Shopware 6 - cache key of logged in storefront user

Is there a way to figure out the full cache key of a logged in frontend user?
The task is to dispay a specific info on the product listing and on the detail page after a customer has bought the product. So we need to clear the specific cache after the customer bought a product.

You can inject CacheInvalidator and invalidate the cache entries tagged with the product id yourself.

use Shopware\Core\Framework\Adapter\Cache\CacheInvalidator;
use Shopware\Core\Framework\DataAbstractionLayer\Cache\EntityCacheKeyGenerator;

class CacheInvalidatorService
{
    public function __construct(private readonly CacheInvalidator $cacheInvalidator) 
    {
    }

    public function invalidate(string $productId): void
    {
        $this->cacheInvalidator->invalidate([EntityCacheKeyGenerator::buildProductTag($productId)]);
    }
}

This should suffice if you have a generic value that updates after each purchase, like a counter increasing with each purchase or something.

The customer itself has no specific cache key. Otherwise there would be a cache entry for every single customer, essentially contradicting the principle of caching. So if you want to display something entirely customer specific, like if a customer has bought the product before, you can listen to the specific page load event, e.g. ProductPageLoadedEvent. In the subscriber you can then use the current customer, fetch additional data and add it as extension. With a logged-in customer the http cache should be circumvented by default and the object-cached response, including the product(s) will be part of the aforementioned events.

use Shopware\Storefront\Page\Product\ProductPageLoadedEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class MySubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents(): array
    {
        return [
            ProductPageLoadedEvent::class => 'onProductPageLoaded',
        ];
    }

    public function onProductPageLoaded(ProductPageLoadedEvent $event): void
    {
        // the object that could've previously been cached
        $page = $event->getPage();
        
        $customer = $event->getSalesChannelContext()->getCustomer();
        $product = $page->getProduct();
        
        if (!$customer) {
            return;
        }
        
        // do something with the customer and product
        // ...
        
        // add additional data as extension for use in the template
        $page->addExtension('myExtension', $someExtension);
    }
}