Avoid leaking the first relation we call #first on

With the previous implementation, the block passed to
define_singleton_method, which will live forever as the method body,
captures the parameters (args and block) in its enclosure.

For the current_scope registry, that can include an AR::Relation.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Avoid leaking the first relation we call `first` on, per model.
+
+    Fixes #21921.
+
+    *Matthew Draper*, *Jean Boussier*
+
 *   Remove unused `pk_and_sequence_for` in AbstractMysqlAdapter.
 
     *Ryuta Kamizono*

activesupport/lib/active_support/per_thread_registry.rb

@@ -43,9 +43,9 @@ def instance
     protected
       def method_missing(name, *args, &block) # :nodoc:
         # Caches the method definition as a singleton method of the receiver.
-        define_singleton_method(name) do |*a, &b|
-          instance.public_send(name, *a, &b)
-        end
+        #
+        # By letting #delegate handle it, we avoid an enclosure that'll capture args.
+        singleton_class.delegate name, to: :instance
 
         send(name, *args, &block)
       end