From de2c2bd60fdce52cc7ba38a25f3e9436442af604 Mon Sep 17 00:00:00 2001
From: Luke Gruber <luke.gruber@shopify.com>
Date: Thu, 4 Dec 2025 16:10:21 -0500
Subject: [PATCH] Take VM lock in `class_switch_superclass` (#15356)

Safe multi-ractor subclass list mutation

We need to lock around mutation and accesses of a class's subclasses
list. Unfortunately we also need to do this when creating singleton
classes, as the singleton class does need to go into `super`'s
subclasses list for CC invalidation purposes.
---
 class.c                 |  6 ++++--
 test/ruby/test_class.rb | 13 +++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/class.c b/class.c
index 0cec526b74..9716ba07da 100644
--- a/class.c
+++ b/class.c
@@ -734,8 +734,10 @@ class_detach_subclasses(VALUE klass, VALUE arg)
 static void
 class_switch_superclass(VALUE super, VALUE klass)
 {
-    class_detach_subclasses(klass, Qnil);
-    rb_class_subclass_add(super, klass);
+    RB_VM_LOCKING() {
+        class_detach_subclasses(klass, Qnil);
+        rb_class_subclass_add(super, klass);
+    }
 }
 
 /**
diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 10b7655e9a..1faecd0cb2 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -917,4 +917,17 @@ CODE
       end
     end;
   end
+
+  def test_safe_multi_ractor_subclasses_list_mutation
+    assert_ractor "#{<<~"begin;"}\n#{<<~'end;'}"
+    begin;
+      4.times.map do
+        Ractor.new do
+          20_000.times do
+            Object.new.singleton_class
+          end
+        end
+      end.each(&:join)
+    end;
+  end
 end