From 1a1b836ded242f56852db078f121d93a41ba7c22 Mon Sep 17 00:00:00 2001
From: Guewen Baconnier <guewen@gmail.com>
Date: Tue, 22 Mar 2022 13:36:46 +0100
Subject: [PATCH] Fix multi-threading bug not firing success callbacks (#778)

`@valid_transitions` is shared across threads, when a thread consumes a
transition, another thread which should execute the same transition for
another model instance may ignore its success callback.

This change avoid to share the same hash of transition across different
model instances. Each instance will have its own hash of transitions.
The hash itself is not subject to thread race conditions as the object ids
are unique. Also, the transitions for an object_id are cleaned after the
transition have been fired, otherwise the hash would grow infinitely.

See https://github.com/aasm/aasm/issues/448#issuecomment-377922368 for
an example of code to reproduce the multi-thread bug.

Fixes #448
---
 lib/aasm/core/event.rb | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/lib/aasm/core/event.rb b/lib/aasm/core/event.rb
index 77747f8..316b127 100644
--- a/lib/aasm/core/event.rb
+++ b/lib/aasm/core/event.rb
@@ -10,7 +10,7 @@ module AASM::Core
       @name = name
       @state_machine = state_machine
       @transitions = []
-      @valid_transitions = {}
+      @valid_transitions = Hash.new { |h, k| h[k] = {} }
       @guards = Array(options[:guard] || options[:guards] || options[:if])
       @unless = Array(options[:unless]) #TODO: This could use a better name
       @default_display_name = name.to_s.gsub(/_/, ' ').capitalize
@@ -79,8 +79,9 @@ module AASM::Core
 
     def fire_transition_callbacks(obj, *args)
       from_state = obj.aasm(state_machine.name).current_state
-      transition = @valid_transitions[from_state]
-      @valid_transitions[from_state].invoke_success_callbacks(obj, *args) if transition
+      transition = @valid_transitions[obj.object_id][from_state]
+      transition.invoke_success_callbacks(obj, *args) if transition
+      @valid_transitions.delete(obj.object_id)
     end
 
     def ==(event)
@@ -153,7 +154,7 @@ module AASM::Core
             result = transition
           else
             result = to_state || Array(transition.to).first
-            Array(transition.to).each {|to| @valid_transitions[to] = transition }
+            Array(transition.to).each {|to| @valid_transitions[obj.object_id][to] = transition }
             transition.execute(obj, *args)
           end