Merge branch 'mysql_drop_all_tables' into 'master'

Disable MySQL foreign key checks before dropping all tables

Fixes #20237. Disables MySQL foreign key checks before dropping all tables in a restore. MySQL doesn't honor `CASCADE` without a special flag when the database/tables are created. In order to drop the tables we need to disable foreign key checks. After the drop, re-enable the key checks. 

WIP: Pending confirmation from customer that this fix works. 

See merge request !5472

CHANGELOG

@@ -16,6 +16,7 @@ v 8.11.0 (unreleased)
 v 8.10.2 (unreleased)
   - User can now search branches by name. !5144
   - Fix backup restore. !5459
+  - Disable MySQL foreign key checks before dropping all tables. !5472
   - Use project ID in repository cache to prevent stale data from persisting across projects. !5460
 
 v 8.10.1

lib/tasks/gitlab/db.rake

@@ -25,6 +25,10 @@ namespace :gitlab do
     desc 'Drop all tables'
     task :drop_tables => :environment do
       connection = ActiveRecord::Base.connection
+
+      # If MySQL, turn off foreign key checks
+      connection.execute('SET FOREIGN_KEY_CHECKS=0') if Gitlab::Database.mysql?
+
       tables = connection.tables
       tables.delete 'schema_migrations'
       # Truncate schema_migrations to ensure migrations re-run
@@ -35,6 +39,9 @@ namespace :gitlab do
       # MySQL: http://dev.mysql.com/doc/refman/5.7/en/drop-table.html
       # Add `IF EXISTS` because cascade could have already deleted a table.
       tables.each { |t| connection.execute("DROP TABLE IF EXISTS #{connection.quote_table_name(t)} CASCADE") }
+
+      # If MySQL, re-enable foreign key checks
+      connection.execute('SET FOREIGN_KEY_CHECKS=1') if Gitlab::Database.mysql?
     end
 
     desc 'Configures the database by running migrate, or by loading the schema and seeding if needed'