Fix ActivityPub replies to local comments not threading

[akirk]

Summary

• Fix replies from the Fediverse to local WordPress comments, federated as ?c=NNN URLs, so they arrive as threaded comments on the cached Friends post.
• Prevent local comment URLs from being misresolved by url_to_postid() before ActivityPub can fall back to url_to_commentid().
• Let ActivityPub persist inbound interaction comments on ActivityPub-capable friend_post_cache posts only while handling the inbound Create activity, while keeping cached posts disabled for the normal ActivityPub post lifecycle.
• Defer Friends feed-item creation for replies to local comments so ActivityPub's comment handler can process them.
• Fix comment reply links on friend_post_cache posts to use the local Friends URL instead of the external source URL.
• Extend remote comment fetching to ActivityPub-capable cached posts and dedupe comments that are already stored locally.
• Add regression tests for the comment URL handling, ActivityPub comment persistence, Friends deferral, reply-link URL, and remote-comment deduplication.

Test plan

• composer test -- --filter ActivityPubTest
• composer check-cs
• Comment on a friend's post that originates from an ActivityPub-capable source.
• Have someone reply to that comment from the Fediverse.
• Verify the reply arrives as a threaded comment on the correct Friends post, not as a standalone cached post.
• Verify the "Reply" link on comments uses the local Friends URL, not the external source URL.
• Verify remote comments load without duplicating locally stored comments.

Test in WordPress Playground

Changelog

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Type

• Fixed

Message

Fix ActivityPub replies to local comments not arriving as threaded comments

[akirk]

@pfefferle I am trying to debug this problem on my personal homepage. I believe this section is problematic:

https://github.com/Automattic/wordpress-activitypub/blob/6821a2b1be700b24d1064785aa358de375770901/includes/collection/class-interactions.php#L74-L82

url_to_postid for a comment url a la home_url( '?c=' . $comment_id ) strips the parameters off and then matches against the homepage. In my case a static page.

Thus, it doesn't return something falsy and thus doesn't go into the fallback. Is this is an oversight? My fix in this PR is to make sure url_to_postid() returns something false by making the URL invalid. But I think it could be solved more elegantly in the ActivityPub plugin.

My case in the friends plugin might be even a bit more complicated because I am replying in the Friends backend to a not-ActivityPub enabled post (it was federated in but it's a friend_post_cache) but I think it's a valid case for overriding activitypub_is_post_disabled, so that makes it work!

[akirk]

@pfefferle maybe you missed this?