Populate blocking issues count

Second attempt to populate issues#blocking_issues count
using backgroung migrations

changelogs/unreleased/issue_237977-2.yml

+---
+title: Populate blocking issues count
+merge_request: 45176
+author:
+type: other

db/post_migrate/20201014142521_schedule_sync_blocking_issues_count.rb

+# frozen_string_literal: true
+
+require 'set'
+
+class ScheduleSyncBlockingIssuesCount < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  BATCH_SIZE = 50
+  DELAY_INTERVAL = 120.seconds.to_i
+  MIGRATION = 'SyncBlockingIssuesCount'.freeze
+
+  disable_ddl_transaction!
+
+  class TmpIssueLink < ActiveRecord::Base
+    self.table_name = 'issue_links'
+
+    include EachBatch
+  end
+
+  def up
+    return unless Gitlab.ee?
+
+    issue_link_ids = SortedSet.new
+
+    TmpIssueLink.distinct.select(:source_id).where(link_type: 1).each_batch(of: 1000, column: :source_id) do |query|
+      issue_link_ids.merge(query.pluck(:source_id))
+    end
+
+    TmpIssueLink.distinct.select(:target_id).where(link_type: 2).each_batch(of: 1000, column: :target_id) do |query|
+      issue_link_ids.merge(query.pluck(:target_id))
+    end
+
+    issue_link_ids.each_slice(BATCH_SIZE).with_index do |items, index|
+      start_id, *, end_id = items
+
+      arguments = [start_id, end_id]
+
+      final_delay = DELAY_INTERVAL * (index + 1)
+      migrate_in(final_delay, MIGRATION, arguments)
+    end
+  end
+
+  def down
+    # NO OP
+  end
+end

db/schema_migrations/20201014142521

+aed103bb25b70eb8f6387d84225a8e51672a83c4586ccc65da3011ef010da4b1
\ No newline at end of file

ee/spec/migrations/schedule_sync_blocking_issues_count_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20201014142521_schedule_sync_blocking_issues_count')
+
+RSpec.describe ScheduleSyncBlockingIssuesCount do
+  let(:issues) { table(:issues) }
+  let(:links) { table(:issue_links) }
+  let(:migration) { described_class::MIGRATION }
+
+  # Blocking issues
+  let!(:issue_1) { issues.create!(description: 'blocking 1', state_id: 1) }
+  let!(:issue_2) { issues.create!(description: 'blocking 2', state_id: 1) }
+  let!(:issue_3) { issues.create!(description: 'blocking 3', state_id: 2) }
+  let!(:issue_4) { issues.create!(description: 'blocking 4', state_id: 1 ) }
+
+  # Blocked issues
+  let!(:issue_5) { issues.create!(description: 'blocked 1', state_id: 1) }
+  let!(:issue_6) { issues.create!(description: 'blocked 2', state_id: 1) }
+  let!(:issue_7) { issues.create!(description: 'blocked 3', state_id: 1) }
+  let!(:issue_8) { issues.create!(description: 'blocked 4', state_id: 1) }
+  let!(:issue_9) { issues.create!(description: 'blocked 5', state_id: 1) }
+  let!(:issue_10) { issues.create!(description: 'blocked 6', state_id: 1) }
+
+  before do
+    stub_const("#{described_class.name}::BATCH_SIZE", 2)
+
+    # Issue links
+    # -----------
+    # issue_1 blocks two issues with link_type BLOCKS
+    # issue_2 blocks two issues with link_type IS_BLOCKED_BY
+    # issue_3 blocks one issue with link_type IS_BLOCKED_BY but it is closed
+    # issue_4 blocks one issue with link_type BLOCKS
+    links.create!(link_type: 1, source_id: issue_1.id, target_id: issue_5.id)
+    links.create!(link_type: 1, source_id: issue_1.id, target_id: issue_6.id)
+    links.create!(link_type: 2, source_id: issue_7.id, target_id: issue_2.id)
+    links.create!(link_type: 2, source_id: issue_8.id, target_id: issue_2.id)
+    links.create!(link_type: 1, source_id: issue_4.id, target_id: issue_9.id)
+    links.create!(link_type: 2, source_id: issue_10.id, target_id: issue_3.id)
+  end
+
+  context 'scheduling migrations' do
+    before do
+      Sidekiq::Worker.clear_all
+    end
+
+    it 'correctly schedules issuable sync background migration' do
+      Sidekiq::Testing.fake! do
+        freeze_time do
+          migrate!
+
+          expect(migration).to be_scheduled_delayed_migration(120.seconds, issue_1.id, issue_2.id)
+          expect(migration).to be_scheduled_delayed_migration(240.seconds, issue_3.id, issue_4.id)
+          expect(BackgroundMigrationWorker.jobs.size).to eq(2)
+        end
+      end
+    end
+  end
+
+  context 'running background migration' do
+    it 'correctly populates issues blocking_issues_count', :sidekiq_might_not_need_inline do
+      migrate!
+
+      expect(issue_1.reload.blocking_issues_count).to eq(2)
+      expect(issue_2.reload.blocking_issues_count).to eq(2)
+      expect(issue_3.reload.blocking_issues_count).to eq(0)
+      expect(issue_4.reload.blocking_issues_count).to eq(1)
+    end
+  end
+end