Comments: Add wp_update_comment_counts() to reconcile stored counts

[adamsilverstein]

Follow-up to the #12310 counter fix, addressing the second half of pfefferle's review on Trac #35214 comment:52: keeping a post's stored comment_count consistent when the set of excluded comment types changes.

The gap

wp_update_comment_count_now() (made filter-aware in WordPress#12310) only refreshes a post's stored comment_count when that post's comments change. So a count written before a type joined the excluded set stays stale until the post sees activity again. Example: a site runs with review comments counting normally, then installs a plugin that excludes review via default_excluded_comment_types - existing posts keep their inflated count.

Why not auto-recount on registration

register_comment_type() runs on every request during init; it is not a persisted state transition, and the exclusion set is driven by the default_excluded_comment_types filter, not by register/unregister. There is no clean event to hook. Investigated alternatives and rejected them:

• Recount on unregister_comment_type - wrong event (exclusion changes come from the filter, not unregister) and per-request.
• "Dirty set" transient + lazy recount on read - would UPDATE during a GET (breaks read replicas / page caches; get_comments_number() is deliberately read-only) and relies on diffing a per-request filter output on the hot path.

The closest core analog confirms the right pattern: taxonomy term counts are recalculated only on data events, never on taxonomy registration. Rewrite rules use the same model - core exposes flush_rewrite_rules() and documents "call it on activation" rather than auto-detecting.

This PR

wp_update_comment_counts( $post_ids = null ) - a bulk recount helper that recomputes one or more posts' counts through the now filter-aware wp_update_comment_count_now(), so it honors the same exclusion set by construction (no new SQL). A plugin that changes the excluded set calls it once, typically from its activation hook; it also gives a future WP-CLI/admin maintenance tool a single correct entry point.

• null (default) recalculates every post that has at least one comment.
• An int or array of post IDs limits the work (deduped, invalid IDs dropped).
• Returns the number of posts recalculated.

Naming follows the core wp_update_*_counts() bulk family (wp_update_user_counts(), wp_update_network_counts()); singular wp_update_comment_count() already exists for a single post.

Tests

New tests/phpunit/tests/comment/wpUpdateCommentCounts.php: empty input returns 0; targeted IDs only touch those posts; duplicate IDs are deduped; null recalculates all posts with comments; and the headline case - a newly-excluded review type drops a previously stored count to 0.

• 562 --group comment tests pass (557 + 5 new).
• PHPCS clean (0 errors), PHPStan clean.

Review updates

• wp_update_comment_counts() now bumps the comment last_changed cache key. Comment query caches are salted only by that key, so the activation-time register-filter-then-recount flow is now fully self-consistent on persistent object caches.
• The null path additionally visits posts with a nonzero stored count but no remaining comment rows - a SELECT DISTINCT on the comments table cannot see those - and iterates in keyset batches of 1000 instead of materializing every post ID.
• Negative IDs are skipped instead of being absint()-coerced into valid ones.
• Docs now name the per-post side effects: the edit_post hooks fire per recount, so cache purgers and indexers run once per post.
• New tests cover the null-path return value, invalid IDs, stale-count resets on commentless posts via both paths, and the last_changed bump.

Stacked on WordPress#12310 (feature/65537-excluded-comment-types-filter); retarget to trunk once that lands.

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