Refactor user request to fix threadsafety issues

osu.Game/Database/UserLookupCache.cs

@@ -1,7 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
@@ -15,103 +14,75 @@ namespace osu.Game.Database
 {
     public class UserLookupCache : MemoryCachingComponent<int, User>
     {
-        private readonly HashSet<int> nextTaskIDs = new HashSet<int>();
-
         [Resolved]
         private IAPIProvider api { get; set; }
 
-        private readonly object taskAssignmentLock = new object();
-
-        private Task<List<User>> pendingRequest;
-
-        /// <summary>
-        /// Whether <see cref="pendingRequest"/> has already grabbed its IDs.
-        /// </summary>
-        private bool pendingRequestConsumedIDs;
-
         public Task<User> GetUserAsync(int userId, CancellationToken token = default) => GetAsync(userId, token);
 
         protected override async Task<User> ComputeValueAsync(int lookup, CancellationToken token = default)
-        {
-            var users = await getQueryTaskForUser(lookup);
-            return users.FirstOrDefault(u => u.Id == lookup);
-        }
+            => await queryUser(lookup);
 
-        /// <summary>
-        /// Return the task responsible for fetching the provided user.
-        /// This may be part of a larger batch lookup to reduce web requests.
-        /// </summary>
-        /// <param name="userId">The user to lookup.</param>
-        /// <returns>The task responsible for the lookup.</returns>
-        private Task<List<User>> getQueryTaskForUser(int userId)
+        private readonly List<(int id, TaskCompletionSource<User>)> pendingUserTasks = new List<(int, TaskCompletionSource<User>)>();
+        private Task pendingRequestTask;
+        private readonly object taskAssignmentLock = new object();
+
+        private Task<User> queryUser(int userId)
         {
             lock (taskAssignmentLock)
             {
-                nextTaskIDs.Add(userId);
+                var tcs = new TaskCompletionSource<User>();
 
-                // if there's a pending request which hasn't been started yet (and is not yet full), we can wait on it.
-                if (pendingRequest != null && !pendingRequestConsumedIDs && nextTaskIDs.Count < 50)
-                    return pendingRequest;
+                // Add to the queue.
+                pendingUserTasks.Add((userId, tcs));
 
-                return queueNextTask(nextLookup);
+                // Create a request task if there's not already one.
+                if (pendingRequestTask == null)
+                    createNewTask();
+
+                return tcs.Task;
+            }
         }
 
-            List<User> nextLookup()
+        private void performLookup()
         {
-                int[] lookupItems;
+            var userTasks = new List<(int id, TaskCompletionSource<User> task)>();
 
+            // Grab at most 50 users from the queue.
             lock (taskAssignmentLock)
             {
-                    pendingRequestConsumedIDs = true;
-                    lookupItems = nextTaskIDs.ToArray();
-                    nextTaskIDs.Clear();
-
-                    if (lookupItems.Length == 0)
+                while (pendingUserTasks.Count > 0 && userTasks.Count < 50)
                 {
-                        queueNextTask(null);
-                        return new List<User>();
+                    (int id, TaskCompletionSource<User> task) next = pendingUserTasks[^1];
+
+                    pendingUserTasks.RemoveAt(pendingUserTasks.Count - 1);
+
+                    // Perform a secondary check for existence, in case the user was queried in a previous batch.
+                    if (CheckExists(next.id, out var existing))
+                        next.task.SetResult(existing);
+                    else
+                        userTasks.Add(next);
                 }
             }
 
-                var request = new GetUsersRequest(lookupItems);
+            // Query the users.
+            var request = new GetUsersRequest(userTasks.Select(t => t.id).ToArray());
 
             // rather than queueing, we maintain our own single-threaded request stream.
             api.Perform(request);
 
-                return request.Result?.Users;
-            }
-        }
-
-        /// <summary>
-        /// Queues new work at the end of the current work tasks.
-        /// Ensures the provided work is eventually run.
-        /// </summary>
-        /// <param name="work">The work to run. Can be null to signify the end of available work.</param>
-        /// <returns>The task tracking this work.</returns>
-        private Task<List<User>> queueNextTask(Func<List<User>> work)
-        {
+            // Create a new request task if there's still more users to query.
             lock (taskAssignmentLock)
             {
-                if (work == null)
-                {
-                    pendingRequest = null;
-                    pendingRequestConsumedIDs = false;
-                }
-                else if (pendingRequest == null)
-                {
-                    // special case for the first request ever.
-                    pendingRequest = Task.Run(work);
-                    pendingRequestConsumedIDs = false;
-                }
-                else
-                {
-                    // append the new request on to the last to be executed.
-                    pendingRequest = pendingRequest.ContinueWith(_ => work());
-                    pendingRequestConsumedIDs = false;
+                pendingRequestTask = null;
+                if (pendingUserTasks.Count > 0)
+                    createNewTask();
             }
 
-                return pendingRequest;
-            }
+            // Notify of completion.
+            foreach (var (id, task) in userTasks)
+                task.SetResult(request.Result?.Users?.FirstOrDefault(u => u.Id == id));
         }
+
+        private void createNewTask() => pendingRequestTask = Task.Run(performLookup);
     }
 }