Ensure namespace settings are backfilled

changelogs/unreleased/ensure-db-consistency-after-36321.yml

+---
+title: Ensure namespace settings are backfilled via migration
+merge_request: 41679
+author:
+type: other

db/post_migrate/20200907124300_complete_namespace_settings_migration.rb

+# frozen_string_literal: true
+
+class CompleteNamespaceSettingsMigration < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+  BATCH_SIZE = 10000
+
+  class Namespace < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'namespaces'
+  end
+
+  def up
+    Gitlab::BackgroundMigration.steal('BackfillNamespaceSettings')
+
+    ensure_data_migration
+  end
+
+  def down
+    # no-op
+  end
+
+  private
+
+  def ensure_data_migration
+    Namespace.each_batch(of: BATCH_SIZE) do |query|
+      missing_count = query.where("NOT EXISTS (SELECT 1 FROM namespace_settings WHERE namespace_settings.namespace_id=namespaces.id)").limit(1).size
+      if missing_count > 0
+        min, max = query.pluck("MIN(id), MAX(id)").flatten
+        # we expect low record count so inline execution is fine.
+        Gitlab::BackgroundMigration::BackfillNamespaceSettings.new.perform(min, max)
+      end
+    end
+  end
+end

db/schema_migrations/20200907124300

+2311967a9f68e1a428662e0231752ad0d844063d66cca895211d38f9ae928d94
\ No newline at end of file

spec/migrations/complete_namespace_settings_migration_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20200907124300_complete_namespace_settings_migration.rb')
+
+RSpec.describe CompleteNamespaceSettingsMigration, :redis do
+  let(:migration) { spy('migration') }
+
+  context 'when still legacy artifacts exist' do
+    let(:namespaces) { table(:namespaces) }
+    let(:namespace_settings) { table(:namespace_settings) }
+    let!(:namespace) { namespaces.create!(name: 'gitlab', path: 'gitlab-org') }
+
+    it 'steals sidekiq jobs from BackfillNamespaceSettings background migration' do
+      expect(Gitlab::BackgroundMigration).to receive(:steal).with('BackfillNamespaceSettings')
+
+      migrate!
+    end
+
+    it 'migrates namespaces without namespace_settings' do
+      expect { migrate! }.to change { namespace_settings.count }.from(0).to(1)
+    end
+  end
+end