Fix nonatomic database update potentially causing project star counts to go negative

The counter_cache decrement function is called when a project star is deleted,
but there was no guarantee multiple workers would not attempt to delete the
same item simultaneously. Use an atomic update to prevent the count from going negative.

Closes #3067

CHANGELOG

@@ -5,6 +5,7 @@ v 8.2.0 (unreleased)
   - Highlight comment based on anchor in URL
 
 v 8.1.0 (unreleased)
+  - Fix nonatomic database update potentially causing project star counts to go negative (Stan Hu)
   - Fix error preventing displaying of commit data for a directory with a leading dot (Stan Hu)
   - Speed up load times of issue detail pages by roughly 1.5x
   - Add a system note and update relevant merge requests when a branch is deleted or re-added (Stan Hu)

app/models/user.rb

@@ -706,14 +706,17 @@ class User < ActiveRecord::Base
   end
 
   def toggle_star(project)
+    UsersStarProject.transaction do
       user_star_project = users_star_projects.
-      where(project: project, user: self).take
+          where(project: project, user: self).lock(true).first
+
       if user_star_project
         user_star_project.destroy
       else
         UsersStarProject.create!(project: project, user: self)
       end
     end
+  end
 
   def manageable_namespaces
     @manageable_namespaces ||= [namespace] + owned_groups + masters_groups