From 4a00662092a13cd1e6352400ec76403dff80f657 Mon Sep 17 00:00:00 2001
From: Dan Balasescu <smoogipoo@smgi.me>
Date: Tue, 4 Mar 2025 14:02:45 +0900
Subject: [PATCH] Fix thread safety when kicking multiplayer users

---
 .../Online/Multiplayer/MultiplayerClient.cs   | 61 ++++++++++---------
 .../OnlinePlay/Multiplayer/Multiplayer.cs     |  8 +--
 2 files changed, 35 insertions(+), 34 deletions(-)

diff --git a/osu.Game/Online/Multiplayer/MultiplayerClient.cs b/osu.Game/Online/Multiplayer/MultiplayerClient.cs
index 59a4547e9e..91b4ed448c 100644
--- a/osu.Game/Online/Multiplayer/MultiplayerClient.cs
+++ b/osu.Game/Online/Multiplayer/MultiplayerClient.cs
@@ -486,18 +486,44 @@ namespace osu.Game.Online.Multiplayer
             }, false);
         }
 
-        Task IMultiplayerClient.UserLeft(MultiplayerRoomUser user) =>
-            handleUserLeft(user, UserLeft);
+        Task IMultiplayerClient.UserLeft(MultiplayerRoomUser user)
+        {
+            Scheduler.Add(() => handleUserLeft(user, UserLeft), false);
+            return Task.CompletedTask;
+        }
 
         Task IMultiplayerClient.UserKicked(MultiplayerRoomUser user)
         {
-            if (LocalUser == null)
-                return Task.CompletedTask;
+            Scheduler.Add(() =>
+            {
+                if (LocalUser == null)
+                    return;
 
-            if (user.Equals(LocalUser))
-                LeaveRoom();
+                if (user.Equals(LocalUser))
+                    LeaveRoom();
 
-            return handleUserLeft(user, UserKicked);
+                handleUserLeft(user, UserKicked);
+            }, false);
+
+            return Task.CompletedTask;
+        }
+
+        private void handleUserLeft(MultiplayerRoomUser user, Action<MultiplayerRoomUser>? callback)
+        {
+            Debug.Assert(ThreadSafety.IsUpdateThread);
+
+            if (Room == null)
+                return;
+
+            Room.Users.Remove(user);
+            PlayingUserIds.Remove(user.UserID);
+
+            Debug.Assert(APIRoom != null);
+            APIRoom.RecentParticipants = APIRoom.RecentParticipants.Where(u => u.Id != user.UserID).ToArray();
+            APIRoom.ParticipantCount--;
+
+            callback?.Invoke(user);
+            RoomUpdated?.Invoke();
         }
 
         async Task IMultiplayerClient.Invited(int invitedBy, long roomID, string password)
@@ -544,27 +570,6 @@ namespace osu.Game.Online.Multiplayer
             APIRoom.ParticipantCount++;
         }
 
-        private Task handleUserLeft(MultiplayerRoomUser user, Action<MultiplayerRoomUser>? callback)
-        {
-            Scheduler.Add(() =>
-            {
-                if (Room == null)
-                    return;
-
-                Room.Users.Remove(user);
-                PlayingUserIds.Remove(user.UserID);
-
-                Debug.Assert(APIRoom != null);
-                APIRoom.RecentParticipants = APIRoom.RecentParticipants.Where(u => u.Id != user.UserID).ToArray();
-                APIRoom.ParticipantCount--;
-
-                callback?.Invoke(user);
-                RoomUpdated?.Invoke();
-            }, false);
-
-            return Task.CompletedTask;
-        }
-
         Task IMultiplayerClient.HostChanged(int userId)
         {
             Scheduler.Add(() =>
diff --git a/osu.Game/Screens/OnlinePlay/Multiplayer/Multiplayer.cs b/osu.Game/Screens/OnlinePlay/Multiplayer/Multiplayer.cs
index dfed32aebc..0b06a16d98 100644
--- a/osu.Game/Screens/OnlinePlay/Multiplayer/Multiplayer.cs
+++ b/osu.Game/Screens/OnlinePlay/Multiplayer/Multiplayer.cs
@@ -28,11 +28,9 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
 
         private void onRoomUpdated()
         {
-            if (client.Room == null)
+            if (client.Room == null || client.LocalUser == null)
                 return;
 
-            Debug.Assert(client.LocalUser != null);
-
             // If the user exits gameplay before score submission completes, we'll transition to idle when results has been prepared.
             if (client.LocalUser.State == MultiplayerUserState.Results && this.IsCurrentScreen())
                 transitionFromResults();
@@ -62,11 +60,9 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
         {
             base.OnResuming(e);
 
-            if (client.Room == null)
+            if (client.Room == null || client.LocalUser == null)
                 return;
 
-            Debug.Assert(client.LocalUser != null);
-
             if (!(e.Last is MultiplayerPlayerLoader playerLoader))
                 return;