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

1 files changed, 9 insertions, 6 deletions

diff --git a/app/models/user.rb b/app/models/user.rb
index 17ccb3b8788..3b346c55edb 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -706,12 +706,15 @@ class User < ActiveRecord::Base
   end
 
   def toggle_star(project)
-    user_star_project = users_star_projects.
-      where(project: project, user: self).take
-    if user_star_project
-      user_star_project.destroy
-    else
-      UsersStarProject.create!(project: project, user: self)
+    UsersStarProject.transaction do
+      user_star_project = users_star_projects.
+          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