General: Improve caching in get_calendar function

[MarcinDudekDev]

Summary

• Replace monolithic calendar cache (single array with all variations) with individual cache keys per variation — reduces deserialization overhead on reads
• Switch invalidation hooks from save_post/delete_post to clean_post_cache — avoids unnecessary flushes on autosaves, revisions, and draft saves
• Use wp_cache_flush_group('calendar') for invalidation with a generation counter fallback for object cache implementations lacking group flush support
• Replace MySQL date arithmetic (DATE_FORMAT/DATE_ADD) with PHP native gmdate() — eliminates unnecessary DB roundtrip

Technical Details

Cache Architecture (before → after)

Before: All calendar variations stored in one serialized array under key get_calendar. Every read deserializes all months/post-types. Every write re-serializes the entire array. delete_get_calendar_cache() nukes everything via wp_cache_delete.

After: Each variation gets its own cache key (get_calendar_{hash}). Reads fetch only the requested entry. delete_get_calendar_cache() uses wp_cache_flush_group('calendar') to clear all entries.

Persistent Cache Compatibility

Object cache implementations that don't support flush_group get a generation counter fallback: delete_get_calendar_cache() increments a generation value, and the cache key includes that generation — effectively invalidating all previous entries without needing to enumerate them.

Raw SQL Queries Preserved

The ticket suggested replacing $wpdb queries with WP_Query, but SELECT DISTINCT DAYOFMONTH(post_date) returns max 31 rows while WP_Query would load all posts in the month. The existing PR #6708 makes this replacement and introduces a performance regression. We keep the efficient raw queries.

Trac ticket: https://core.trac.wordpress.org/ticket/61343
Supersedes: #6708

Test plan

• All 10 existing + new tests pass (27 assertions)
• PHPCS clean on all 3 modified files
• Cache invalidation works via delete_get_calendar_cache()
• Individual cache keys: different post types cached independently
• flush_group clears all variations at once
• Backwards compatibility: delete_get_calendar_cache() function signature unchanged
• Verify with a persistent object cache plugin (Redis/Memcached)

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AI assistance: Yes
Tool(s): Claude Code (Anthropic)
Model(s): Claude Opus 4.6
Used for: Performance analysis, identifying regression in existing PR #6708, implementation of cache restructure with generation counter fallback, and writing unit tests. All code was reviewed and tested.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

🤖 Generated with Claude Code

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