From 372714c9b603e6e06ac1f0792c96d4066b7413d5 Mon Sep 17 00:00:00 2001
From: Hazelnoot <acomputerdog@gmail.com>
Date: Sat, 7 Jun 2025 21:32:55 -0400
Subject: [PATCH] implement userFollowersCache

---
 packages/backend/src/core/CacheService.ts | 17 +++++++++++++++--
 packages/backend/test/misc/noOpCaches.ts  |  6 ++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/packages/backend/src/core/CacheService.ts b/packages/backend/src/core/CacheService.ts
index 38a93e57f4..e8b26f8b9b 100644
--- a/packages/backend/src/core/CacheService.ts
+++ b/packages/backend/src/core/CacheService.ts
@@ -47,6 +47,7 @@ export class CacheService implements OnApplicationShutdown {
 	public userBlockedCache: QuantumKVCache<Set<string>>; // NOTE: 「被」Blockキャッシュ
 	public renoteMutingsCache: QuantumKVCache<Set<string>>;
 	public userFollowingsCache: QuantumKVCache<Map<string, { withReplies: boolean }>>;
+	public userFollowersCache: QuantumKVCache<Set<string>>;
 	protected userFollowStatsCache = new MemoryKVCache<FollowStats>(1000 * 60 * 10); // 10 minutes
 	protected translationsCache: RedisKVCache<CachedTranslationEntity>;
 
@@ -115,6 +116,11 @@ export class CacheService implements OnApplicationShutdown {
 			fetcher: (key) => this.followingsRepository.find({ where: { followerId: key }, select: ['followeeId', 'withReplies'] }).then(xs => new Map(xs.map(f => [f.followeeId, { withReplies: f.withReplies }]))),
 		});
 
+		this.userFollowersCache = new QuantumKVCache<Set<string>>(this.internalEventService, 'userFollowers', {
+			lifetime: 1000 * 60 * 30, // 30m
+			fetcher: (key) => this.followingsRepository.find({ where: { followeeId: key }, select: ['followerId'] }).then(xs => new Set(xs.map(x => x.followerId))),
+		});
+
 		this.translationsCache = new RedisKVCache<CachedTranslationEntity>(this.redisClient, 'translations', {
 			lifetime: 1000 * 60 * 60 * 24 * 7, // 1 week,
 			memoryCacheLifetime: 1000 * 60, // 1 minute
@@ -154,6 +160,7 @@ export class CacheService implements OnApplicationShutdown {
 								this.userBlockedCache.delete(body.id),
 								this.renoteMutingsCache.delete(body.id),
 								this.userFollowingsCache.delete(body.id),
+								this.userFollowersCache.delete(body.id),
 							]);
 						}
 					} else {
@@ -193,7 +200,10 @@ export class CacheService implements OnApplicationShutdown {
 					if (follower) follower.followingCount++;
 					const followee = this.userByIdCache.get(body.followeeId);
 					if (followee) followee.followersCount++;
-					await this.userFollowingsCache.delete(body.followerId);
+					await Promise.all([
+						this.userFollowingsCache.delete(body.followerId),
+						this.userFollowersCache.delete(body.followeeId),
+					]);
 					this.userFollowStatsCache.delete(body.followerId);
 					this.userFollowStatsCache.delete(body.followeeId);
 					break;
@@ -203,7 +213,10 @@ export class CacheService implements OnApplicationShutdown {
 					if (follower) follower.followingCount--;
 					const followee = this.userByIdCache.get(body.followeeId);
 					if (followee) followee.followersCount--;
-					await this.userFollowingsCache.delete(body.followerId);
+					await Promise.all([
+						this.userFollowingsCache.delete(body.followerId),
+						this.userFollowersCache.delete(body.followeeId),
+					]);
 					this.userFollowStatsCache.delete(body.followerId);
 					this.userFollowStatsCache.delete(body.followeeId);
 					break;
diff --git a/packages/backend/test/misc/noOpCaches.ts b/packages/backend/test/misc/noOpCaches.ts
index 7e8c27503c..40c5d2dc65 100644
--- a/packages/backend/test/misc/noOpCaches.ts
+++ b/packages/backend/test/misc/noOpCaches.ts
@@ -112,6 +112,12 @@ export class NoOpCacheService extends CacheService {
 			onSet: this.userFollowingsCache.onSet,
 			onDelete: this.userFollowingsCache.onDelete,
 		});
+		this.userFollowersCache = new NoOpQuantumKVCache<Set<string>>({
+			internalEventService: fakeInternalEventService,
+			fetcher: this.userFollowersCache.fetcher,
+			onSet: this.userFollowersCache.onSet,
+			onDelete: this.userFollowersCache.onDelete,
+		});
 		this.userFollowStatsCache = new NoOpMemoryKVCache<FollowStats>();
 		this.translationsCache = new NoOpRedisKVCache<CachedTranslationEntity>({
 			redis: fakeRedis,