DEV: Put a mutex around `Upload.migrate_to_new_scheme`.

This ensures that only one migration is running at any given point in
time across the instances.

app/models/upload.rb

@@ -205,12 +205,19 @@ class Upload < ActiveRecord::Base
   def self.migrate_to_new_scheme(limit: nil)
     problems = []
 
+    DistributedMutex.synchronize("migrate_upload_to_new_scheme") do
       if SiteSetting.migrate_to_new_scheme
-      max_file_size_kb = [SiteSetting.max_image_size_kb, SiteSetting.max_attachment_size_kb].max.kilobytes
+        max_file_size_kb = [
+          SiteSetting.max_image_size_kb,
+          SiteSetting.max_attachment_size_kb
+        ].max.kilobytes
+
         local_store = FileStore::LocalStore.new
         db = RailsMultisite::ConnectionManagement.current_db
 
-      scope = Upload.by_users.where("url NOT LIKE '%/original/_X/%' AND url LIKE '%/uploads/#{db}%'").order(id: :desc)
+        scope = Upload.by_users
+          .where("url NOT LIKE '%/original/_X/%' AND url LIKE '%/uploads/#{db}%'")
+          .order(id: :desc)
 
         scope = scope.limit(limit) if limit
 
@@ -299,6 +306,7 @@ class Upload < ActiveRecord::Base
           end
         end
       end
+    end
 
     problems
   end