Schedule background migration

Schedule artifact expiry backfill migration for execution.

db/post_migrate/20201208175117_schedule_backfilling_artifact_expiry_migration.rb

+# frozen_string_literal: true
+
+class ScheduleBackfillingArtifactExpiryMigration < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'expired_artifacts_temp_index'
+  INDEX_CONDITION = 'expire_at IS NULL'
+
+  disable_ddl_transaction!
+
+  def up
+    # Create temporary index for expired artifacts
+    # Needs to be removed in a later migration
+    add_concurrent_index(:ci_job_artifacts, :expire_at, where: INDEX_CONDITION, name: INDEX_NAME)
+
+    queue_background_migration_jobs_by_range_at_intervals(
+      ::Ci::JobArtifact.where(expire_at: nil),
+      ::Gitlab::BackgroundMigration::BackfillArtifactExpiryDate,
+      2.minutes,
+      batch_size: 100_000
+    )
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20201208175117

+68971e7f9a722e98d9e611f614b5465de83ff3d4dc8c7a8078ed1db8f21e6590
\ No newline at end of file

db/structure.sql

@@ -20756,6 +20756,8 @@ CREATE UNIQUE INDEX epic_user_mentions_on_epic_id_and_note_id_index ON epic_user
 
 CREATE UNIQUE INDEX epic_user_mentions_on_epic_id_index ON epic_user_mentions USING btree (epic_id) WHERE (note_id IS NULL);
 
+CREATE INDEX expired_artifacts_temp_index ON ci_job_artifacts USING btree (expire_at) WHERE (expire_at IS NULL);
+
 CREATE INDEX finding_links_on_vulnerability_occurrence_id ON vulnerability_finding_links USING btree (vulnerability_occurrence_id);
 
 CREATE INDEX idx_audit_events_on_entity_id_desc_author_id_created_at ON audit_events_archived USING btree (entity_id, entity_type, id DESC, author_id, created_at);