Add cluster.blocks.read.auto_release setting

[drtootsie]

Addresses #20539

Problem

DiskThresholdMonitor unconditionally auto-releases index.blocks.read on all indices, even when manually set by users. This setting was added to support warm node file cache threshold protection, but the implementation assumes index.blocks.read is only used by the system.

Unlike cluster.blocks.create_index which has cluster.blocks.create_index.auto_release to control this behavior, there is no equivalent setting for index.blocks.read.

Solution

Add cluster.blocks.read.auto_release setting to control read block auto-release behavior, following the same pattern as cluster.blocks.create_index.auto_release.

Changes

+ public static final Setting<Boolean> CLUSTER_READ_BLOCK_AUTO_RELEASE = Setting.boolSetting(
+     "cluster.blocks.read.auto_release",
+     true,
+     Setting.Property.Dynamic,
+     Setting.Property.NodeScope
+ );

+ private volatile boolean readBlockAutoReleaseEnabled;

+ public boolean isReadBlockAutoReleaseEnabled() {
+     return readBlockAutoReleaseEnabled;
+ }

In DiskThresholdMonitor.handleReadBlocks():

+ if (diskThresholdSettings.isReadBlockAutoReleaseEnabled()) {
      indicesToReleaseReadBlock = StreamSupport.stream(...)
          .filter(index -> indicesToBlockRead.contains(index) == false)
          .filter(index -> state.getBlocks().hasIndexBlock(index, IndexMetadata.INDEX_READ_BLOCK))
          .collect(Collectors.toSet());
+ } else {
+     indicesToReleaseReadBlock = Set.of();
+ }

Usage

Users can disable auto-release to preserve manually-set read blocks:

PUT /_cluster/settings
{
  "persistent": {
    "cluster.blocks.read.auto_release": false
  }
}

Testing

• Set index.blocks.read: true manually on an index
• With default setting (true): block is auto-released
• With cluster.blocks.read.auto_release: false: block persists
• Warm node file cache protection still works (sets blocks that get auto-released)

Impact

• Default behavior unchanged (auto-release enabled)
• Users can now control read block auto-release
• Manually-set read blocks can persist until explicitly removed

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📝 Walkthrough

Walkthrough

This PR adds conditional auto-release functionality for read-blocks in disk threshold management. It introduces a new dynamic cluster setting that gates automatic read-block release logic, allowing operators to control whether blocked indices are automatically unblocked when disk conditions improve.

Changes

Cohort / File(s)	Summary
Read-Block Auto-Release Configuration server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdSettings.java	Introduces new cluster setting CLUSTER_READ_BLOCK_AUTO_RELEASE (boolean, dynamic, node-scoped) with corresponding volatile field, getter, and settings update listener for dynamic updates.
Read-Block Release Logic server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdMonitor.java	Adds conditional branching in handleReadBlocks() that computes indicesToReleaseReadBlock based on the isReadBlockAutoReleaseEnabled() flag; when disabled, release set is empty, otherwise computed as currently blocked indices not in the block-list that retain INDEX_READ_BLOCK.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~18 minutes

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and concisely summarizes the main change: adding a new cluster setting to control read block auto-release behavior.
Description check	✅ Passed	The description includes all required sections: Problem statement, Solution, Changes with code examples, Usage instructions, and Testing details. It comprehensively addresses the issue and follows the template structure.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdMonitor.java (1)

496-527: ⚠️ Potential issue | 🟠 Major

Apply indicesNotToAutoRelease filter to indicesToReleaseReadBlock in handleReadBlocks.

The read-only block auto-release in handleReadOnlyBlocks (line 456) filters out indicesNotToAutoRelease to prevent releasing blocks on flood-stage, high-watermark, or nodes with missing usage data. However, handleReadBlocks does not apply this same filter to indicesToReleaseReadBlock, allowing read blocks to be auto-released on indices that should remain blocked. This creates an asymmetry where read-only blocks are protected but read blocks are not.

Pass indicesNotToAutoRelease to handleReadBlocks and apply the same filter to indicesToReleaseReadBlock (around line 499-506) to match the pattern in handleReadOnlyBlocks.

📜 Review details

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 8c244c0 and 10ce8c8.

📒 Files selected for processing (2)

• server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdMonitor.java
• server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdSettings.java

🧰 Additional context used

🧠 Learnings (1)

📚 Learning: 2026-01-13T17:40:27.167Z

Learnt from: reta
Repo: opensearch-project/OpenSearch PR: 20411
File: server/src/main/java/org/opensearch/index/codec/CodecService.java:112-133
Timestamp: 2026-01-13T17:40:27.167Z
Learning: Avoid capturing or evaluating a supplier (e.g., this::defaultCodec) upfront when passing it to a registry during object construction. If registries may replace defaults during iteration (as in EnginePlugin.getAdditionalCodecs), pass the supplier itself and only resolve it at use time. This ensures dynamic behavior is preserved during initialization and prevents premature binding of defaults in codecs/registry setup. This pattern should apply to similar initialization paths in Java server code where registries may mutate defaults during construction.

Applied to files:

• server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdMonitor.java
• server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdSettings.java

🔇 Additional comments (2)

server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdSettings.java (1)

117-122: Clean, consistent addition mirroring the existing createIndexBlockAutoReleaseEnabled pattern.

Setting definition, volatile field, constructor init, update consumer registration, setter, and getter all follow the established convention. LGTM.

Also applies to: 132-132, 163-163, 175-175, 377-379, 439-441

server/src/main/java/org/opensearch/cluster/routing/allocation/DiskThresholdMonitor.java (1)

497-509: Correct conditional gating of read-block auto-release.

The toggle cleanly wraps the existing release computation and falls back to an immutable empty set when disabled. This is consistent with how isCreateIndexBlockAutoReleaseEnabled() gates the create-index block release in handleClusterCreateIndexBlocks.

[github-actions]

❌ Gradle check result for 10ce8c8: 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 a493f8b: 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?