Swap FK merge_requests to ci_pipelines for LFK

Swaps FK for merge_requests.head_pipeline_id to ci_pipelines

Changelog: changed

db/post_migrate/20220119154442_remove_ci_pipelines_merge_requests_head_pipeline_id_fk.rb

+# frozen_string_literal: true
+
+class RemoveCiPipelinesMergeRequestsHeadPipelineIdFk < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  def up
+    with_lock_retries do
+      execute('LOCK ci_pipelines, merge_requests IN ACCESS EXCLUSIVE MODE')
+
+      remove_foreign_key_if_exists(:merge_requests, :ci_pipelines, name: "fk_fd82eae0b9")
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key(:merge_requests, :ci_pipelines, name: "fk_fd82eae0b9", column: :head_pipeline_id, target_column: :id, on_delete: :nullify)
+  end
+end

db/schema_migrations/20220119154442

+7cb06f695c7e8e733ea50f0b2e8fbb115d5ac0d7e7f000228bd5ced58ab362c9
\ No newline at end of file

db/structure.sql

@@ -30010,9 +30010,6 @@ ALTER TABLE ONLY system_note_metadata
 ALTER TABLE ONLY vulnerability_remediations
     ADD CONSTRAINT fk_fc61a535a0 FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE;
 
-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 project_import_data
     ADD CONSTRAINT fk_ffb9ee3a10 FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE;
 

lib/gitlab/database/gitlab_loose_foreign_keys.yml

@@ -98,6 +98,9 @@ merge_requests:
   - table: ci_pipelines
     column: head_pipeline_id
     on_delete: async_nullify
+  - table: ci_pipelines
+    column: head_pipeline_id
+    on_delete: async_nullify
 vulnerability_statistics:
   - table: ci_pipelines
     column: latest_pipeline_id

spec/lib/gitlab/database/no_cross_db_foreign_keys_spec.rb

@@ -48,7 +48,6 @@ RSpec.describe 'cross-database foreign keys' do
       dast_site_profiles_builds.ci_build_id
       dast_site_profiles_pipelines.ci_pipeline_id
       external_pull_requests.project_id
-      merge_requests.head_pipeline_id
       merge_trains.pipeline_id
       requirements_management_test_reports.build_id
       security_scans.build_id

spec/models/merge_request_spec.rb

@@ -4965,4 +4965,11 @@ RSpec.describe MergeRequest, factory_default: :keep do
   it_behaves_like 'it has loose foreign keys' do
     let(:factory_name) { :merge_request }
   end
+
+  context 'loose foreign key on merge_requests.head_pipeline_id' do
+    it_behaves_like 'cleanup by a loose foreign key' do
+      let!(:parent) { create(:ci_pipeline) }
+      let!(:model) { create(:merge_request, head_pipeline: parent) }
+    end
+  end
 end