Remove foreign key ci_pending_builds.namespace_id

The loose foreign key was already added for this in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77636 .  We've
confirmed in https://gitlab.com/gitlab-org/gitlab/-/issues/348269 that
it is indeed safe to swap these foreign keys for loose foreign keys.

Changelog: other

db/post_migrate/20220106233459_remove_foreign_key_ci_pending_builds_namespace_id.rb

+# frozen_string_literal: true
+
+class RemoveForeignKeyCiPendingBuildsNamespaceId < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  CONSTRAINT_NAME = 'fk_fdc0137e4a'
+
+  def up
+    with_lock_retries do
+      remove_foreign_key_if_exists(:ci_pending_builds, :namespaces, name: CONSTRAINT_NAME)
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key :ci_pending_builds, :namespaces, column: :namespace_id, on_delete: :cascade, name: CONSTRAINT_NAME
+  end
+end

db/schema_migrations/20220106233459

+fc44084057c5fd30f0e7918b4f856a60409f08ad1eb5c3fbc2d8ad5bad7f1ffd
\ No newline at end of file

db/structure.sql

@@ -29884,9 +29884,6 @@ ALTER TABLE ONLY ci_daily_build_group_report_results
 ALTER TABLE ONLY merge_requests
     ADD CONSTRAINT fk_fd82eae0b9 FOREIGN KEY (head_pipeline_id) REFERENCES ci_pipelines(id) ON DELETE SET NULL;
 
-ALTER TABLE ONLY ci_pending_builds
-    ADD CONSTRAINT fk_fdc0137e4a FOREIGN KEY (namespace_id) REFERENCES namespaces(id) ON DELETE CASCADE;
-
 ALTER TABLE ONLY project_import_data
     ADD CONSTRAINT fk_ffb9ee3a10 FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE;
 

spec/models/ci/pending_build_spec.rb

@@ -223,4 +223,9 @@ RSpec.describe Ci::PendingBuild do
       end
     end
   end
+
+  it_behaves_like 'cleanup by a loose foreign key' do
+    let!(:parent) { create(:namespace) }
+    let!(:model) { create(:ci_pending_build, namespace: parent) }
+  end
 end