Rolls back the current transaction.
This is just a wrapper method to rollback whatever transaction stack we are currently in, which is managed by the connection object itself. Note that logging (preferable with watchdog_exception()) needs to happen after a transaction has been rolled back or the log messages will be rolled back too.
Drupal\Core\Database\Connection::rollback()
public function rollback() {
$this->rolledBack = TRUE;
$this->connection
->rollback($this->name);
}