Background migration to replace blocked_by links

`blocked_by` links will be replaced with `blocks` links
where source and target is swapped.

changelogs/unreleased/migrate_blocked_by.yml

+---
+title: Migrate blocked_by issue links to blocks type by swapping source and target
+merge_request: 45262
+author:
+type: other

db/post_migrate/20201015073808_schedule_blocked_by_links_replacement.rb

+# frozen_string_literal: true
+
+class ScheduleBlockedByLinksReplacement < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INTERVAL = 2.minutes
+  # at the time of writing there were 47600 blocked_by issues:
+  # estimated time is 48 batches * 2 minutes -> 100 minutes
+  BATCH_SIZE = 1000
+  MIGRATION = 'ReplaceBlockedByLinks'
+
+  disable_ddl_transaction!
+
+  class IssueLink < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'issue_links'
+  end
+
+  def up
+    relation = IssueLink.where(link_type: 2)
+
+    queue_background_migration_jobs_by_range_at_intervals(
+      relation, MIGRATION, INTERVAL, batch_size: BATCH_SIZE)
+  end
+
+  def down
+  end
+end

db/schema_migrations/20201015073808

+e44ab2a7b3014b44d7d84de1f7e618d2fc89f98b8d59f5f6fa331544e206355f
\ No newline at end of file

lib/gitlab/background_migration/replace_blocked_by_links.rb

+# frozen_string_literal: true
+# rubocop:disable Style/Documentation
+
+module Gitlab
+  module BackgroundMigration
+    class ReplaceBlockedByLinks
+      class IssueLink < ActiveRecord::Base
+        self.table_name = 'issue_links'
+      end
+
+      def perform(start_id, stop_id)
+        blocked_by_links = IssueLink.where(id: start_id..stop_id).where(link_type: 2)
+
+        ActiveRecord::Base.transaction do
+          # if there is duplicit bi-directional relation (issue2 is blocked by issue1
+          # and issue1 already links issue2), then we can just delete 'blocked by'.
+          # This should be rare as we have a pre-create check which checks if issues are
+          # already linked
+          blocked_by_links
+            .joins('INNER JOIN issue_links as opposite_links ON issue_links.source_id = opposite_links.target_id AND issue_links.target_id = opposite_links.source_id')
+            .where('opposite_links.link_type': 1)
+            .delete_all
+
+          blocked_by_links.update_all('source_id=target_id,target_id=source_id,link_type=1')
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/background_migration/replace_blocked_by_links_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::BackgroundMigration::ReplaceBlockedByLinks, schema: 20201015073808 do
+  let(:namespace) { table(:namespaces).create!(name: 'gitlab', path: 'gitlab-org') }
+  let(:project) { table(:projects).create!(namespace_id: namespace.id, name: 'gitlab') }
+  let(:issue1) { table(:issues).create!(project_id: project.id, title: 'a') }
+  let(:issue2) { table(:issues).create!(project_id: project.id, title: 'b') }
+  let(:issue3) { table(:issues).create!(project_id: project.id, title: 'c') }
+  let(:issue_links) { table(:issue_links) }
+  let!(:blocks_link) { issue_links.create!(source_id: issue1.id, target_id: issue2.id, link_type: 1) }
+  let!(:bidirectional_link) { issue_links.create!(source_id: issue2.id, target_id: issue1.id, link_type: 2) }
+  let!(:blocked_link) { issue_links.create!(source_id: issue1.id, target_id: issue3.id, link_type: 2) }
+
+  subject { described_class.new.perform(issue_links.minimum(:id), issue_links.maximum(:id)) }
+
+  it 'deletes issue links where opposite relation already exists' do
+    expect { subject }.to change { issue_links.count }.by(-1)
+  end
+
+  it 'ignores issue links other than blocked_by' do
+    subject
+
+    expect(blocks_link.reload.link_type).to eq(1)
+  end
+
+  it 'updates blocked_by issue links' do
+    subject
+
+    link = blocked_link.reload
+    expect(link.link_type).to eq(1)
+    expect(link.source_id).to eq(issue3.id)
+    expect(link.target_id).to eq(issue1.id)
+  end
+end

spec/migrations/schedule_blocked_by_links_replacement_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20201015073808_schedule_blocked_by_links_replacement')
+
+RSpec.describe ScheduleBlockedByLinksReplacement do
+  let(:namespace) { table(:namespaces).create!(name: 'gitlab', path: 'gitlab-org') }
+  let(:project) { table(:projects).create!(namespace_id: namespace.id, name: 'gitlab') }
+  let(:issue1) { table(:issues).create!(project_id: project.id, title: 'a') }
+  let(:issue2) { table(:issues).create!(project_id: project.id, title: 'b') }
+  let(:issue3) { table(:issues).create!(project_id: project.id, title: 'c') }
+  let!(:issue_links) do
+    [
+      table(:issue_links).create!(source_id: issue1.id, target_id: issue2.id, link_type: 1),
+      table(:issue_links).create!(source_id: issue2.id, target_id: issue1.id, link_type: 2),
+      table(:issue_links).create!(source_id: issue1.id, target_id: issue3.id, link_type: 2)
+    ]
+  end
+
+  before do
+    stub_const("#{described_class.name}::BATCH_SIZE", 1)
+  end
+
+  it 'schedules jobs for blocked_by links' do
+    Sidekiq::Testing.fake! do
+      freeze_time do
+        migrate!
+
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(
+          2.minutes, issue_links[1].id, issue_links[1].id)
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(
+          4.minutes, issue_links[2].id, issue_links[2].id)
+        expect(BackgroundMigrationWorker.jobs.size).to eq(2)
+      end
+    end
+  end
+end