Add cascading namespace setting database migration helper

Add a migration helper to simplify adding a new cascading setting.
The helper will create the four required columns, 2 each in
namespace_settings and application_settings.

changelogs/unreleased/dblessing_cascading_setting_migration_helper.yml

+---
+title: Add cascading namespace setting database migration helper
+merge_request: 58940
+author:
+type: added

lib/gitlab/database/migration_helpers/cascading_namespace_settings.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module MigrationHelpers
+      module CascadingNamespaceSettings
+        include Gitlab::Database::MigrationHelpers
+
+        # Creates the four required columns that constitutes a single cascading
+        # namespace settings attribute. This helper is only appropriate if the
+        # setting is not already present as a non-cascading attribute.
+        #
+        # Creates the `setting_name` column along with the `lock_setting_name`
+        # column in both `namespace_settings` and `application_settings`.
+        #
+        # This helper is not reversible and must be defined in conjunction with
+        # `remove_cascading_namespace_setting` in separate up and down directions.
+        #
+        # setting_name - The name of the cascading attribute - same as defined
+        #                in `NamespaceSetting` with the `cascading_attr` method.
+        # type - The column type for the setting itself (:boolean, :integer, etc.)
+        # options - Standard Rails column options hash. Accepts keys such as
+        #           `null` and `default`.
+        #
+        # `null` and `default` options will only be applied to the `application_settings`
+        # column. In most cases, a non-null default value should be specified.
+        def add_cascading_namespace_setting(setting_name, type, **options)
+          lock_column_name = "lock_#{setting_name}".to_sym
+
+          check_cascading_namespace_setting_consistency(setting_name, lock_column_name)
+
+          namespace_options = options.merge(null: true, default: nil)
+
+          with_lock_retries do
+            add_column(:namespace_settings, setting_name, type, namespace_options)
+            add_column(:namespace_settings, lock_column_name, :boolean, default: false, null: false)
+          end
+
+          add_column(:application_settings, setting_name, type, options)
+          add_column(:application_settings, lock_column_name, :boolean, default: false, null: false)
+        end
+
+        def remove_cascading_namespace_setting(setting_name)
+          lock_column_name = "lock_#{setting_name}".to_sym
+
+          with_lock_retries do
+            remove_column(:namespace_settings, setting_name) if column_exists?(:namespace_settings, setting_name)
+            remove_column(:namespace_settings, lock_column_name) if column_exists?(:namespace_settings, lock_column_name)
+          end
+
+          remove_column(:application_settings, setting_name) if column_exists?(:application_settings, setting_name)
+          remove_column(:application_settings, lock_column_name) if column_exists?(:application_settings, lock_column_name)
+        end
+
+        private
+
+        def check_cascading_namespace_setting_consistency(setting_name, lock_name)
+          existing_columns = []
+
+          %w(namespace_settings application_settings).each do |table|
+            existing_columns << "#{table}.#{setting_name}" if column_exists?(table.to_sym, setting_name)
+            existing_columns << "#{table}.#{lock_name}" if column_exists?(table.to_sym, lock_name)
+          end
+
+          return if existing_columns.empty?
+
+          raise <<~ERROR
+          One or more cascading namespace columns already exist. `add_cascading_namespace_setting` helper
+          can only be used for new settings, when none of the required columns already exist.
+          Existing columns: #{existing_columns.join(', ')}
+          ERROR
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/database/migration_helpers/cascading_namespace_settings_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::MigrationHelpers::CascadingNamespaceSettings do
+  let(:migration) do
+    ActiveRecord::Migration.new.extend(described_class)
+  end
+
+  describe '#add_cascading_namespace_setting' do
+    it 'creates the required columns', :aggregate_failures do
+      expect(migration).to receive(:add_column).with(:namespace_settings, :some_setting, :integer, null: true, default: nil)
+      expect(migration).to receive(:add_column).with(:namespace_settings, :lock_some_setting, :boolean, null: false, default: false)
+
+      expect(migration).to receive(:add_column).with(:application_settings, :some_setting, :integer, null: false, default: 5)
+      expect(migration).to receive(:add_column).with(:application_settings, :lock_some_setting, :boolean, null: false, default: false)
+
+      migration.add_cascading_namespace_setting(:some_setting, :integer, null: false, default: 5)
+    end
+
+    context 'when columns already exist' do
+      before do
+        migration.add_column(:namespace_settings, :cascading_setting, :integer)
+        migration.add_column(:application_settings, :lock_cascading_setting, :boolean)
+      end
+
+      it 'raises an error when some columns already exist' do
+        expect do
+          migration.add_cascading_namespace_setting(:cascading_setting, :integer)
+        end.to raise_error %r/Existing columns: namespace_settings.cascading_setting, application_settings.lock_cascading_setting/
+      end
+    end
+  end
+
+  describe '#remove_cascading_namespace_setting' do
+    before do
+      allow(migration).to receive(:column_exists?).and_return(true)
+    end
+
+    it 'removes the columns', :aggregate_failures do
+      expect(migration).to receive(:remove_column).with(:namespace_settings, :some_setting)
+      expect(migration).to receive(:remove_column).with(:namespace_settings, :lock_some_setting)
+
+      expect(migration).to receive(:remove_column).with(:application_settings, :some_setting)
+      expect(migration).to receive(:remove_column).with(:application_settings, :lock_some_setting)
+
+      migration.remove_cascading_namespace_setting(:some_setting)
+    end
+  end
+end