From 11dd7f12a5cd8cbc604f2a1693d0cf5801f3c5f4 Mon Sep 17 00:00:00 2001
From: Vadim Kononov <vadim@poetic.com>
Date: Mon, 28 Aug 2023 12:31:10 -0500
Subject: [PATCH] Fix defining object finalizer in Ruby 3.1 to reference a UUID
 string

In Ruby 3.1, accessing callback_queue from a finalizer causes an "undefined
local variable or method `callback_queue' for FiniteMachine::Observer:Class
(NameError)", and callback_queue is never cleaned up. However, making the
finalizer an instance method rather than a class method throws a warning
about the improper use of finalizers. Hence, using the UUID string as
an object to check before it is destroyed by garbage collection for when
to call the finalizer proc.

---------

Co-authored-by: Piotr Murach <pmurach@gmail.com>
---
 CHANGELOG.md                   |  4 +++
 lib/finite_machine/observer.rb | 50 +++++++++++++++++++++-------------
 2 files changed, 35 insertions(+), 19 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b181f60..b251dbf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Change Log
 
+### Fixed
+* Fix defining object finalizer in Ruby 3.1 to reference a UUID string
+  by Vadim Kononov(@vkononov)
+
 ## [v0.14.0] - 2020-09-12
 
 ### Added
diff --git a/lib/finite_machine/observer.rb b/lib/finite_machine/observer.rb
index 4591d57..ec31b2a 100644
--- a/lib/finite_machine/observer.rb
+++ b/lib/finite_machine/observer.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "securerandom"
+
 require_relative "async_call"
 require_relative "callable"
 require_relative "hook_event"
@@ -13,20 +15,6 @@ module FiniteMachine
   class Observer < GenericDSL
     include Safety
 
-    # Clean up callback queue
-    #
-    # @api private
-    def self.cleanup_callback_queue
-      proc do
-        begin
-          if callback_queue.alive?
-            callback_queue.shutdown
-          end
-        rescue MessageQueueDeadError
-        end
-      end
-    end
-
     # The current state machine
     attr_reader :machine
 
@@ -44,11 +32,6 @@ def initialize(machine)
       @hooks   = Hooks.new
 
       @machine.subscribe(self)
-      ObjectSpace.define_finalizer(self, self.class.cleanup_callback_queue)
-    end
-
-    def callback_queue
-      @callback_queue ||= MessageQueue.new
     end
 
     # Evaluate in current context
@@ -204,6 +187,35 @@ def defer(callable, trans_event, *data)
       callback_queue << async_call
     end
 
+    # Get an existing callback queue or create a new one
+    #
+    # @return [FiniteMachine::MessageQueue]
+    #
+    # @api private
+    def callback_queue
+      @callback_queue ||= MessageQueue.new.tap do
+        @queue_id = SecureRandom.uuid
+        ObjectSpace.define_finalizer(@queue_id, proc do
+          cleanup_callback_queue
+        end)
+      end
+    end
+
+    # Clean up the callback queue
+    #
+    # @return [Boolean, nil]
+    #
+    # @api private
+    def cleanup_callback_queue
+      ObjectSpace.undefine_finalizer(@queue_id) if @queue_id
+      return unless @callback_queue && callback_queue.alive?
+
+      begin
+        callback_queue.shutdown
+      rescue MessageQueueDeadError
+      end
+    end
+
     # Create callable instance
     #
     # @api private