From fdf025942b526ad34ce1a8cd25bd55f0acb6980c Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Tue, 8 Dec 2020 01:24:38 +0900
Subject: [PATCH] Ensure room is locked when mutating users

---
 .../RealtimeMultiplayer/MultiplayerRoom.cs    | 25 ++++++++++++++++---
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs b/osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs
index f42f8d7f5f..1a02dab5e0 100644
--- a/osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs
+++ b/osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs
@@ -9,16 +9,33 @@ namespace osu.Game.Online.RealtimeMultiplayer
     [Serializable]
     public class MultiplayerRoom
     {
+        private object writeLock = new object();
+
         public long RoomID { get; set; }
 
         public MultiplayerRoomState State { get; set; }
 
-        public IReadOnlyList<MultiplayerRoomUser> Users => users;
-
         private List<MultiplayerRoomUser> users = new List<MultiplayerRoomUser>();
 
-        public void Join(int user) => users.Add(new MultiplayerRoomUser(user));
+        public IReadOnlyList<MultiplayerRoomUser> Users
+        {
+            get
+            {
+                lock (writeLock)
+                    return users.ToArray();
+            }
+        }
 
-        public void Leave(int user) => users.RemoveAll(u => u.UserID == user);
+        public void Join(int user)
+        {
+            lock (writeLock)
+                users.Add(new MultiplayerRoomUser(user));
+        }
+
+        public void Leave(int user)
+        {
+            lock (writeLock)
+                users.RemoveAll(u => u.UserID == user);
+        }
     }
 }