From 0bcfb8e199004cb123598d9d5972d97ebf383cd2 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Mon, 29 Nov 2021 18:52:29 +0900
Subject: [PATCH 1/2] Fix regression in implementation of fix

---
 osu.Game/Database/RealmContextFactory.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/osu.Game/Database/RealmContextFactory.cs b/osu.Game/Database/RealmContextFactory.cs
index 6948918fe7..a20139e830 100644
--- a/osu.Game/Database/RealmContextFactory.cs
+++ b/osu.Game/Database/RealmContextFactory.cs
@@ -160,6 +160,7 @@ namespace osu.Game.Database
                 if (!currentThreadCanCreateContexts.Value)
                 {
                     contextCreationLock.Wait();
+                    currentThreadCanCreateContexts.Value = true;
                     tookSemaphoreLock = true;
                 }
                 else
@@ -167,7 +168,6 @@ namespace osu.Game.Database
                     // the semaphore is used to handle blocking of all context creation during certain periods.
                     // once the semaphore has been taken by this code section, it is safe to create further contexts on the same thread.
                     // this can happen if a realm subscription is active and triggers a callback which has user code that calls `CreateContext`.
-                    currentThreadCanCreateContexts.Value = true;
                 }
 
                 contexts_created.Value++;

From 566e10f8ccff29b3c0780832ec1811c13f1e4276 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Mon, 29 Nov 2021 18:54:01 +0900
Subject: [PATCH 2/2] Refactor test to be easier to follow

---
 osu.Game.Tests/Database/GeneralUsageTests.cs | 36 +++++++++++---------
 1 file changed, 19 insertions(+), 17 deletions(-)

diff --git a/osu.Game.Tests/Database/GeneralUsageTests.cs b/osu.Game.Tests/Database/GeneralUsageTests.cs
index fec1a0dedd..d15e34f2da 100644
--- a/osu.Game.Tests/Database/GeneralUsageTests.cs
+++ b/osu.Game.Tests/Database/GeneralUsageTests.cs
@@ -36,34 +36,36 @@ namespace osu.Game.Tests.Database
             });
         }
 
+        /// <summary>
+        /// Test to ensure that a `CreateContext` call nested inside a subscription doesn't cause any deadlocks
+        /// due to context fetching semaphores.
+        /// </summary>
         [Test]
-        public void TestNestedContextCreation()
+        public void TestNestedContextCreationWithSubscription()
         {
             RunTestWithRealm((realmFactory, _) =>
             {
-                var mainContext = realmFactory.Context;
                 bool callbackRan = false;
 
-                var subscription = mainContext.All<RealmBeatmap>().SubscribeForNotifications((sender, changes, error) =>
+                using (var context = realmFactory.CreateContext())
                 {
-                    realmFactory.CreateContext();
-                    callbackRan = true;
-                });
-
-                Task.Factory.StartNew(() =>
-                {
-                    using (var threadContext = realmFactory.CreateContext())
+                    var subscription = context.All<RealmBeatmap>().SubscribeForNotifications((sender, changes, error) =>
                     {
-                        threadContext.Write(r => r.Add(new RealmBeatmap(CreateRuleset(), new RealmBeatmapDifficulty(), new RealmBeatmapMetadata())));
-                    }
-                }, TaskCreationOptions.LongRunning | TaskCreationOptions.HideScheduler).Wait();
+                        using (realmFactory.CreateContext())
+                        {
+                            callbackRan = true;
+                        }
+                    });
 
-                // will create a context but also run the callback above (Refresh is implicitly run when getting a new context).
-                realmFactory.CreateContext();
+                    // Force the callback above to run.
+                    using (realmFactory.CreateContext())
+                    {
+                    }
+
+                    subscription.Dispose();
+                }
 
                 Assert.IsTrue(callbackRan);
-
-                subscription.Dispose();
             });
         }