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    <h3>Editing: RoundRobinTransport.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier &lt;fabien@symfony.com&gt;
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\Mailer\Transport;

use Symfony\Component\Mailer\Envelope;
use Symfony\Component\Mailer\Exception\TransportException;
use Symfony\Component\Mailer\Exception\TransportExceptionInterface;
use Symfony\Component\Mailer\SentMessage;
use Symfony\Component\Mime\RawMessage;

/**
 * Uses several Transports using a round robin algorithm.
 *
 * @author Fabien Potencier &lt;fabien@symfony.com&gt;
 */
class RoundRobinTransport implements TransportInterface
{
    /**
     * @var \SplObjectStorage&lt;TransportInterface, float&gt;
     */
    private \SplObjectStorage $deadTransports;
    private array $transports = [];
    private int $retryPeriod;
    private int $cursor = -1;

    /**
     * @param TransportInterface[] $transports
     */
    public function __construct(array $transports, int $retryPeriod = 60)
    {
        if (!$transports) {
            throw new TransportException(sprintf('&quot;%s&quot; must have at least one transport configured.', static::class));
        }

        $this-&gt;transports = $transports;
        $this-&gt;deadTransports = new \SplObjectStorage();
        $this-&gt;retryPeriod = $retryPeriod;
    }

    public function send(RawMessage $message, Envelope $envelope = null): ?SentMessage
    {
        $exception = null;

        while ($transport = $this-&gt;getNextTransport()) {
            try {
                return $transport-&gt;send($message, $envelope);
            } catch (TransportExceptionInterface $e) {
                $exception ??= new TransportException('All transports failed.');
                $exception-&gt;appendDebug(sprintf(&quot;Transport \&quot;%s\&quot;: %s\n&quot;, $transport, $e-&gt;getDebug()));
                $this-&gt;deadTransports[$transport] = microtime(true);
            }
        }

        throw $exception ?? new TransportException('No transports found.');
    }

    public function __toString(): string
    {
        return $this-&gt;getNameSymbol().'('.implode(' ', array_map('strval', $this-&gt;transports)).')';
    }

    /**
     * Rotates the transport list around and returns the first instance.
     */
    protected function getNextTransport(): ?TransportInterface
    {
        if (-1 === $this-&gt;cursor) {
            $this-&gt;cursor = $this-&gt;getInitialCursor();
        }

        $cursor = $this-&gt;cursor;
        while (true) {
            $transport = $this-&gt;transports[$cursor];

            if (!$this-&gt;isTransportDead($transport)) {
                break;
            }

            if ((microtime(true) - $this-&gt;deadTransports[$transport]) &gt; $this-&gt;retryPeriod) {
                $this-&gt;deadTransports-&gt;detach($transport);

                break;
            }

            if ($this-&gt;cursor === $cursor = $this-&gt;moveCursor($cursor)) {
                return null;
            }
        }

        $this-&gt;cursor = $this-&gt;moveCursor($cursor);

        return $transport;
    }

    protected function isTransportDead(TransportInterface $transport): bool
    {
        return $this-&gt;deadTransports-&gt;contains($transport);
    }

    protected function getInitialCursor(): int
    {
        // the cursor initial value is randomized so that
        // when are not in a daemon, we are still rotating the transports
        return mt_rand(0, \count($this-&gt;transports) - 1);
    }

    protected function getNameSymbol(): string
    {
        return 'roundrobin';
    }

    private function moveCursor(int $cursor): int
    {
        return ++$cursor &gt;= \count($this-&gt;transports) ? 0 : $cursor;
    }
}
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