Change locking method to better allow cross-thread locking

osu.Game/Online/RealtimeMultiplayer/LockUntilDisposal.cs

@@ -1,24 +0,0 @@
-// 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.Threading;
-
-namespace osu.Game.Online.RealtimeMultiplayer
-{
-    public readonly struct LockUntilDisposal : IDisposable
-    {
-        private readonly object lockTarget;
-
-        public LockUntilDisposal(object lockTarget)
-        {
-            this.lockTarget = lockTarget;
-            Monitor.Enter(lockTarget);
-        }
-
-        public void Dispose()
-        {
-            Monitor.Exit(lockTarget);
-        }
-    }
-}

osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs

@@ -5,7 +5,9 @@
 
 using System;
 using System.Collections.Generic;
+using System.Threading;
 using Newtonsoft.Json;
+using osu.Framework.Allocation;
 
 namespace osu.Game.Online.RealtimeMultiplayer
 {
@@ -48,10 +50,26 @@ namespace osu.Game.Online.RealtimeMultiplayer
             RoomID = roomId;
         }
 
+        private object updateLock = new object();
+
+        private ManualResetEventSlim freeForWrite = new ManualResetEventSlim(true);
+
         /// <summary>
         /// Request a lock on this room to perform a thread-safe update.
         /// </summary>
-        public LockUntilDisposal LockForUpdate() => new LockUntilDisposal(writeLock);
+        public IDisposable LockForUpdate()
+        {
+            // ReSharper disable once InconsistentlySynchronizedField
+            freeForWrite.Wait();
+
+            lock (updateLock)
+            {
+                freeForWrite.Wait();
+                freeForWrite.Reset();
+
+                return new ValueInvokeOnDisposal<MultiplayerRoom>(this, r => freeForWrite.Set());
+            }
+        }
 
         public override string ToString() => $"RoomID:{RoomID} Host:{Host?.UserID} Users:{Users.Count} State:{State} Settings: [{Settings}]";
     }