Twig embed leads to recursion and InvalidComponentException

I'm not sure what I should add/change in MR at this point. I can't understand where this bug comes from. It seems to me that it is potentially a bug in Twig itself. I'm not familiar with Twig's internal code, but I think I should try to find out where the template is compiled to find out why it compiles it with a wrong template reference which leads to a recursion.

I just don't know where to approach this task. It's definitely not a ComponentValidator problem. Everything hints and leads to Twig. Basically, that 'example:foo' component tried to be rendered twice, while it is a single call. And the second call comes from a wrong Twig compiled template, which is a little hard to follow and understand, so I want to try to find out where and how these templates are built in the first place.

Maybe I'll try to make a failtest later.

Added a failtest which highlights the issue as well: https://git.drupalcode.org/issue/drupal-3464719/-/jobs/2292129#L3250

Did a quick and dirty test with just Twig, and it works fine.

script.php:

<?php

use Twig\Environment;
use Twig\Loader\FilesystemLoader;

$loader = new FilesystemLoader(__DIR__ . '/templates');
$twig = new Environment($loader, [
  'cache' => __DIR__ . '/cache',
]);
echo $twig->render('foo.twig');

./templates/foo.twig:

<div class="wrapper">
  {% embed 'bar.twig' with {} only %}{% endembed %}
</div>

./templates/bar.twig:

Hello from Bar!

Output:

$ drush scr var/scripts/twig/test.php 
<div class="wrapper">
  Hello from Bar!
</div>

I'm almost given up, but found a major clue. It seems that \Drupal\Core\Template\ComponentNodeVisitor::leaveNode should exit if the node has a parent.

It seems like when you render a node with a parent set, this filter validate_component_props receives a wrong context, which leads to an exception during validation.

I've tested locally, and everything seems to be working fine now.

Well, the pipeline confirms that it fixes a problem and doesn't break anything else (at least what has been tested). On my project, everything works as expected now as well. Honestly, for me, it is hard to explain why this helps.

Just an assumption from debugging: It looks like such components are represented twice in the Twig Node Tree, one of them is some kind of container (wrapper), it doesn't have a payload with variables, but \Drupal\Core\Template\ComponentNodeVisitor::leaveNode still adds a validation function to it, which leads to an exception. Twig also adds 'extends' to 'embed' tag, which can also be a potential hint to a source of the problem. It internally compiles something like include 'sdc_test:wrapper' extends 'sdc_test:wrapper', and that extends have a different context, but passing checks for component name in \Drupal\Core\Template\ComponentNodeVisitor::leaveNode.

Updated summary and simplified it a little, since there is a failtest and a possible solution exists.

Tested locally and it seems to have resolved the issue as well.