core: user_create: add optimistic loop

packages/hydrooj/src/model/user.ts

@@ -312,12 +312,16 @@ class UserModel {
         mail: string, uname: string, password: string,
         uid?: number, regip: string = '127.0.0.1', priv: number = system.get('default.priv'),
     ) {
-        const salt = String.random();
+        let autoAlloc = false;
         if (typeof uid !== 'number') {
             const [udoc] = await coll.find({}).sort({ _id: -1 }).limit(1).toArray();
             uid = Math.max((udoc?._id || 0) + 1, 2);
+            autoAlloc = true;
         }
+        const salt = String.random();
+        while (true) { // eslint-disable-line no-constant-condition
             try {
+                // eslint-disable-next-line no-await-in-loop
                 await coll.insertOne({
                     _id: uid,
                     mail,
@@ -334,14 +338,16 @@ class UserModel {
                     priv,
                     avatar: `gravatar:${mail}`,
                 });
+                return uid;
             } catch (e) {
                 if (e?.code === 11000) {
                     // Duplicate Key Error
                     throw new UserAlreadyExistError(Object.values(e?.keyValue || {}));
                 }
-            throw e;
+                if (!autoAlloc || JSON.stringify(e.keyPattern) !== '{"_id":1}') throw e;
+            }
+            uid++;
         }
-        return uid;
     }
 
     @ArgMethod