[backend] Use a semaphore around populateMentions

This fixes a user-generated DoS payload for giant webring-style trees of mentions in user bios that could cause backend stalls.

.pnp.cjs

@@ -1722,6 +1722,10 @@ function $$SETUP_STATE(hydrateRuntimeState, basePath) {
         "async-lock",\
         "npm:1.4.0"\
       ],\
+      [\
+        "async-mutex",\
+        "npm:0.4.0"\
+      ],\
       [\
         "async-settle",\
         "npm:1.0.0"\
@@ -13423,6 +13427,16 @@ function $$SETUP_STATE(hydrateRuntimeState, basePath) {
           "linkType": "HARD"\
         }]\
       ]],\
+      ["async-mutex", [\
+        ["npm:0.4.0", {\
+          "packageLocation": "./.yarn/cache/async-mutex-npm-0.4.0-f5a25d4255-813a71728b.zip/node_modules/async-mutex/",\
+          "packageDependencies": [\
+            ["async-mutex", "npm:0.4.0"],\
+            ["tslib", "npm:2.6.2"]\
+          ],\
+          "linkType": "HARD"\
+        }]\
+      ]],\
       ["async-settle", [\
         ["npm:1.0.0", {\
           "packageLocation": "./.yarn/cache/async-settle-npm-1.0.0-5d08fbf926-d2382ad4b9.zip/node_modules/async-settle/",\
@@ -13862,6 +13876,7 @@ function $$SETUP_STATE(hydrateRuntimeState, basePath) {
             ["archiver", "npm:5.3.1"],\
             ["argon2", "npm:0.30.3"],\
             ["async-lock", "npm:1.4.0"],\
+            ["async-mutex", "npm:0.4.0"],\
             ["autolinker", "npm:4.0.0"],\
             ["autwh", "npm:0.1.0"],\
             ["aws-sdk", "npm:2.1413.0"],\
@@ -30575,6 +30590,13 @@ function $$SETUP_STATE(hydrateRuntimeState, basePath) {
             ["tslib", "npm:2.6.1"]\
           ],\
           "linkType": "HARD"\
+        }],\
+        ["npm:2.6.2", {\
+          "packageLocation": "./.yarn/cache/tslib-npm-2.6.2-4fc8c068d9-329ea56123.zip/node_modules/tslib/",\
+          "packageDependencies": [\
+            ["tslib", "npm:2.6.2"]\
+          ],\
+          "linkType": "HARD"\
         }]\
       ]],\
       ["tsscmp", [\

packages/backend/package.json

@@ -41,6 +41,7 @@
 		"archiver": "5.3.1",
 		"argon2": "^0.30.3",
 		"async-lock": "1.4.0",
+		"async-mutex": "^0.4.0",
 		"autolinker": "4.0.0",
 		"autwh": "0.1.0",
 		"aws-sdk": "2.1413.0",

packages/backend/src/models/repositories/user-profile.ts

@@ -6,8 +6,12 @@ import { resolveMentionToUserAndProfile } from "@/remote/resolve-user.js";
 import { IMentionedRemoteUsers } from "@/models/entities/note.js";
 import { unique } from "@/prelude/array.js";
 import config from "@/config/index.js";
+import { Semaphore } from "async-mutex";
+
+const queue = new Semaphore(10);
 
 export const UserProfileRepository = db.getRepository(UserProfile).extend({
+    // We must never await this without promiseEarlyReturn, otherwise giant webring-style profile mention trees will cause the queue to stop working
     async updateMentions(id: UserProfile["userId"]){
         const profile = await this.findOneBy({ userId: id });
         if (!profile) return;
@@ -18,11 +22,12 @@ export const UserProfileRepository = db.getRepository(UserProfile).extend({
         if (profile.fields.length > 0)
             tokens.push(...profile.fields.map(p => mfm.parse(p.value).concat(mfm.parse(p.name))).flat());
 
-        const partial = {
-            mentions: await populateMentions(tokens, profile.userHost)
-        };
-
-        return UserProfileRepository.update(profile.userId, partial)
+        return queue.runExclusive(async () => {
+            const partial = {
+                mentions: await populateMentions(tokens, profile.userHost)
+            };
+            return UserProfileRepository.update(profile.userId, partial);
+        });
     },
 });
 

yarn.lock

@@ -4779,6 +4779,15 @@ __metadata:
   languageName: node
   linkType: hard
 
+"async-mutex@npm:^0.4.0":
+  version: 0.4.0
+  resolution: "async-mutex@npm:0.4.0"
+  dependencies:
+    tslib: ^2.4.0
+  checksum: 813a71728b35a4fbfd64dba719f04726d9133c67b577fcd951b7028c4a675a13ee34e69beb82d621f87bf81f5d4f135c4c44be0448550c7db728547244ef71fc
+  languageName: node
+  linkType: hard
+
 "async-settle@npm:^1.0.0":
   version: 1.0.0
   resolution: "async-settle@npm:1.0.0"
@@ -5129,6 +5138,7 @@ __metadata:
     archiver: 5.3.1
     argon2: ^0.30.3
     async-lock: 1.4.0
+    async-mutex: ^0.4.0
     autolinker: 4.0.0
     autwh: 0.1.0
     aws-sdk: 2.1413.0
@@ -19712,6 +19722,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"tslib@npm:^2.4.0":
+  version: 2.6.2
+  resolution: "tslib@npm:2.6.2"
+  checksum: 329ea56123005922f39642318e3d1f0f8265d1e7fcb92c633e0809521da75eeaca28d2cf96d7248229deb40e5c19adf408259f4b9640afd20d13aecc1430f3ad
+  languageName: node
+  linkType: hard
+
 "tsscmp@npm:1.0.6":
   version: 1.0.6
   resolution: "tsscmp@npm:1.0.6"