Implement final review comments from @DouweM and @rymai

- Have `Uniquify` take a block instead of a Proc/function. This is more
  idiomatic than passing around a function in Ruby.

- Block a user before moving their issues to the ghost user. This avoids a data
  race where an issue is created after the issues are migrated to the ghost user,
  and before the destroy takes place.

- No need to migrate issues (to the ghost user) in a transaction, because
  we're using `update_all`

- Other minor changes

1 files changed, 9 insertions, 4 deletions

diff --git a/app/services/users/destroy_service.rb b/app/services/users/destroy_service.rb
index bf1e49734cc..523279944ae 100644
--- a/app/services/users/destroy_service.rb
+++ b/app/services/users/destroy_service.rb
@@ -37,13 +37,18 @@ module Users
     end
 
     private
-    
+
     def move_issues_to_ghost_user(user)
+      # Block the user before moving issues to prevent a data race.
+      # If the user creates an issue after `move_issues_to_ghost_user`
+      # runs and before the user is destroyed, the destroy will fail with
+      # an exception. We block the user so that issues can't be created
+      # after `move_issues_to_ghost_user` runs and before the destroy happens.
+      user.block
+
       ghost_user = User.ghost
 
-      Issue.transaction do
-        user.issues.update_all(author_id: ghost_user.id)
-      end
+      user.issues.update_all(author_id: ghost_user.id)
 
       user.reload
     end