FIX: Prevent race condition when post processing post (#8819)

If a post is being cooked twice (for example after an edit), there is a
chance the 'raw' and 'cooked' column to be inconsistent. This reduces
the chances of that happening.

app/jobs/regular/process_post.rb

@@ -7,6 +7,7 @@ module Jobs
   class ProcessPost < ::Jobs::Base
 
     def execute(args)
+      DistributedMutex.synchronize("process_post_#{args[:post_id]}") do
         post = Post.find_by(id: args[:post_id])
         # two levels of deletion
         return unless post.present? && post.topic.present?
@@ -47,6 +48,7 @@ module Jobs
           end
         end
       end
+    end
 
     # onebox may have added some links, so extract them now
     def extract_links(post)