[Feature] Add grammar bundle generation API for PPL language features

[mengweieric]

Description

Implements the backend grammar metadata API for PPL autocomplete support.

This endpoint serves a versioned grammar bundle containing ANTLR metadata required for downstream consumers (for example OpenSearch Dashboards) to reconstruct a functional PPL lexer/parser at runtime using antlr4ng interpreter APIs. This enables full client-side parsing/autocomplete with zero per-keystroke server calls, while keeping backend grammar as the source of truth.

What the bundle contains:

• Serialized lexer and parser ATNs via ATNSerializer.serialize().toArray(), compatible with antlr4ng ATNDeserializer.deserialize()
• Token vocabulary (literal/symbolic names), lexer/parser rule names, channel names, and mode names for interpreter reconstruction
• tokenDictionary, ignoredTokens, and rulesToVisit for autocomplete behavior
• grammarHash (ATNs + lexer/parser rule names + literal/symbolic vocabulary + ANTLR version) for client-side change detection
• bundleVersion and antlrVersion for compatibility validation

Backend behavior:

• Synchronized lazy initialization (bundle built on first request and cached for node lifecycle)
• Deterministic output (same plugin JAR -> same bundle shape/content)
• Endpoint marked @ExperimentalApi
• Grammar endpoint now performs authorization via PPL transport action before serving bundle data (aligns with existing PPL permission model)

Also included:

• REST API spec registration for ppl.grammar + YAML REST response-shape test
• Unit tests for grammar REST handler (including authorization-failure path)
• Security integration tests for grammar endpoint access with/without PPL permission
• THIRD-PARTY updated to reflect ANTLR 4.13.2

Related Issues

Resolves #5218

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a new GET endpoint /_plugins/_ppl/_grammar that serves a cached, serialized ANTLR grammar bundle; introduces GrammarBundle and PPLGrammarBundleBuilder, a RestPPLGrammarAction handler with tests, unit tests for the builder, and updates ANTLR third‑party metadata to 4.13.2.

Changes

Cohort / File(s)	Summary
Third‑party notices THIRD-PARTY	Updated ANTLR version from 4.7.1 to 4.13.2 and copyright year range from 2012-2017 to 2012-2024 in license headers.
Plugin registration plugin/src/main/java/org/opensearch/sql/plugin/SQLPlugin.java	Imported and registered RestPPLGrammarAction to expose the new PPL grammar endpoint.
REST handler plugin/src/main/java/org/opensearch/sql/plugin/rest/RestPPLGrammarAction.java	New REST handler serving GET /_plugins/_ppl/_grammar with lazy thread‑safe caching (double‑checked locking), JSON serialization via XContentBuilder, error handling (500), and test hooks (buildBundle, invalidateCache).
REST handler tests plugin/src/test/java/org/opensearch/sql/plugin/rest/RestPPLGrammarActionTest.java	Unit tests for route/name, successful response structure, caching behavior, cache invalidation, and error path (500). Includes MockRestChannel to capture responses.
Grammar data model ppl/src/main/java/org/opensearch/sql/ppl/autocomplete/GrammarBundle.java	New immutable Lombok @Value/@Builder container for serialized ANTLR grammar data (bundleVersion, antlrVersion, grammarHash, lexer/parser ATNs, rule names, channels, modes, vocabulary).
Bundle builder ppl/src/main/java/org/opensearch/sql/ppl/autocomplete/PPLGrammarBundleBuilder.java	New builder that instantiates ANTLR lexer/parser, serializes ATNs, extracts token/rule/channel/mode names, computes SHA‑256 grammar hash, and constructs GrammarBundle.
Builder tests ppl/src/test/java/org/opensearch/sql/ppl/autocomplete/PPLGrammarBundleBuilderTest.java	Tests validating bundle contents, deterministic builds, hash format, start rule resolution, and non‑empty ATNs/names.
Manifest pom.xml	Minor manifest changes related to dependency/metadata updates (ANTLR bump).

Sequence Diagram

sequenceDiagram
    participant Client
    participant Handler as RestPPLGrammarAction
    participant Cache
    participant Builder as PPLGrammarBundleBuilder
    participant Serializer as XContentBuilder

    Client->>Handler: GET /_plugins/_ppl/_grammar
    Handler->>Cache: check cached bundle
    alt cache hit
        Cache-->>Handler: return Bundle
    else cache miss
        Handler->>Builder: buildBundle()
        Builder->>Builder: inspect lexer/parser, serialize ATNs, compute hash
        Builder-->>Handler: Bundle
        Handler->>Cache: store Bundle
    end
    Handler->>Serializer: serialize Bundle to JSON
    Serializer-->>Handler: JSON payload
    Handler-->>Client: HTTP 200 + JSON

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• Bump ANTLR Version to 4.13.2 #5159 — Also updates ANTLR version and related ANTLR-dependent code.

Suggested labels

enhancement

Suggested reviewers

• ps48
• kavithacm
• derek-ho
• joshuali925
• penghuo
• GumpacG
• Swiddis
• dai-chen

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.25% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly describes the main feature being added: a grammar bundle generation API for PPL language autocomplete support, which aligns with the primary changeset focus.
Description check	✅ Passed	The pull request description clearly outlines the implementation of a backend grammar metadata API for PPL autocomplete support, detailing bundle contents, backend behavior, and related updates.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

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

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Persistent review updated to latest commit da140ef

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Persistent review updated to latest commit 090171a

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