Cancel artifact expiry backfill background jobs.

Those background migrations will be re-scheduled with
a follow-up MR (gitlab-org/gitlab!51822)

changelogs/unreleased/mc-backstage-cleanup-artifact-expiry-migration-problem.yml

+---
+title: Cancel artifact expiry backfill background jobs.
+merge_request: 51821
+author:
+type: fixed

db/post_migrate/20201208175117_schedule_backfilling_artifact_expiry_migration.rb

@@ -24,12 +24,18 @@ class ScheduleBackfillingArtifactExpiryMigration < ActiveRecord::Migration[6.0]
     # Needs to be removed in a later migration
     add_concurrent_index(:ci_job_artifacts, %i(id created_at), where: INDEX_CONDITION, name: INDEX_NAME)
 
-    queue_background_migration_jobs_by_range_at_intervals(
-      JobArtifact.without_expiry_date.before_switch,
-      ::Gitlab::BackgroundMigration::BackfillArtifactExpiryDate,
-      2.minutes,
-      batch_size: 200_000
-    )
+    # queue_background_migration_jobs_by_range_at_intervals(
+    #   JobArtifact.without_expiry_date.before_switch,
+    #   ::Gitlab::BackgroundMigration::BackfillArtifactExpiryDate,
+    #   2.minutes,
+    #   batch_size: 200_000
+    # )
+    # The scheduling code was using the full class symbol
+    # (`::Gitlab::BackgroundMigration::BackfillArtifactExpiryDate`) instead of a
+    # string with the class name (`BackfillArtifactExpiryDate`) by mistake,
+    # which resulted in an error. It is commented out so it's a no-op to prevent
+    # errors and will be reintroduced with
+    # https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51822.
   end
 
   def down

db/post_migrate/20210115215854_cancel_artifact_expiry_backfill.rb

+# frozen_string_literal: true
+
+class CancelArtifactExpiryBackfill < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  MIGRATION = 'BackfillArtifactExpiryDate'
+
+  disable_ddl_transaction!
+
+  def up
+    Gitlab::BackgroundMigration.steal(MIGRATION) do |job|
+      job.delete
+
+      false
+    end
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20210115215854

+cb846ce5f6270cfdc543c3d4ad3e861b2a92445b952ee8f0a02f4171b9792411
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