Comments: Generalize ping handling with an is_ping comment type flag (Trac #35214)

[adamsilverstein]

Summary

This is item 4 of the Trac #35214 "below the hood" follow-up work: generalizing the hard-coded pingback/trackback handling so registered comment types can opt into ping behaviour.

Previously, pingbacks and trackbacks were singled out by hard-coded comment_type string comparisons:

• separate_comments() bucketed 'pingback'/'trackback' into the 'pings' group via if ( 'trackback' === $type || 'pingback' === $type ).
• Walker_Comment::start_el() rendered the compact ping markup via if ( ( 'pingback' === ... || 'trackback' === ... ) && $args['short_ping'] ).
• WP_Comment_Query expanded a 'pings' type token to ['pingback','trackback'].

None of these could be expressed by a registered comment type, so a plugin registering (say) a webmention type could not be grouped, queried, or rendered as a ping.

What changed

• WP_Comment_Type::$is_ping (default false) - a new flag marking a type as a ping (a notification from another site) rather than a human-authored comment.
• The built-in pingback and trackback types are registered with 'is_ping' => true.
• separate_comments() now groups any registered ping type into the pings bucket.
• Walker_Comment::start_el() now renders any registered ping type with the compact ping markup (reusing the comment type object it already looks up for render_callback).
• WP_Comment_Query now expands a 'pings' type to every registered ping type.

Built-in behaviour is unchanged: pingback/trackback still group, query, and render exactly as before, and the default comment/note types are not pings.

Review updates

Following a review pass over the stack:

• The query side landed here rather than as a follow-up. An earlier draft left WP_Comment_Query( 'type' => 'pings' ) on the built-in pair, which meant a theme reading $wp_query->comments_by_type['pings'] and a plugin calling get_comments( array( 'type' => 'pings' ) ) would disagree about which comments are pings. Rather than ship that window in a stable release, 'pings' now expands from the registry on both sides.

  The resolved set goes into the comment query cache key: which types are pings depends on what is registered, that changes when a plugin is activated, and the comment last_changed salt only moves when a comment does. Without it a 'pings' query would keep serving results cached from before the type existed. There is a test for exactly that.

• A registered ping type is now labeled with its own name. Walker_Comment::ping() prints a literal "Pingback:", which has been imprecise since 3.6 when trackbacks started sharing the markup. Once any type can opt in, "Pingback: example.com" for a webmention is plainly wrong for the audience this flag is for. Registered non-built-in types use their singular_name, matching what comment_type() already does; the built-ins keep the literal string so their markup is byte-identical.

  The existing test asserted 'Pingback:' for a webmention, which pinned the wrong label, so it was replaced by one test for the compact shape (label, author link, no comment body) and one for the label itself.

• The is_ping docs now say where the flag stops. It drives grouping, querying, and display. It does not reach validation, default comment status, or notification emails - those still key on the pingback and trackback type names, so a custom ping type gains none of their handling and, just as importantly, none of their bypasses.

• New test: the end-to-end case the suite was missing, wp_list_comments() with type => 'pings' listing a registered ping type, which is where a theme actually meets the grouping change.

The pings type name is reserved against registration in #12311, so a type literally named pings can no longer collide with the meta bucket.

Still out of scope

• check_comment() author/email validation skip for pings, get_default_comment_status() ping status mapping, wp_notify_postauthor() / wp_notify_moderator() wording, and the dashboard recent-comments labels all still key on the two built-in names. These are moderation and notification semantics rather than grouping, and each carries its own behaviour-regression risk.
• A generic ping class in get_comment_class(), so themes have one stable selector for custom ping types (today a webmention only gets .webmention, missing the .pingback / .trackback styling classic themes ship).

These are tracked as the ping generalization backlog on #35214.

Testing

$ phpunit --group comment
OK (717 tests, 1742 assertions)

$ phpunit --group feed
OK (44 tests, 318 assertions)

New coverage in tests/phpunit/tests/comment/separateComments.php, walker.php, query.php, and wpCommentType.php. PHPCS reports no new warnings on the changed files and PHPStan is clean.

Stacking

Based on feature/comment-type-render-callback (fork PR #53) because it touches the same Walker_Comment::start_el() method. Retarget to trunk once the registration API (#12311) and the display-callback PR land.

See #35214.

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