Make BackgroundMigrationWorker backward compatible

changelogs/unreleased/backward-compatibility-for-background-migrations.yml

+---
+title: Make BackgroundMigrationWorker backward compatible
+merge_request: 22271
+author:
+type: fixed

lib/gitlab/background_migration.rb

@@ -78,6 +78,20 @@ module Gitlab
     end
 
     def self.migration_class_for(class_name)
+      # We don't pass class name with Gitlab::BackgroundMigration:: prefix anymore
+      # but some jobs could be already spawned so we need to have some backward compatibility period.
+      # Can be removed since 13.x
+      full_class_name_prefix_regexp = /\A(::)?Gitlab::BackgroundMigration::/
+
+      if class_name.match(full_class_name_prefix_regexp)
+        Gitlab::ErrorTracking.track_and_raise_for_dev_exception(
+          StandardError.new("Full class name is used"),
+          class_name: class_name
+        )
+
+        class_name = class_name.sub(full_class_name_prefix_regexp, '')
+      end
+
       const_get(class_name, false)
     end
 

spec/lib/gitlab/background_migration_spec.rb

@@ -152,6 +152,17 @@ describe Gitlab::BackgroundMigration do
 
       described_class.perform('Foo', [10, 20])
     end
+
+    context 'backward compatibility' do
+      it 'performs a background migration for fully-qualified job classes' do
+        expect(migration).to receive(:perform).with(10, 20).once
+        expect(Gitlab::ErrorTracking)
+          .to receive(:track_and_raise_for_dev_exception)
+          .with(instance_of(StandardError), hash_including(:class_name))
+
+        described_class.perform('Gitlab::BackgroundMigration::Foo', [10, 20])
+      end
+    end
   end
 
   describe '.exists?' do