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

@@ -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

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