MapDynamicPageRoute doesn't work with Index routes

This repository is to demonstrate a bug in ASP.NET Core 6 routing when using a DynamicRouteValueTransformer

To reproduce:

1. Create a new Razor Pages project

2. Create this DynamicRouteValueTransformer

public class RouteTransformer : DynamicRouteValueTransformer
{   
    public override ValueTask<RouteValueDictionary> TransformAsync(HttpContext httpContext, RouteValueDictionary values)
    {   
        values["page"] = "/Index";
        return ValueTask.FromResult(values);
    }
}

3. Register it in Program.cs

app.MapDynamicPageRoute<RouteTransformer>("{**dynamicRoute}");

4. Visit a URL that would get routed by the DynamicRouteValueTransformer (like /foobar) and it will say that there are two routes for /Index

AmbiguousMatchException: The request matched multiple endpoints. Matches:

/Index
/Index
Microsoft.AspNetCore.Routing.Matching.DefaultEndpointSelector.ReportAmbiguity(CandidateState[] candidateState)

This only impacts Index routing (and only Razor Pages, not MVC)

For example, change RouteTransformer to have values["page"] = "/Privacy"; and it will route /foobar to /Privacy and the Privacy Policy page will show correctly. If you move the files to /Home/Index and /Home/Privacy and set values["page"] = "/Home/Index";, you'll get the same AmbiguousMatchException.

---

I don't know enough about ASP.NET's routing internals so the following might be wrong.

It's getting two routes for /Index rather than a route for "/Index" and a route for "".

When applying the DynamicPageEndpointMatcherPolicy and specifically selector.SelectEndpoints, we end up matching two routes because ActionSelectionTable<Endpoint> Table has two entries for new string[] {"/Index"} via ActionSelectionTable.Select. This gets populated off the ActionDescriptor.RouteValues via ActionSelectionTable.Create. tl;dr: We have two entries that have a RouteValues that match /Index.

So where does this happen? It looks like PageActionDescriptorProvider takes the /Index route with two selectors and sets two Index routes in AddActionDescriptors (one for each selector). The problem is that it uses model.RouteValues for each selector and so it gets two /Index routes rather than a /Index and /.

So how should this be fixed? I'm not sure. It seems like we need something like TransformPageRoute that takes into consideration the selector (but as I noted, I don't know enough about ASP.NET's routing internals to speak well on this topic).

Maybe:

var pageRouteMetadata = selectorModel.EndpointMetadata.OfType<PageRouteMetadata>().SingleOrDefault();
if (pageRouteMetadata != null)
{
    descriptor.RouteValues.Add("page", pageRouteMetadata.PageRoute);
}

But this is where I'm running into a wall since Rider doesn't seem to want to open the aspnetcore repository on my Apple Silocon Mac so I can't really test this.

There's a test which seems to think the descriptor.RouteValues["page"] should have /Index for both descriptors, despite the descriptor.AttributeRouteInfo.Template only having /Index in one of the two. So I could simply be wrong about the RouteValues.

I can use a custom IPageRouteModelConvention to remove the /Index selector and reset the model.RouteValues["page"] to exclude /Index. I'm not sure I'd ever want to route to /People/Index instead of just /People so there is a workaround.