From 7df5cabb7685ea23c136d8b8e333b12fa3b40107 Mon Sep 17 00:00:00 2001
From: Julian Lam <julian@nodebb.org>
Date: Fri, 22 Mar 2024 14:39:18 -0400
Subject: [PATCH] feat: have category actor send Announce(Note) activity on
 posts from that cid

re: #12434
---
 src/activitypub/notes.js |  8 ++++++++
 src/api/activitypub.js   | 25 ++++++++++++++++++-------
 2 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/src/activitypub/notes.js b/src/activitypub/notes.js
index 51ad12f45c17..ef53e813d3cc 100644
--- a/src/activitypub/notes.js
+++ b/src/activitypub/notes.js
@@ -254,3 +254,11 @@ Notes.syncUserInboxes = async function (tid) {
 	winston.verbose(`[activitypub/syncUserInboxes] Syncing tid ${tid} with ${uids.size} inboxes`);
 	await db.sortedSetsAdd(keys, keys.map(() => score || Date.now()), tid);
 };
+
+Notes.getCategoryFollowers = async (cid) => {
+	// Retrieves remote users who have followed a category; used to build recipient list
+	let uids = await db.getSortedSetRangeByScore(`cid:${cid}:uid:watch:state`, 0, -1, categories.watchStates.tracking, categories.watchStates.tracking);
+	uids = uids.filter(uid => !utils.isNumber(uid));
+
+	return uids;
+};
diff --git a/src/api/activitypub.js b/src/api/activitypub.js
index 8edbe3eb24aa..bd8d700908fd 100644
--- a/src/api/activitypub.js
+++ b/src/api/activitypub.js
@@ -104,15 +104,26 @@ activitypubApi.create.post = enabledCheck(async (caller, { pid }) => {
 
 	const object = await activitypub.mocks.note(post);
 	const { targets } = await buildRecipients(object, post.user.uid);
-
-	const payload = {
-		type: 'Create',
-		to: object.to,
-		cc: object.cc,
-		object,
+	const { cid } = post.category;
+	const followers = await activitypub.notes.getCategoryFollowers(cid);
+
+	const payloads = {
+		create: {
+			type: 'Create',
+			to: object.to,
+			cc: object.cc,
+			object,
+		},
+		announce: {
+			type: 'Announce',
+			to: [`${nconf.get('url')}/category/${cid}/followers`],
+			cc: [activitypub._constants.publicAddress],
+			object,
+		},
 	};
 
-	await activitypub.send('uid', caller.uid, Array.from(targets), payload);
+	await activitypub.send('uid', caller.uid, Array.from(targets), payloads.create);
+	await activitypub.send('cid', cid, followers, payloads.announce);
 });
 
 activitypubApi.update = {};