Allow realm subscriptions to be initiated from a non-update thread

osu.Game/Database/RealmAccess.cs

@@ -1,6 +1,8 @@
 // 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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
@@ -17,6 +19,7 @@ using osu.Framework.Input.Bindings;
 using osu.Framework.Logging;
 using osu.Framework.Platform;
 using osu.Framework.Statistics;
+using osu.Framework.Threading;
 using osu.Game.Beatmaps;
 using osu.Game.Configuration;
 using osu.Game.Input.Bindings;
@@ -28,8 +31,6 @@ using osu.Game.Stores;
 using Realms;
 using Realms.Exceptions;
 
-#nullable enable
-
 namespace osu.Game.Database
 {
     /// <summary>
@@ -46,6 +47,8 @@ namespace osu.Game.Database
 
         private readonly IDatabaseContextFactory? efContextFactory;
 
+        private readonly SynchronizationContext? updateThreadSyncContext;
+
         /// <summary>
         /// Version history:
         /// 6    ~2021-10-18   First tracked version.
@@ -143,12 +146,15 @@ namespace osu.Game.Database
         /// </summary>
         /// <param name="storage">The game storage which will be used to create the realm backing file.</param>
         /// <param name="filename">The filename to use for the realm backing file. A ".realm" extension will be added automatically if not specified.</param>
+        /// <param name="updateThread">The game update thread, used to post realm operations into a thread-safe context.</param>
         /// <param name="efContextFactory">An EF factory used only for migration purposes.</param>
-        public RealmAccess(Storage storage, string filename, IDatabaseContextFactory? efContextFactory = null)
+        public RealmAccess(Storage storage, string filename, GameThread? updateThread = null, IDatabaseContextFactory? efContextFactory = null)
         {
             this.storage = storage;
             this.efContextFactory = efContextFactory;
 
+            updateThreadSyncContext = updateThread?.SynchronizationContext ?? SynchronizationContext.Current;
+
             Filename = filename;
 
             if (!Filename.EndsWith(realm_extension, StringComparison.Ordinal))
@@ -379,9 +385,6 @@ namespace osu.Game.Database
         public IDisposable RegisterForNotifications<T>(Func<Realm, IQueryable<T>> query, NotificationCallbackDelegate<T> callback)
             where T : RealmObjectBase
         {
-            if (!ThreadSafety.IsUpdateThread)
-                throw new InvalidOperationException(@$"{nameof(RegisterForNotifications)} must be called from the update thread.");
-
             lock (realmLock)
             {
                 Func<Realm, IDisposable?> action = realm => query(realm).QueryAsyncWithNotifications(callback);
@@ -459,23 +462,24 @@ namespace osu.Game.Database
         /// <returns>An <see cref="IDisposable"/> which should be disposed to unsubscribe any inner subscription.</returns>
         public IDisposable RegisterCustomSubscription(Func<Realm, IDisposable?> action)
         {
-            if (!ThreadSafety.IsUpdateThread)
+            if (updateThreadSyncContext == null)
-                throw new InvalidOperationException(@$"{nameof(RegisterForNotifications)} must be called from the update thread.");
+                throw new InvalidOperationException("Attempted to register a realm subscription before update thread registration.");
-
-            var syncContext = SynchronizationContext.Current;
 
             total_subscriptions.Value++;
 
-            registerSubscription(action);
+            if (ThreadSafety.IsUpdateThread)
+                updateThreadSyncContext.Send(_ => registerSubscription(action), null);
+            else
+                updateThreadSyncContext.Post(_ => registerSubscription(action), null);
 
             // This token is returned to the consumer.
             // When disposed, it will cause the registration to be permanently ceased (unsubscribed with realm and unregistered by this class).
             return new InvokeOnDisposal(() =>
             {
                 if (ThreadSafety.IsUpdateThread)
-                    syncContext.Send(_ => unsubscribe(), null);
+                    updateThreadSyncContext.Send(_ => unsubscribe(), null);
                 else
-                    syncContext.Post(_ => unsubscribe(), null);
+                    updateThreadSyncContext.Post(_ => unsubscribe(), null);
 
                 void unsubscribe()
                 {

osu.Game/OsuGameBase.cs

@@ -200,7 +200,7 @@ namespace osu.Game
             if (Storage.Exists(DatabaseContextFactory.DATABASE_NAME))
                 dependencies.Cache(EFContextFactory = new DatabaseContextFactory(Storage));
 
-            dependencies.Cache(realm = new RealmAccess(Storage, "client", EFContextFactory));
+            dependencies.Cache(realm = new RealmAccess(Storage, "client", Host.UpdateThread, EFContextFactory));
 
             dependencies.CacheAs<RulesetStore>(RulesetStore = new RealmRulesetStore(realm, Storage));
             dependencies.CacheAs<IRulesetStore>(RulesetStore);