server: optimise follower inboxes query

Use the distinct query thingy so we don't have to make the Set work
so hard. This is also uniform code with the "everyone" above so should
hopefully be easier to understand.

packages/backend/src/remote/activitypub/deliver-manager.ts

@@ -119,26 +119,18 @@ export default class DeliverManager {
 
 		if (this.recipes.some(r => isFollowers(r))) {
 			// followers deliver
-			// TODO: SELECT DISTINCT ON ("followerSharedInbox") "followerSharedInbox" みたいな問い合わせにすればよりパフォーマンス向上できそう
-			// ただ、sharedInboxがnullなリモートユーザーも稀におり、その対応ができなさそう？
-			const followers = await Followings.find({
-				where: {
-					followeeId: this.actor.id,
-					followerHost: Not(IsNull()),
-				},
-				select: {
-					followerSharedInbox: true,
-					followerInbox: true,
-				},
-			}) as {
-				followerSharedInbox: string | null;
-				followerInbox: string;
-			}[];
+			const followers = await Followings.createQueryBuilder('followings')
+				// return either the shared inbox (if available) or the individual inbox
+				.select('COALESCE(followings.followerSharedInbox, followings.followerInbox)', 'inbox')
+				// so we don't have to make our inboxes Set work as hard
+				.distinct(true)
+				// ...for the specific actors followers
+				.where('followings.followeeId = :actorId', { actorId: this.actor.id })
+				// don't deliver to ourselves
+				.andWhere('followings.followerHost IS NOT NULL')
+				.getRawMany();
 
-			for (const following of followers) {
-				const inbox = following.followerSharedInbox || following.followerInbox;
-				inboxes.add(inbox);
-			}
+			followers.forEach(({ inbox }) => inboxes.add(inbox));
 		}
 
 		this.recipes.filter((recipe): recipe is IDirectRecipe =>