server: fix rendering of Follow activity when removing follow

closes FoundKeyGang/FoundKey#263

Changelog: Fixed

packages/backend/src/remote/activitypub/renderer/follow.ts

@@ -2,9 +2,29 @@ import config from '@/config/index.js';
 import { User } from '@/models/entities/user.js';
 import { Users } from '@/models/index.js';
 
+/**
+ * Renders a follow activity.
+ * @param follower user that is (trying to) follow someone
+ * @param followee the user that is (trying to) be followed
+ * @param requestId: ID of this follow. If undefined and follower is local, will be generated.
+ */
 export default (follower: { id: User['id']; host: User['host']; uri: User['host'] }, followee: { id: User['id']; host: User['host']; uri: User['host'] }, requestId?: string) => {
+	let id = requestId;
+
+	if (id == null && follower.host == null) {
+		/*
+		Generate an id only if the follower is local.
+		Otherwise we may try to generate an ID for a remote activity,
+		in which case its better not to put an id at all.
+
+		Why IDs must be generated: https://github.com/misskey-dev/misskey/issues/8655
+		Why IDs must not be generated for remote activities: https://akkoma.dev/FoundKeyGang/FoundKey/issues/263
+		*/
+		id = `${config.url}/follows/${follower.id}/${followee.id}`;
+	}
+
 	const follow = {
-		id: requestId ?? `${config.url}/follows/${follower.id}/${followee.id}`,
+		id,
 		type: 'Follow',
 		actor: Users.isLocalUser(follower) ? `${config.url}/users/${follower.id}` : follower.uri,
 		object: Users.isLocalUser(followee) ? `${config.url}/users/${followee.id}` : followee.uri,