bug symfony#59596 [Mime] use isRendered method to ensure we can avoid rendering an email twice (walva)

This PR was merged into the 6.4 branch.

Discussion
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[Mime] use `isRendered` method to ensure we can avoid rendering an email twice

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | yes? Not sure
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Issues        |
| License       | MIT

# Description:

After upgrading my project from Symfony 5.4 to 6.4, due this [change](https://github.com/symfony/symfony/pull/47075/files), I encountered an issue where asynchronous email sending failed due to serialization problems with `TemplatedEmail`. According to the [Symfony documentation](https://symfony.com/doc/current/mailer.html), when sending an email asynchronously, the email instance must be serializable. While Mailer instances are inherently serializable, `TemplatedEmail `requires 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 Symfony `Mailer` to force the rendering of emails using the `BodyRendererInterface::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 same `TemplatedEmail `object was rendered again later in the execution process, specifically in the [MessageListener](https://github.com/symfony/mailer/blob/6.4/EventListener/MessageListener.php#L72).

To prevent this double rendering, I utilized the `TemplatedEmail::isRendered(): bool` method, as seen in the [AbstractTransport](https://github.com/symfony/symfony/blob/7.3/src/Symfony/Component/Mailer/Transport/AbstractTransport.php#L83). 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 the `TemplatedEmail` has already been rendered before attempting to render it again. By using the `isRendered()` 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 extending `TemplatedEmail` 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](https://symfony.com/doc/current/mailer.html)
[MessageListener Implementation](https://github.com/symfony/mailer/blob/6.4/EventListener/MessageListener.php#L72)
[AbstractTransport Implementation](https://github.com/symfony/symfony/blob/7.3/src/Symfony/Component/Mailer/Transport/AbstractTransport.php#L83)
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 on https://github.com/symfony/symfony/pull/47075/files.

I would recommend using the method TemplatedEmail::isRendered() the same way it is used in [AbstractTransport.php](https://github.com/symfony/symfony/blob/7.3/src/Symfony/Component/Mailer/Transport/AbstractTransport.php#L83).

Keeping like this prevents subclass of TemplatedEmail with computed html template to not be rendered twice, such as [NotificationEmail](https://github.com/symfony/symfony/blob/7.3/src/Symfony/Bridge/Twig/Mime/NotificationEmail.php#L202).

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

Commits
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2d37c79 [Mime] use isRendered method to avoid rendering an email twice

Author: nicolas-grekas

src/Symfony/Bridge/Twig/Mime/BodyRenderer.php

@@ -45,7 +45,7 @@ public function render(Message $message): void
             return;
         }
 
-        if (null === $message->getTextTemplate() && null === $message->getHtmlTemplate()) {
+        if ($message->isRendered()) {
             // email has already been rendered
             return;
         }

src/Symfony/Bridge/Twig/Tests/Mime/BodyRendererTest.php

@@ -105,10 +105,14 @@ public function testRenderedOnce()
         ;
         $email->textTemplate('text');
 
+        $this->assertFalse($email->isRendered());
         $renderer->render($email);
+        $this->assertTrue($email->isRendered());
+
         $this->assertEquals('Text', $email->getTextBody());
 
         $email->text('reset');
+        $this->assertTrue($email->isRendered());
 
         $renderer->render($email);
         $this->assertEquals('reset', $email->getTextBody());