue where replica shards can temporarily return stale results during promotion to primary. This happens in resetEngineToGlobalCheckpoint because the shard temporarily flips to a ReadOnly engine and initializes its reader from disk. This is possible because NRTReplicationEngine did not previously implement flush and the freshest data is only active on the reader. Fixed by implementing flush and also honoring acquireLatestCommit's flushFirst parameter.

Related Issues

related #8985

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/23247/
• CommitID: 8b73d3e
  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 (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/23251/
• CommitID: 8b73d3e

[codecov]

Codecov Report

Merging #9495 (3606e06) into main (569d5c2) will decrease coverage by 0.04%.
Report is 2 commits behind head on main.
The diff coverage is 65.71%.

@@             Coverage Diff              @@
##               main    #9495      +/-   ##
============================================
- Coverage     71.15%   71.11%   -0.04%     
+ Complexity    57536    57451      -85     
============================================
  Files          4781     4781              
  Lines        271197   271227      +30     
  Branches      39595    39599       +4     
============================================
- Hits         192975   192891      -84     
- Misses        62011    62119     +108     
- Partials      16211    16217       +6     

Files Changed	Coverage Δ
.../opensearch/telemetry/tracing/noop/NoopTracer.java	57.14% <0.00%> (-9.53%)	⬇️
...rg/opensearch/telemetry/tracing/WrappedTracer.java	78.57% <50.00%> (-6.05%)	⬇️
.../opensearch/index/engine/NRTReplicationEngine.java	77.14% <58.82%> (-2.24%)	⬇️
...elemetry/tracing/OTelTracingContextPropagator.java	73.33% <70.00%> (-1.67%)	⬇️
...rg/opensearch/telemetry/tracing/DefaultTracer.java	93.10% <100.00%> (+0.51%)	⬆️
...racing/ThreadContextBasedTracerContextStorage.java	74.19% <100.00%> (+0.86%)	⬆️

... and 503 files with indirect coverage changes

[opensearch-trigger-bot]

Compatibility status:

Checks if related components are compatible with change d1678ba

Incompatible components

Incompatible components: [https://github.com/opensearch-project/index-management.git, https://github.com/opensearch-project/asynchronous-search.git, https://github.com/opensearch-project/k-nn.git, https://github.com/opensearch-project/notifications.git, https://github.com/opensearch-project/security-analytics.git]

Skipped components

Compatible components

Compatible components: [https://github.com/opensearch-project/security.git, https://github.com/opensearch-project/alerting.git, https://github.com/opensearch-project/anomaly-detection.git, https://github.com/opensearch-project/sql.git, https://github.com/opensearch-project/job-scheduler.git, https://github.com/opensearch-project/observability.git, https://github.com/opensearch-project/common-utils.git, https://github.com/opensearch-project/reporting.git, https://github.com/opensearch-project/cross-cluster-replication.git, https://github.com/opensearch-project/geospatial.git, https://github.com/opensearch-project/performance-analyzer.git, https://github.com/opensearch-project/ml-commons.git, https://github.com/opensearch-project/performance-analyzer-rca.git, https://github.com/opensearch-project/neural-search.git, https://github.com/opensearch-project/opensearch-oci-object-storage.git]

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/23331/
• CommitID: fab021a

[dreamer-89]

because the shard temporarily flips to a ReadOnly engine and initializes its reader from disk.

Is it convenient to add unit test which mimics this state

[maosuhan]

@mch2 I have copied the MR to my local repo, it does solve my problem now. The search result on bumping replica will not produce stale data now.

https://github.com/maosuhan/OpenSearch/tree/fix_sr
org.opensearch.indices.replication.SegmentReplicationPrimaryPromotionIT

[mch2]

because the shard temporarily flips to a ReadOnly engine and initializes its reader from disk.

Is it convenient to add unit test which mimics this state

added a better test to SegmentReplicationIndexShardTests

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/23653/
• CommitID: 3608f50
  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?

[mch2]

pushed a rebase - test failures look unrelated.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/23658/
• CommitID: 3606e06

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-9495-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 012c4fa4ec8b719cecd4e163c5fdc0e4f42679d3
# Push it to GitHub
git push --set-upstream origin backport/backport-9495-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-9495-to-2.x.