Add migration to enqueue background worker

Changelog: added

db/migrate/20211214012507_backfill_incident_issue_escalation_statuses.rb

+# frozen_string_literal: true
+
+class BackfillIncidentIssueEscalationStatuses < Gitlab::Database::Migration[1.0]
+  MIGRATION = 'BackfillIncidentIssueEscalationStatuses'
+  DELAY_INTERVAL = 2.minutes
+  BATCH_SIZE = 10_000
+
+  disable_ddl_transaction!
+
+  class Issue < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'issues'
+
+    scope :incidents, -> { where(issue_type: 1) }
+  end
+
+  def up
+    relation = Issue.incidents
+
+    queue_background_migration_jobs_by_range_at_intervals(
+      relation, MIGRATION, DELAY_INTERVAL, batch_size: BATCH_SIZE)
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20211214012507

+a7aa1ffccce785d365720309e3773f167075a9d06805eea941e6cd47bc918471
\ No newline at end of file

spec/migrations/20211214012507_backfill_incident_issue_escalation_statuses_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!
+
+RSpec.describe BackfillIncidentIssueEscalationStatuses do
+  let(:namespaces)  { table(:namespaces) }
+  let(:projects)    { table(:projects) }
+  let(:issues)      { table(:issues) }
+  let(:namespace)   { namespaces.create!(name: 'foo', path: 'foo') }
+  let(:project)     { projects.create!(namespace_id: namespace.id) }
+
+  before do
+    stub_const("#{described_class.name}::BATCH_SIZE", 1)
+  end
+
+  it 'schedules jobs for incident issues' do
+    issues.create!(project_id: project.id) # non-incident issue
+    incident_1 = issues.create!(project_id: project.id, issue_type: 1)
+    incident_2 = issues.create!(project_id: project.id, issue_type: 1)
+
+    Sidekiq::Testing.fake! do
+      freeze_time do
+        migrate!
+
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(
+          2.minutes, incident_1.id, incident_1.id)
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(
+          4.minutes, incident_2.id, incident_2.id)
+        expect(BackgroundMigrationWorker.jobs.size).to eq(2)
+      end
+    end
+  end
+end