From ff0c289e1da5baf77f732b911b416419e342c08a Mon Sep 17 00:00:00 2001
From: Julian Lam <julian@nodebb.org>
Date: Tue, 16 Jul 2024 11:37:38 -0400
Subject: [PATCH] feat: #12695 Topic Synchronization via resolvable `context`

- Generation of a context collection digest via object ids
- Sending of said digest in ETag header
- Parsing of digests via If-None-Match header
- Update note assertion logic to handle 304 response
---
 src/activitypub/contexts.js           | 24 +++++++++++++++++--
 src/activitypub/helpers.js            | 19 +++++++++++++++
 src/activitypub/notes.js              |  6 ++++-
 src/controllers/activitypub/actors.js | 33 ++++++++++++++++++++++-----
 4 files changed, 73 insertions(+), 9 deletions(-)

diff --git a/src/activitypub/contexts.js b/src/activitypub/contexts.js
index c84ac30bc8..b93cd15f33 100644
--- a/src/activitypub/contexts.js
+++ b/src/activitypub/contexts.js
@@ -2,7 +2,9 @@
 
 const winston = require('winston');
 
+const db = require('../database');
 const posts = require('../posts');
+const topics = require('../topics');
 
 const activitypub = module.parent.exports;
 const Contexts = module.exports;
@@ -13,20 +15,38 @@ Contexts.get = async (uid, id) => {
 	let context;
 	let type;
 
+	// Generate digest for If-None-Match if locally cached
+	const tid = await posts.getPostField(id, 'tid');
+	const headers = {};
+	if (tid) {
+		const [mainPid, pids] = await Promise.all([
+			topics.getTopicField(tid, 'mainPid'),
+			db.getSortedSetMembers(`tid:${tid}:posts`),
+		]);
+		pids.push(mainPid);
+		const digest = activitypub.helpers.generateDigest(new Set(pids));
+		headers['If-None-Match'] = `"${digest}"`;
+	}
+
 	try {
-		({ context } = await activitypub.get('uid', uid, id));
+		({ context } = await activitypub.get('uid', uid, id, { headers }));
 		if (!context) {
 			winston.verbose(`[activitypub/context] ${id} contains no context.`);
 			return false;
 		}
 		({ type } = await activitypub.get('uid', uid, context));
 	} catch (e) {
+		if (e.code === 'ap_get_304') {
+			winston.verbose(`[activitypub/context] ${id} context unchanged.`);
+			return { tid };
+		}
+
 		winston.verbose(`[activitypub/context] ${id} context not resolvable.`);
 		return false;
 	}
 
 	if (acceptableTypes.includes(type)) {
-		return context;
+		return { context };
 	}
 
 	return false;
diff --git a/src/activitypub/helpers.js b/src/activitypub/helpers.js
index 23830ed060..78a1a0d06e 100644
--- a/src/activitypub/helpers.js
+++ b/src/activitypub/helpers.js
@@ -4,6 +4,7 @@ const { generateKeyPairSync } = require('crypto');
 const nconf = require('nconf');
 const validator = require('validator');
 const cheerio = require('cheerio');
+const crypto = require('crypto');
 
 const meta = require('../meta');
 const posts = require('../posts');
@@ -19,6 +20,7 @@ const webfingerCache = ttl({
 	max: 5000,
 	ttl: 1000 * 60 * 60 * 24, // 24 hours
 });
+const sha256 = payload => crypto.createHash('sha256').update(payload).digest('hex');
 
 const Helpers = module.exports;
 
@@ -403,3 +405,20 @@ Helpers.generateCollection = async ({ set, method, page, perPage, url }) => {
 
 	return object;
 };
+
+Helpers.generateDigest = (set) => {
+	if (!(set instanceof Set)) {
+		throw new Error('[[error:invalid-data]]');
+	}
+
+	return Array
+		.from(set)
+		.map(item => sha256(item))
+		.reduce((memo, cur) => {
+			const a = Buffer.from(memo, 'hex');
+			const b = Buffer.from(cur, 'hex');
+			// eslint-disable-next-line no-bitwise
+			const result = a.map((x, i) => x ^ b[i]);
+			return result.toString('hex');
+		});
+};
diff --git a/src/activitypub/notes.js b/src/activitypub/notes.js
index 5e396efa62..315622acea 100644
--- a/src/activitypub/notes.js
+++ b/src/activitypub/notes.js
@@ -43,7 +43,11 @@ Notes.assert = async (uid, input, options = { skipChecks: false }) => {
 
 	let chain;
 	const context = await activitypub.contexts.get(uid, id);
-	if (context) {
+	if (context.tid) {
+		unlock(id);
+		const { tid } = context;
+		return { tid, count: 0 };
+	} else if (context.context) {
 		chain = Array.from(await activitypub.contexts.getItems(uid, context));
 	} else {
 		// Fall back to inReplyTo traversal
diff --git a/src/controllers/activitypub/actors.js b/src/controllers/activitypub/actors.js
index 3ce4df4ab2..beaf346e1b 100644
--- a/src/controllers/activitypub/actors.js
+++ b/src/controllers/activitypub/actors.js
@@ -2,6 +2,7 @@
 
 const nconf = require('nconf');
 
+const db = require('../../database');
 const meta = require('../../meta');
 const privileges = require('../../privileges');
 const posts = require('../../posts');
@@ -100,13 +101,33 @@ Actors.topic = async function (req, res, next) {
 	const perPage = meta.config.postsPerPage;
 	const { cid, titleRaw: name, mainPid, slug } = await topics.getTopicFields(req.params.tid, ['cid', 'title', 'mainPid', 'slug']);
 	try {
-		const collection = await activitypub.helpers.generateCollection({
-			set: `tid:${req.params.tid}:posts`,
-			method: posts.getPidsFromSet,
-			page,
-			perPage,
-			url: `${nconf.get('url')}/topic/${req.params.tid}`,
+		let [collection, pids] = await Promise.all([
+			activitypub.helpers.generateCollection({
+				set: `tid:${req.params.tid}:posts`,
+				method: posts.getPidsFromSet,
+				page,
+				perPage,
+				url: `${nconf.get('url')}/topic/${req.params.tid}`,
+			}),
+			db.getSortedSetMembers(`tid:${req.params.tid}:posts`),
+		]);
+
+		// Generate digest for ETag
+		pids.push(mainPid);
+		pids = pids.map(pid => (utils.isNumber(pid) ? `${nconf.get('url')}/post/${pid}` : pid));
+		const digest = activitypub.helpers.generateDigest(new Set(pids));
+		const ifNoneMatch = req.get('If-None-Match').split(',').map((tag) => {
+			tag = tag.trim();
+			if (tag.startsWith('"') && tag.endsWith('"')) {
+				return tag.slice(1, tag.length - 1);
+			}
+
+			return tag;
 		});
+		if (ifNoneMatch.includes(digest)) {
+			return res.sendStatus(304);
+		}
+		res.set('ETag', digest);
 
 		// Convert pids to urls
 		collection.totalItems += 1;