Schedule full build stage migration in the background

For builds that are still missing `stage_id`.

db/post_migrate/20171205101928_schedule_build_stage_migration.rb

@@ -0,0 +1,20 @@
+class ScheduleBuildStageMigration < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  MIGRATION = 'MigrateBuildStage'.freeze
+  BATCH = 10_000
+
+  class Build < ActiveRecord::Base
+    include EachBatch
+    self.table_name = 'ci_builds'
+  end
+
+  def change
+    Build.where('stage_id IS NULL').each_batch(of: BATCH) do |builds, index|
+      builds.pluck(:id).map { |id| [MIGRATION, [id]] }.tap do |migrations|
+        BackgroundMigrationWorker.perform_bulk_in(index.minutes, migrations)
+      end
+    end
+  end
+end

spec/migrations/schedule_build_stage_migration_spec.rb

@@ -0,0 +1,43 @@
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20171205101928_schedule_build_stage_migration')
+
+describe ScheduleBuildStageMigration, :migration do
+  let(:projects) { table(:projects) }
+  let(:pipelines) { table(:ci_pipelines) }
+  let(:stages) { table(:ci_stages) }
+  let(:jobs) { table(:ci_builds) }
+
+  before do
+    ##
+    # Dependencies
+    #
+    projects.create!(id: 123, name: 'gitlab', path: 'gitlab-ce')
+    pipelines.create!(id: 1, project_id: 123, ref: 'master', sha: 'adf43c3a')
+    stages.create!(id: 1, project_id: 123, pipeline_id: 1, name: 'test')
+
+    ##
+    # CI/CD jobs
+    #
+    jobs.create!(id: 10, commit_id: 1, project_id: 123, stage_id: nil)
+    jobs.create!(id: 20, commit_id: 1, project_id: 123, stage_id: nil)
+    jobs.create!(id: 30, commit_id: 1, project_id: 123, stage_id: nil)
+    jobs.create!(id: 40, commit_id: 1, project_id: 123, stage_id: 1)
+  end
+
+  before do
+    stub_const("#{described_class}::BATCH", 1)
+  end
+
+  it 'schedules background migrations in batches in bulk' do
+    Sidekiq::Testing.fake! do
+      Timecop.freeze do
+        migrate!
+
+        expect(described_class::MIGRATION).to be_scheduled_migration(1.minutes, 10)
+        expect(described_class::MIGRATION).to be_scheduled_migration(2.minutes, 20)
+        expect(described_class::MIGRATION).to be_scheduled_migration(3.minutes, 30)
+        expect(BackgroundMigrationWorker.jobs.size).to eq 3
+      end
+    end
+  end
+end