Add C extension spec for rb_undefine_finalizer

Author: peterzhu2118

optional/capi/ext/finalizer_spec.c

@@ -9,10 +9,15 @@ static VALUE define_finalizer(VALUE self, VALUE obj, VALUE finalizer) {
   return rb_define_finalizer(obj, finalizer);
 }
 
+static VALUE undefine_finalizer(VALUE self, VALUE obj) {
+  return rb_undefine_finalizer(obj);
+}
+
 void Init_finalizer_spec(void) {
   VALUE cls = rb_define_class("CApiFinalizerSpecs", rb_cObject);
 
   rb_define_method(cls, "rb_define_finalizer", define_finalizer, 2);
+  rb_define_method(cls, "rb_undefine_finalizer", undefine_finalizer, 1);
 }
 
 #ifdef __cplusplus

optional/capi/finalizer_spec.rb

@@ -20,4 +20,21 @@
       ruby_exe(code).should == "finalizer run\n"
     end
   end
+
+  describe "rb_undefine_finalizer" do
+    ruby_bug "#20981", "3.4.0"..."3.4.2" do
+      it "removes finalizers from the object" do
+        code = <<~RUBY
+          require #{extension_path.dump}
+
+          obj = Object.new
+          finalizer = Proc.new { puts "finalizer run" }
+          ObjectSpace.define_finalizer(obj, finalizer)
+          CApiFinalizerSpecs.new.rb_undefine_finalizer(obj)
+        RUBY
+
+        ruby_exe(code).should.empty?
+      end
+    end
+  end
 end