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

2 files changed, 10 insertions, 6 deletions

diff --git a/CHANGELOG b/CHANGELOG
index a1161a7a527..2c8df909441 100644
--- a/CHANGELOG
+++ b/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)
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