TYPO3 Core

@Georg: Sorry, I don't see how config.disableHrefLang also disables canonical URLs because the patch disables alternate language URLs:

<link rel="alternate" hreflang="de" href="https://..." />

<link rel="canonical" href="https://..." />

Will add this to my todo.

But what is the reason that you don't use the official API's? You can use a PSR-14 event to define the canonical yourself. You can use the event ModifyUrlForCanonicalTagEvent for that. I would not recommend doing it "yourself" as this will lead into the issue we always had: there is no way extensions can work together and define which extension should render the tag and might lead into duplicate tags (as you have now). I'm really looking for use cases which can't be done by the official API.

Thanks for the patch!
We create a bunch of header tags in Aimeos if their content directly depends on the shop product data. Using the TYPO3 API would require us to fetch the data again from the database in the TYPO3 MVC controller which we want to avoid. Thus, in our case it's for performance reasons.

So basically you want to be able to set the canonical (and href I guess) by a controller? I will think about if this is something that is already possible or otherwise if we can make this easily possible.

No, in our case we want to avoid setting the canonical URL in the controller for performance reasons. If we can disable it in the controller, we are totally fine.

Well, my goal is that you should never ever set those tags manually yourself, but doing it by the official API. If I understand you correctly, using an EventListener will cause an extra db call in your situation while you have that information in the controller action isnt it? So if you can set the canonical link in your controller, that would be good isn't it? I assume you always want to have a canonical especially with products and categories in ecommerce.

Aimeos is a bit special because it supports multiple frameworks and applications. For that, the "controller" logic for retrieving the data and passing it to the views is encapsulated in a view model object (called "HTML Cllient"). In the TYPO3 MVC controller we don't have access to the product data any more because all TYPO3 MVC controller actions are dumb and only returning the response from the view model. Here's the TYPO3 MVC catalog controller where you can see the implementation:
https://github.com/aimeos/aimeos-typo3/blob/master/Classes/Controller/CatalogController.php#L48-L53

We pass a PSR-7 request to the view model and would return a PSR-7 response to TYPO3 but Extbase doesn't support that so it's still a string and the generated headers are added to the TYPO3 page here:
https://github.com/aimeos/aimeos-typo3/blob/master/Classes/Controller/AbstractController.php#L159-L167

As you can see, we don't have data from the view model in the TYPO3 MVC controller so offering a method to set the canonical tag there makes no sense for us. The ability to disable generating canonical tags is all we need.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68258

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68258