Fix concurrent unfollowing decrementing follower count more than once (#18527)

app/services/unfollow_service.rb

@@ -2,6 +2,8 @@
 
 class UnfollowService < BaseService
   include Payloadable
+  include Redisable
+  include Lockable
 
   # Unfollow and notify the remote user
   # @param [Account] source_account Where to unfollow from
@@ -13,8 +15,10 @@ class UnfollowService < BaseService
     @target_account = target_account
     @options        = options
 
+    with_lock("relationship:#{[source_account.id, target_account.id].sort.join(':')}") do
       unfollow! || undo_follow_request!
     end
+  end
 
   private