From c98451a7bf2a75af6dab4ed82818c80f72c994c8 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Mon, 29 Nov 2021 16:18:57 +0900
Subject: [PATCH] Fix potential deadlock on nested context creation requests

---
 osu.Game/Database/RealmContextFactory.cs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/osu.Game/Database/RealmContextFactory.cs b/osu.Game/Database/RealmContextFactory.cs
index 2bc77934a8..08d7580db7 100644
--- a/osu.Game/Database/RealmContextFactory.cs
+++ b/osu.Game/Database/RealmContextFactory.cs
@@ -52,6 +52,8 @@ namespace osu.Game.Database
         /// </summary>
         private readonly SemaphoreSlim contextCreationLock = new SemaphoreSlim(1);
 
+        private bool canCreateContexts;
+
         private static readonly GlobalStatistic<int> refreshes = GlobalStatistics.Get<int>(@"Realm", @"Dirty Refreshes");
         private static readonly GlobalStatistic<int> contexts_created = GlobalStatistics.Get<int>(@"Realm", @"Contexts (Created)");
 
@@ -153,7 +155,13 @@ namespace osu.Game.Database
 
             try
             {
-                contextCreationLock.Wait();
+                if (!canCreateContexts)
+                    contextCreationLock.Wait();
+
+                // the semaphore is used to stop all context creation.
+                // once the semaphore has been taken by this code section, it is safe to create further contexts.
+                // this can happen if a realm subscription is active and triggers a callback which has user code that calls `CreateContext`.
+                canCreateContexts = true;
 
                 contexts_created.Value++;
 
@@ -162,6 +170,7 @@ namespace osu.Game.Database
             finally
             {
                 contextCreationLock.Release();
+                canCreateContexts = false;
             }
         }