FIX: add mutex around un/mark as solved

To prevent any race conditions when two users toggle the state of a reply.

(cf. https://meta.discourse.org/161104)

plugin.rb

@@ -77,6 +77,8 @@ SQL
 
     def self.accept_answer!(post, acting_user, topic: nil)
       topic ||= post.topic
+
+      DistributedMutex.synchronize("discourse_solved_toggle_answer_#{topic.id}") do
         accepted_id = topic.custom_fields["accepted_answer_post_id"].to_i
 
         if accepted_id > 0
@@ -157,9 +159,12 @@ SQL
 
         DiscourseEvent.trigger(:accepted_solution, post)
       end
+    end
 
     def self.unaccept_answer!(post, topic: nil)
       topic ||= post.topic
+
+      DistributedMutex.synchronize("discourse_solved_toggle_answer_#{topic.id}") do
         post.custom_fields["is_accepted_answer"] = nil
         topic.custom_fields["accepted_answer_post_id"] = nil
 
@@ -198,12 +203,13 @@ SQL
         DiscourseEvent.trigger(:unaccepted_solution, post)
       end
     end
+  end
 
   require_dependency "application_controller"
+
   class DiscourseSolved::AnswerController < ::ApplicationController
 
     def accept
-
       limit_accepts
 
       post = Post.find(params[:id].to_i)
@@ -219,7 +225,6 @@ SQL
     end
 
     def unaccept
-
       limit_accepts
 
       post = Post.find(params[:id].to_i)
@@ -234,12 +239,11 @@ SQL
     end
 
     def limit_accepts
-      unless current_user.staff?
+      return if current_user.staff?
       RateLimiter.new(nil, "accept-hr-#{current_user.id}", 20, 1.hour).performed!
       RateLimiter.new(nil, "accept-min-#{current_user.id}", 4, 30.seconds).performed!
     end
   end
-  end
 
   DiscourseSolved::Engine.routes.draw do
     post "/accept" => "answer#accept"
@@ -477,7 +481,6 @@ SQL
       end
 
       topic.user_id == current_user.id && !topic.closed
-
     end
   end
 
@@ -486,9 +489,7 @@ SQL
     attributes :can_accept_answer, :can_unaccept_answer, :accepted_answer
 
     def can_accept_answer
-      topic = (topic_view && topic_view.topic) || object.topic
-
-      if topic
+      if topic = (topic_view && topic_view.topic) || object.topic
         return scope.can_accept_answer?(topic, object) && object.post_number > 1 && !accepted_answer
       end
 
@@ -496,8 +497,7 @@ SQL
     end
 
     def can_unaccept_answer
-      topic = (topic_view && topic_view.topic) || object.topic
-      if topic
+      if topic = (topic_view && topic_view.topic) || object.topic
         scope.can_accept_answer?(topic, object) && (post_custom_fields["is_accepted_answer"] == 'true')
       end
     end