Issue #3394123: Add Symfony Messenger support for async messsages (emails as queues)

src/Connections.php

+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\symfony_mailer;
+
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+
+/**
+ * Repository of mailer transport DSNs.
+ *
+ * This is needed since we need to be able to reverse DSN to the transport
+ * key due to concepts introduced by this project. EmailAdjuster is able to call
+ * \Drupal\symfony_mailer\EmailInterface::setTransportDsn, rather than setting
+ * the transport name.
+ */
+final class Connections {
+
+  public function __construct(
+    private EntityTypeManagerInterface $entityTypeManager,
+  )
+  {
+  }
+
+  /**
+   * Get DSNs by name.
+   *
+   * By convention, the first item is the default.
+   *
+   * @see https://symfony.com/doc/current/mailer.html#multiple-email-transports
+   */
+  public function getDsns(): array {
+    /** @var \Drupal\symfony_mailer\Entity\MailerTransport[] $transports */
+    $transports = $this->entityTypeManager
+      ->getStorage('mailer_transport')
+      ->loadMultiple();
+
+    $result = [];
+    foreach ($transports as $transport) {
+      $result[$transport->id()] = $transport->getDsn();
+    }
+    return $result;
+  }
+
+  public function getTransportNameFromDsn(string $dsn): ?string {
+    $key = array_search($dsn, $this->getDsns());
+    return $key !== FALSE ? $key : NULL;
+  }
+
+  public function getDefaultTransportName(): ?string {
+    return array_key_first($this->getDsns());
+  }
+
+}