Add foreign key to ci_builds runner_id

When a runner gets deleted, jobs that were run with it still have
runner_id. With this commit, they will start to be nullified.

There will be 3 steps;
1. Add a NOT VALID foreign key constraint to the column to ensure
GitLab doesn't create inconsistent records.
2. Add a data migration, to fix or clean up existing records.
3. Validate the whole table by making the foreign key VALID.

This is the 1st step.

Changelog: other

db/migrate/20220208170445_add_not_valid_foreign_key_to_ci_builds_runner_id.rb

+# frozen_string_literal: true
+
+class AddNotValidForeignKeyToCiBuildsRunnerId < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_foreign_key :ci_builds, :ci_runners, column: :runner_id, on_delete: :nullify, validate: false
+  end
+
+  def down
+    with_lock_retries do
+      remove_foreign_key_if_exists :ci_builds, column: :runner_id
+    end
+  end
+end

db/schema_migrations/20220208170445

+e00dd618ca393596f3ff05b44b1a9a36183729a864a5cf4b8f1a262dfcdb932b
\ No newline at end of file

db/structure.sql

@@ -30140,6 +30140,9 @@ ALTER TABLE ONLY ci_builds_metadata
 ALTER TABLE ONLY gitlab_subscriptions
     ADD CONSTRAINT fk_e2595d00a1 FOREIGN KEY (namespace_id) REFERENCES namespaces(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY ci_builds
+    ADD CONSTRAINT fk_e4ef9c2f27 FOREIGN KEY (runner_id) REFERENCES ci_runners(id) ON DELETE SET NULL NOT VALID;
+
 ALTER TABLE ONLY merge_requests
     ADD CONSTRAINT fk_e719a85f8a FOREIGN KEY (author_id) REFERENCES users(id) ON DELETE SET NULL;
 

spec/db/schema_spec.rb

@@ -26,7 +26,7 @@ RSpec.describe 'Database schema' do
     boards: %w[milestone_id iteration_id],
     chat_names: %w[chat_id team_id user_id],
     chat_teams: %w[team_id],
-    ci_builds: %w[erased_by_id runner_id trigger_request_id],
+    ci_builds: %w[erased_by_id trigger_request_id],
     ci_namespace_monthly_usages: %w[namespace_id],
     ci_runner_projects: %w[runner_id],
     ci_trigger_requests: %w[commit_id],