Remove ci_pending_builds FK to projects

We already have a ci_pending_builds FK to ci_builds.
ci_builds in turn have a FK to projects. So this FK should
be safe to remove.

With this change when project is deleted, dependent ci_pending_builds
will either be:

- deleted via loose foreign keys
- deleted via FK cascade projects -> ci_builds -> ci_pending_builds

Changelog: other

db/post_migrate/20220111221516_remove_projects_ci_pending_builds_fk.rb

+# frozen_string_literal: true
+
+class RemoveProjectsCiPendingBuildsFk < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  def up
+    with_lock_retries do
+      remove_foreign_key_if_exists(:ci_pending_builds, :projects, name: "fk_rails_480669c3b3")
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key(:ci_pending_builds, :projects, name: "fk_rails_480669c3b3", column: :project_id, target_column: :id, on_delete: "cascade")
+  end
+end

db/schema_migrations/20220111221516

+0cb120b0036b3f472edb57fcb8a52877d399edf8ff1f416ce76497d6aa8265d7
\ No newline at end of file

db/structure.sql

@@ -30368,9 +30368,6 @@ ALTER TABLE ONLY vulnerability_feedback
 ALTER TABLE ONLY user_custom_attributes
     ADD CONSTRAINT fk_rails_47b91868a8 FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
 
-ALTER TABLE ONLY ci_pending_builds
-    ADD CONSTRAINT fk_rails_480669c3b3 FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE;
-
 ALTER TABLE ONLY upcoming_reconciliations
     ADD CONSTRAINT fk_rails_497b4938ac FOREIGN KEY (namespace_id) REFERENCES namespaces(id) ON DELETE CASCADE;
 

spec/models/ci/pending_build_spec.rb

@@ -228,4 +228,9 @@ RSpec.describe Ci::PendingBuild do
     let!(:parent) { create(:namespace) }
     let!(:model) { create(:ci_pending_build, namespace: parent) }
   end
+
+  it_behaves_like 'cleanup by a loose foreign key' do
+    let!(:parent) { create(:project) }
+    let!(:model) { create(:ci_pending_build, project: parent) }
+  end
 end