Rollover: scope checkBlock to write index only, skipping non-write alias members

[mishail]

Disclaimer: parts of this PR were generated by LLM

Description

The rollover precheck in TransportRolloverAction.checkBlock resolved the rollover target to every alias member and aborted on any METADATA_WRITE block on any of them. That means a non-write alias member carrying such a block — for example a cross-cluster-replication follower's index-replication block — kills otherwise-valid rollovers against a sibling write index. The same shape applies to data-stream rollovers when an older backing carries a metadata-write block.

This PR scopes the precheck to the single write index via IndexAbstraction.getWriteIndex() — the same path MetadataRolloverService uses to perform the rollover — so the precheck and the actual rollover agree on which index matters. The same code path handles alias and data-stream targets and works regardless of which plugin placed the block.

The block check itself still goes through ClusterBlocks.indicesWithRemoteSnapshotBlockedException(added by #16483) — only the input changes from "all alias members" to "the single write index". The helper's existing carve-out for remote_snapshot indices (skip the abort unless the user has set index.blocks.read_only, index.blocks.metadata, or index.blocks.read_only_allow_delete) therefore continues to apply, so searchable-snapshot indices keep working.

ClusterBlocks.indicesWithRemoteSnapshotBlockedException itself is unchanged; TransportIndicesAliasesAction still uses it for alias-modification blocking and is unaffected.

Related Issues

Resolves #21629

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]

PR Reviewer Guide 🔍

(Review updated until commit 83fed8f)

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🧪 PR contains tests

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[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
Possible issue	Add null-safety for metadata access Add null-safety check for state.metadata() before accessing getIndicesLookup(). If the cluster state metadata is null, this will throw a NullPointerException. Consider adding a guard clause to return null early if metadata is unavailable. server/src/main/java/org/opensearch/action/admin/indices/rollover/TransportRolloverAction.java [130-137] +if (state.metadata() == null) { + return null; +} IndexAbstraction indexAbstraction = state.metadata().getIndicesLookup().get(request.getRolloverTarget()); if (indexAbstraction == null) { return null; } IndexMetadata writeIndex = indexAbstraction.getWriteIndex(); if (writeIndex == null) { return null; } Suggestion importance[1-10]: 2 __ Why: While adding null-safety checks is generally good practice, ClusterState.metadata() is highly unlikely to return null in OpenSearch's architecture. The ClusterState object always contains metadata, and this would represent a fundamental system failure. The suggestion addresses a theoretical edge case that doesn't occur in practice, making it low-impact defensive programming rather than fixing an actual issue.	Low

[github-actions]

❌ Gradle check result for bca10e0: 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]

Persistent review updated to latest commit 78dfd3e

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✅ Gradle check result for 78dfd3e: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.48%. Comparing base (097bf13) to head (83fed8f).

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #21838      +/-   ##
============================================
- Coverage     73.50%   73.48%   -0.03%     
+ Complexity    75572    75532      -40     
============================================
  Files          6034     6034              
  Lines        342604   342606       +2     
  Branches      49279    49281       +2     
============================================
- Hits         251826   251751      -75     
- Misses        70758    70862     +104     
+ Partials      20020    19993      -27     

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[github-actions]

Persistent review updated to latest commit 27f18eb

[github-actions]

❌ Gradle check result for 27f18eb: 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]

Persistent review updated to latest commit 27f18eb

[github-actions]

❌ Gradle check result for 27f18eb: 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]

Persistent review updated to latest commit 83fed8f

[github-actions]

✅ Gradle check result for 83fed8f: SUCCESS

[mishail]

@andrross it's been a month already. Are there anything wrong with these changes?