1 files changed, 52 insertions, 0 deletions
diff --git a/lib/gitlab/background_migration/cleanup_concurrent_schema_change.rb b/lib/gitlab/background_migration/cleanup_concurrent_schema_change.rb
new file mode 100644
index 00000000000..54f77f184d5
--- /dev/null
+++ b/lib/gitlab/background_migration/cleanup_concurrent_schema_change.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # Base class for cleaning up concurrent schema changes.
+    class CleanupConcurrentSchemaChange
+      include Database::MigrationHelpers
+
+      # table - The name of the table the migration is performed for.
+      # old_column - The name of the old (to drop) column.
+      # new_column - The name of the new column.
+      def perform(table, old_column, new_column)
+        return unless column_exists?(table, new_column)
+
+        rows_to_migrate = define_model_for(table)
+          .where(new_column => nil)
+          .where
+          .not(old_column => nil)
+
+        if rows_to_migrate.any?
+          BackgroundMigrationWorker.perform_in(
+            RESCHEDULE_DELAY,
+            self.class.name,
+            [table, old_column, new_column]
+          )
+        else
+          cleanup_concurrent_schema_change(table, old_column, new_column)
+        end
+      end
+
+      # These methods are necessary so we can re-use the migration helpers in
+      # this class.
+      def connection
+        ActiveRecord::Base.connection
+      end
+
+      def method_missing(name, *args, &block)
+        connection.__send__(name, *args, &block) # rubocop: disable GitlabSecurity/PublicSend
+      end
+
+      def respond_to_missing?(*args)
+        connection.respond_to?(*args) || super
+      end
+
+      def define_model_for(table)
+        Class.new(ActiveRecord::Base) do
+          self.table_name = table
+        end
+      end
+    end
+  end
+end