feat: show topic follower counts (#13326)

fix upgrade script dates
add upgrade script to count topic followers for each topic

src/upgrades/4.3.0/normalize_thumbs_uploads.js

@@ -7,7 +7,7 @@ const crypto = require('crypto');
 
 module.exports = {
 	name: 'Normalize topic thumbnails, post & user uploads to same format',
-	timestamp: Date.UTC(2021, 1, 7),
+	timestamp: Date.UTC(2025, 3, 4),
 	method: async function () {
 		const { progress } = this;
 

src/upgrades/4.3.0/topic_follower_counts.js

@@ -0,0 +1,35 @@
+'use strict';
+
+const db = require('../../database');
+const batch = require('../../batch');
+
+
+module.exports = {
+	name: 'Set "followercount" on each topic object',
+	timestamp: Date.UTC(2025, 3, 15),
+	method: async function () {
+		const { progress } = this;
+
+		progress.total = await db.sortedSetCard('topics:tid');
+
+		await batch.processSortedSet('topics:tid', async (tids) => {
+			const keys = tids.map(tid => `tid:${tid}:followers`);
+			const followerCounts = await db.setsCount(keys);
+
+			const bulkSet = [];
+
+			followerCounts.forEach((count, idx) => {
+				const tid = tids[idx];
+				if (count > 0) {
+					bulkSet.push([`topic:${tid}`, {followercount: count}]);
+				}
+			});
+
+			await db.setObjectBulk(bulkSet);
+
+			progress.incr(tids.length);
+		}, {
+			batch: 500,
+		});
+	},
+};