feat: have category actor send Announce(Note) activity on posts from that cid

re: #12434

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;
+};

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 { cid } = post.category;
+	const followers = await activitypub.notes.getCategoryFollowers(cid);
 
-	const payload = {
-		type: 'Create',
-		to: object.to,
-		cc: object.cc,
-		object,
+	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 = {};