SendEmailCommand.php

<?php

namespace XXX\XXX\Command;

use Symfony\Component\Console\Input\InputInterface;
use Symfony\Component\Console\Output\OutputInterface;
use Symfony\Component\Console\Input\InputOption;

use Symfony\Bundle\SwiftmailerBundle\Command\SendEmailCommand as OriginalSendEmailCommand;

class SendEmailCommand extends OriginalSendEmailCommand
{
    /**
     * @see Command
     */
    protected function configure()
    {
        parent::configure();

        $this
            ->setName('xxx:spool:send')
            ->setDescription('Sends emails from the spool repeatedly')
            ->addOption('waiting-time', null, InputOption::VALUE_OPTIONAL, 'Time to wait between two calls', 1000)
        ;
    }

    /**
     * {@inheritdoc}
     */
    protected function execute(InputInterface $input, OutputInterface $output)
    {
        while (true) {
            parent::execute($input, $output);
            usleep($input->getOption('waiting-time') * 1000);
        }
    }
}

This crontab ensures the infinite loop is always running. Start it if it's not, do nothing if the pid file is valid.

* * * * * /sbin/start-stop-daemon --start --make-pidfile --pidfile /opt/xxx/run/worker01_mail.pid --chuid worker --oknodo --exec /usr/bin/php -- /opt/xxx/app/console xxx:spool:send --env=cli --no-debug > /dev/null