Add test coverage of potential deeadlock scenario with nested realm context fetching

osu.Game.Tests/Database/GeneralUsageTests.cs

@@ -5,6 +5,9 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 using NUnit.Framework;
+using osu.Game.Database;
+using osu.Game.Models;
+using Realms;
 
 #nullable enable
 
@@ -33,6 +36,37 @@ namespace osu.Game.Tests.Database
             });
         }
 
+        [Test]
+        public void TestNestedContextCreation()
+        {
+            RunTestWithRealm((realmFactory, _) =>
+            {
+                var mainContext = realmFactory.Context;
+                bool callbackRan = false;
+
+                var subscription = mainContext.All<RealmBeatmap>().SubscribeForNotifications((sender, changes, error) =>
+                {
+                    realmFactory.CreateContext();
+                    callbackRan = true;
+                });
+
+                Task.Factory.StartNew(() =>
+                {
+                    using (var threadContext = realmFactory.CreateContext())
+                    {
+                        threadContext.Write(r => r.Add(new RealmBeatmap(CreateRuleset(), new RealmBeatmapDifficulty(), new RealmBeatmapMetadata())));
+                    }
+                }, TaskCreationOptions.LongRunning | TaskCreationOptions.HideScheduler).Wait();
+
+                // will create a context but also run the callback above (Refresh is implicitly run when getting a new context).
+                realmFactory.CreateContext();
+
+                Assert.IsTrue(callbackRan);
+
+                subscription.Dispose();
+            });
+        }
+
         [Test]
         public void TestBlockOperationsWithContention()
         {