Create post migration script to schedule migration in batches

This post migration will schedule background jobs to populate recently
introduced `resolved_on_default_branch` column of already existing
vulnerability records.

db/post_migrate/20200806100713_schedule_populate_resolved_on_default_branch_column.rb

+# frozen_string_literal: true
+
+class SchedulePopulateResolvedOnDefaultBranchColumn < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  BATCH_SIZE = 100
+  DELAY_INTERVAL = 2.minutes.to_i
+  MIGRATION_CLASS = 'PopulateResolvedOnDefaultBranchColumn'
+
+  disable_ddl_transaction!
+
+  class Project < ActiveRecord::Base
+    include EachBatch
+
+    scope :has_vulnerabilities, -> { joins('INNER JOIN vulnerabilities v ON v.project_id = projects.id').group(:id) }
+  end
+
+  def up
+    return unless run_migration?
+
+    Project.has_vulnerabilities.each_batch(of: BATCH_SIZE) do |batch, index|
+      project_ids = batch.pluck(:id)
+      migrate_in(index * DELAY_INTERVAL, MIGRATION_CLASS, project_ids)
+    end
+  end
+
+  def down; end
+
+  private
+
+  def run_migration?
+    Gitlab.ee? && table_exists?(:projects) && table_exists?(:vulnerabilities)
+  end
+end

db/schema_migrations/20200806100713

+fdcce45050f972d8edf2c645022f517ff6b9f4c76767e6cebe45a11fe34dd388
\ No newline at end of file

ee/spec/migrations/schedule_populate_resolved_on_default_branch_column_spec.rb

+require 'spec_helper'
+
+require Rails.root.join('db', 'post_migrate', '20200806100713_schedule_populate_resolved_on_default_branch_column.rb')
+
+RSpec.describe SchedulePopulateResolvedOnDefaultBranchColumn do
+  before do
+    allow_any_instance_of(Gitlab).to receive(:ee?).and_return(ee?)
+  end
+
+  around do |example|
+    Timecop.freeze { Sidekiq::Testing.fake! { example.run } }
+  end
+
+  context 'when the Gitlab instance is CE' do
+    let(:ee?) { false }
+
+    it 'does not run the migration' do
+      expect { migrate! }.not_to change { BackgroundMigrationWorker.jobs.size }
+    end
+  end
+
+  context 'when the Gitlab instance is EE' do
+    let(:ee?) { true }
+    let!(:project_1) { create(:project) }
+    let!(:project_2) { create(:project) }
+    let!(:project_3) { create(:project) }
+
+    before do
+      create(:vulnerability, project: project_1)
+      create(:vulnerability, project: project_2)
+
+      stub_const("#{described_class.name}::BATCH_SIZE", 1)
+    end
+
+    it 'schedules the background jobs', :aggregate_failures do
+      migrate!
+
+      expect(BackgroundMigrationWorker.jobs.size).to be(2)
+      expect(described_class::MIGRATION_CLASS).to be_scheduled_delayed_migration(2.minutes, project_1.id)
+      expect(described_class::MIGRATION_CLASS).to be_scheduled_delayed_migration(4.minutes, project_2.id)
+    end
+  end
+end

lib/gitlab/background_migration/populate_resolved_on_default_branch_column.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # rubocop:disable Style/Documentation
+    class PopulateResolvedOnDefaultBranchColumn
+      def perform(*); end
+    end
+  end
+end
+
+Gitlab::BackgroundMigration::UpdateVulnerabilityConfidence.prepend_if_ee('EE::Gitlab::BackgroundMigration::PopulateResolvedOnDefaultBranchColumn')