Ensure realm subscriptions always fire initial callback with null ChangeSet

osu.Game.Tests/Database/RealmSubscriptionRegistrationTests.cs

@@ -71,6 +71,35 @@ void onChanged(IRealmCollection<BeatmapSetInfo> sender, ChangeSet? changes)
             }
         }
 
+        [Test]
+        public void TestSubscriptionInitialChangeSetNull()
+        {
+            ChangeSet? firstChanges = null;
+            int receivedChangesCount = 0;
+
+            RunTestWithRealm((realm, _) =>
+            {
+                var registration = realm.RegisterForNotifications(r => r.All<BeatmapSetInfo>(), onChanged);
+
+                realm.WriteAsync(r => r.Add(TestResources.CreateTestBeatmapSetInfo())).WaitSafely();
+
+                realm.Run(r => r.Refresh());
+
+                Assert.That(receivedChangesCount, Is.EqualTo(1));
+                Assert.That(firstChanges, Is.Null);
+
+                registration.Dispose();
+            });
+
+            void onChanged(IRealmCollection<BeatmapSetInfo> sender, ChangeSet? changes)
+            {
+                if (receivedChangesCount == 0)
+                    firstChanges = changes;
+
+                receivedChangesCount++;
+            }
+        }
+
         [Test]
         public void TestSubscriptionWithAsyncWrite()
         {

osu.Game/Database/RealmObjectExtensions.cs

@@ -65,7 +65,8 @@ public static class RealmObjectExtensions
                          if (!d.Beatmaps.Contains(existingBeatmap))
                          {
                              Debug.Fail("Beatmaps should never become detached under normal circumstances. If this ever triggers, it should be investigated further.");
-                             Logger.Log("WARNING: One of the difficulties in a beatmap was detached from its set. Please save a copy of logs and report this to devs.", LoggingTarget.Database, LogLevel.Important);
+                             Logger.Log("WARNING: One of the difficulties in a beatmap was detached from its set. Please save a copy of logs and report this to devs.", LoggingTarget.Database,
+                                 LogLevel.Important);
                              d.Beatmaps.Add(existingBeatmap);
                          }
 
@@ -291,7 +292,24 @@ public static IDisposable QueryAsyncWithNotifications<T>(this IRealmCollection<T
             if (!RealmAccess.CurrentThreadSubscriptionsAllowed)
                 throw new InvalidOperationException($"Make sure to call {nameof(RealmAccess)}.{nameof(RealmAccess.RegisterForNotifications)}");
 
-            return collection.SubscribeForNotifications(callback);
+            bool initial = true;
+            return collection.SubscribeForNotifications(((sender, changes) =>
+            {
+                if (initial)
+                {
+                    initial = false;
+
+                    // Realm might coalesce the initial callback, meaning we never receive a `ChangeSet` of `null` marking the first callback.
+                    // Let's decouple it for simplicity in handling.
+                    if (changes != null)
+                    {
+                        callback(sender, null);
+                        return;
+                    }
+                }
+
+                callback(sender, changes);
+            }));
         }
 
         /// <summary>