Remove foreign key constraint ci_runner_namespaces.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 this foreign key for a loose foreign key.

Changelog: other

db/post_migrate/20220106235626_remove_foreign_key_ci_runner_namespaces_namespace_id.rb

+# frozen_string_literal: true
+
+class RemoveForeignKeyCiRunnerNamespacesNamespaceId < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  CONSTRAINT_NAME = 'fk_rails_f9d9ed3308'
+
+  def up
+    with_lock_retries do
+      remove_foreign_key_if_exists(:ci_runner_namespaces, :namespaces, name: CONSTRAINT_NAME)
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key :ci_runner_namespaces, :namespaces, column: :namespace_id, on_delete: :cascade, name: CONSTRAINT_NAME
+  end
+end

db/schema_migrations/20220106235626

+34f966723cae63e831f7fc9d965cda90f1fd7bca522fc58e78a0de4b959a47a2
\ No newline at end of file

db/structure.sql

@@ -31438,9 +31438,6 @@ ALTER TABLE ONLY merge_requests_closing_issues
 ALTER TABLE ONLY merge_trains
     ADD CONSTRAINT fk_rails_f90820cb08 FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE SET NULL;
 
-ALTER TABLE ONLY ci_runner_namespaces
-    ADD CONSTRAINT fk_rails_f9d9ed3308 FOREIGN KEY (namespace_id) REFERENCES namespaces(id) ON DELETE CASCADE;
-
 ALTER TABLE ONLY banned_users
     ADD CONSTRAINT fk_rails_fa5bb598e5 FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
 

spec/models/ci/runner_namespace_spec.rb

@@ -6,4 +6,10 @@ RSpec.describe Ci::RunnerNamespace do
   it_behaves_like 'includes Limitable concern' do
     subject { build(:ci_runner_namespace, group: create(:group, :nested), runner: create(:ci_runner, :group)) }
   end
+
+  it_behaves_like 'cleanup by a loose foreign key' do
+    let!(:model) { create(:ci_runner_namespace) }
+
+    let!(:parent) { model.namespace }
+  end
 end