Description:

After upgrading my project from Symfony 5.4 to 6.4, due thischange, I encountered an issue where asynchronous email sending failed due to serialization problems withTemplatedEmail. According to theSymfony documentation, when sending an email asynchronously, the email instance must be serializable. While Mailer instances are inherently serializable,TemplatedEmailrequires that its context be serializable. If the context contains non-serializable variables, such as Doctrine entities, it's recommended to either replace them with serializable variables or render the email before calling$mailer->send($email).

To address this, I decorated the SymfonyMailer to force the rendering of emails using theBodyRendererInterface::render(Message $message): void method. However, this approach led to an exception indicating that the context was empty. The method successfully rendered the email, clearing its context, but the sameTemplatedEmailobject was rendered again later in the execution process, specifically in theMessageListener.

To prevent this double rendering, I utilized theTemplatedEmail::isRendered(): bool method, as seen in theAbstractTransport. This ensures that if an email is already rendered, it won't be rendered again, preserving the context and preventing serialization issues.

Solution

The proposed fix involves checking if theTemplatedEmail has already been rendered before attempting to render it again. By using theisRendered() method, we can avoid unnecessary re-rendering, which can lead to empty contexts and serialization problems during asynchronous email sending.

Impact

This correction allows classes extendingTemplatedEmail to be sent asynchronously without encountering errors related to double rendering or context loss due to serialization issues, as outlined in the documentation.

References

Symfony Mailer Documentation
MessageListener Implementation
AbstractTransport Implementation
This pull request aims to enhance the robustness of asynchronous email handling in Symfony by ensuring that TemplatedEmail instances are not rendered multiple times, thereby maintaining their context and serializability.

Original message:

Following up my comment onhttps://github.com/symfony/symfony/pull/47075/files.

I would recommend using the method TemplatedEmail::isRendered() the same way it is used inAbstractTransport.php.

Keeping like this prevents subclass of TemplatedEmail with computed html template to not be rendered twice, such asNotificationEmail.

This change will allow developers to create a subclass of template emails and render them before sending them async, without rendering them twice.