Add background migration that migrates stages indices

lib/gitlab/background_migration/migrate_stage_index.rb

+# frozen_string_literal: true
+# rubocop:disable Style/Documentation
+
+module Gitlab
+  module BackgroundMigration
+    class MigrateStageIndex
+      module Migratable
+        class Stage < ActiveRecord::Base
+          self.table_name = 'ci_stages'
+        end
+      end
+
+      def perform(start_id, stop_id)
+        sql = <<~SQL
+          UPDATE ci_stages
+            SET index =
+              (SELECT stage_idx FROM ci_builds
+                WHERE ci_builds.stage_id = ci_stages.id
+                GROUP BY ci_builds.stage_idx ORDER BY COUNT(*) DESC LIMIT 1)
+          WHERE ci_stages.id BETWEEN #{start_id.to_i} AND #{stop_id.to_i}
+        SQL
+
+        ActiveRecord::Base.connection.execute(sql)
+      end
+    end
+  end
+end

spec/lib/gitlab/background_migration/migrate_stage_index_spec.rb

+require 'spec_helper'
+
+describe Gitlab::BackgroundMigration::MigrateStageIndex, :migration, schema: 20180420080616 do
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+  let(:pipelines) { table(:ci_pipelines) }
+  let(:stages) { table(:ci_stages) }
+  let(:jobs) { table(:ci_builds) }
+
+
+  before do
+    namespaces.create(id: 10, name: 'gitlab-org', path: 'gitlab-org')
+    projects.create!(id: 11, namespace_id: 10, name: 'gitlab', path: 'gitlab')
+    pipelines.create!(id: 12, project_id: 11, ref: 'master', sha: 'adf43c3a')
+
+    stages.create(id: 100, project_id: 11, pipeline_id: 12, name: 'build')
+    stages.create(id: 101, project_id: 11, pipeline_id: 12, name: 'test')
+
+    jobs.create!(id: 121, commit_id: 12, project_id: 11,
+                 stage_idx: 2, stage_id: 100)
+    jobs.create!(id: 122, commit_id: 12, project_id: 11,
+                 stage_idx: 2, stage_id: 100)
+    jobs.create!(id: 123, commit_id: 12, project_id: 11,
+                 stage_idx: 10, stage_id: 100)
+    jobs.create!(id: 124, commit_id: 12, project_id: 11,
+                 stage_idx: 3, stage_id: 101)
+  end
+
+  it 'correctly migrates stages indices' do
+    expect(stages.all.pluck(:index)).to all(be_nil)
+
+    described_class.new.perform(100, 101)
+
+    expect(stages.all.pluck(:index)).to eq [2, 3]
+  end
+end