Change terraform_states.locked_by_user_id foreign key to SET NULL

Fixes an edge case where a user could not be deleted if they
held a lock (in the Terraform sense, not a database lock) on a
Terraform state, as it would violate the foreign key constraint.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55813

changelogs/unreleased/update-terraform-states-user-foreign-key.yml

+---
+title: Change terraform_states.locked_by_user_id foreign key to SET NULL
+merge_request: 55813
+author:
+type: fixed

db/migrate/20210305002016_update_terraform_states_locked_by_user_id_foreign_key.rb

+# frozen_string_literal: true
+
+class UpdateTerraformStatesLockedByUserIdForeignKey < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  OLD_FOREIGN_KEY = 'fk_rails_558901b030'
+  NEW_FOREIGN_KEY = 'fk_558901b030'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_foreign_key :terraform_states, :users,
+      column: :locked_by_user_id, on_delete: :nullify, name: NEW_FOREIGN_KEY
+
+    with_lock_retries do
+      remove_foreign_key :terraform_states, :users, name: OLD_FOREIGN_KEY
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key :terraform_states, :users,
+      column: :locked_by_user_id, on_delete: nil, name: OLD_FOREIGN_KEY
+
+    with_lock_retries do
+      remove_foreign_key :terraform_states, :users, name: NEW_FOREIGN_KEY
+    end
+  end
+end

db/schema_migrations/20210305002016

+c91541048b119bcbbe9454760c6a90ddb6c4dc90e121f3ee7f4cdd6a1bde2c9e
\ No newline at end of file

db/structure.sql

@@ -24451,6 +24451,9 @@ ALTER TABLE ONLY path_locks
 ALTER TABLE ONLY clusters_applications_prometheus
     ADD CONSTRAINT fk_557e773639 FOREIGN KEY (cluster_id) REFERENCES clusters(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY terraform_states
+    ADD CONSTRAINT fk_558901b030 FOREIGN KEY (locked_by_user_id) REFERENCES users(id) ON DELETE SET NULL;
+
 ALTER TABLE ONLY merge_request_metrics
     ADD CONSTRAINT fk_56067dcb44 FOREIGN KEY (target_project_id) REFERENCES projects(id) ON DELETE CASCADE;
 
@@ -25477,9 +25480,6 @@ ALTER TABLE ONLY geo_node_namespace_links
 ALTER TABLE ONLY clusters_applications_knative
     ADD CONSTRAINT fk_rails_54fc91e0a0 FOREIGN KEY (cluster_id) REFERENCES clusters(id) ON DELETE CASCADE;
 
-ALTER TABLE ONLY terraform_states
-    ADD CONSTRAINT fk_rails_558901b030 FOREIGN KEY (locked_by_user_id) REFERENCES users(id);
-
 ALTER TABLE ONLY issuable_metric_images
     ADD CONSTRAINT fk_rails_56417a5a7f FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE;