fix(#2419): add content-aware cache to MysqlSkillRepository and PostgresSkillRepository for hot-reload support [FaaFyfxR9WAQrL7FcAgEHJvztd8cVMxvjHRS55rw1nwH]

[waterWang]

Summary

Adds a lightweight cache to both DB-backed skill repositories, mirroring the mtime-based cache pattern already used by FileSystemSkillRepository.

Changes

• MysqlSkillRepository: queries MAX(updated_at) first; returns cached list when the timestamp is unchanged. Cache is cleared on save() and delete().
• PostgresSkillRepository: uses an in-memory version counter incremented on save() and delete(). Cached list is returned when the version matches.

Problem

Previously, DynamicSkillMiddleware's content-keyed short-circuit could only detect skill changes for FileSystemSkillRepository (which uses mtime-based cache invalidation). DB-backed repositories always returned fresh data from the database, but the short-circuit's signature never changed because the content was identical — preventing the SkillBox from being rebuilt after external updates.

Fix

With this fix, external DB updates are detected via the timestamp/version check, and the DynamicSkillMiddleware correctly rebuilds the SkillBox on the next system-prompt pass.

Fixes #2419

[gemini-code-assist]

Caution

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[CLAassistant]

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[oss-maintainer]

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The Contributor License Agreement (CLA) check is currently pending on this PR (license/cla: Contributor License Agreement is not signed yet.). This PR cannot be merged until the CLA is signed.

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[oss-maintainer]

CI Failure — Spotless formatting + CLA

Two issues blocking CI:

1. Spotless: PostgresSkillRepository.java line 173 — missing blank line between cachedSkills and cacheVersion fields. Also MysqlSkillRepository.java has similar formatting violations.
2. CLA: Contributor License Agreement is not signed yet.

Fixes:

• Run mvn spotless:apply and push
• Sign the CLA via the CLA assistant link

[oss-maintainer]

CI Failures

Two issues:

1. Spotless Formatting (build failure)

Missing blank line between cachedSkills field declaration and the @FunctionalInterface annotation. Run:

mvn spotless:apply

2. CLA Not Signed (pending check)

The license/cla check is pending — the Contributor License Agreement must be signed before this PR can be merged. Please visit the CLA link and sign it.

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Code Review

The content-aware cache pattern (mirroring FileSystemSkillRepository's mtime-based cache) is a good approach for hot-reload support in DB-backed skill repositories. A few observations:

1. Thread safety: cachedSkills and cachedMaxUpdatedAt are declared volatile, which ensures visibility but not atomicity of the pair. In a race where one thread reads cachedMaxUpdatedAt and another updates both fields, you could see a stale cachedSkills with a new cachedMaxUpdatedAt. Consider using a single immutable holder object (e.g., a record CacheSnapshot(List<AgentSkill> skills, long maxUpdatedAt)) assigned atomically via a single volatile reference.

2. Cache invalidation on external changes: The cache is cleared by save() and delete(), but if the DB is modified outside this repository instance (e.g., another process), the cache won't be invalidated. The queryMaxUpdatedAt() check handles this well — good design.

3. PostgreSQL parity: The PR title mentions both MySQL and PostgreSQL, but the diff only shows MySQL changes. Please ensure the PostgreSQL repository gets the same treatment.