RenderController should support async/await downstream

[mortenbock]

When inheriting RenderController the Index method is implemented in a "sync" pattern.

https://github.com/umbraco/Umbraco-CMS/blob/3f6ebe7656d6ded634db2c268a4b60d11fa076c7/src/Umbraco.Web.Common/Controllers/RenderController.cs#L47C1-L50

/// <summary>
///     The default action to render the front-end view.
/// </summary>
public virtual IActionResult Index() => CurrentTemplate(new ContentModel(CurrentPage));

This means that any downstream code that needs to do async IO work will have to block the threads with .Result or similar, which can (and have) lead to thread pool exhaustion, deadlocks and general performance issues.

So, as a suggestion, the method signature should be something like:

public virtual Task<IActionResult> Index(CancellationToken cancellationToken);

[bergmania]

That will be super breaking to change that signature.

IIRC you can add the signature in your superclass and add a [NonAction] attribute. Then you can next to it have an async version and the original sync-version will be ignored.

[mortenbock]

I appreciate that it is a breaking change, but I think there are ways around that. It could be by also having a AsyncRenderController with the new signature, so developers can inherit that when needed, or some other way of wrapping implementations of the old class.

I do think it is important to signal that the stack is inherently async, by also having the entry point reflect that, so people don't get caught out doing .Result.

Thank you for the suggestion on [NonAction]. I will test that out for our use case.

In any situation, I think it would be good to expand the documentation with best practice examples of how to support async downstream:
https://docs.umbraco.com/umbraco-cms/reference/routing/custom-controllers#replace-umbracos-default-rendercontroller