Request body transforms only allowed on non-empty requests already

[vtali]

Describe the bug

Per documentation we are able to modify the body.
https://learn.microsoft.com/en-us/aspnet/core/fundamentals/servers/yarp/extensibility-transforms?view=aspnetcore-9.0#request-body-transforms

This only true if the request already comes in with a body.

I have a requirement where I need to on a specific POST route, add a default body.
The POST request will come from a source that I'm not able to modify, but need to then, add a body before sending it to its downstream destination.

To Reproduce

Submitting a POST with no body results in the: Replacing the YARP outgoing request HttpContent is not supported.
This works if I do have a body of course.

context.AddRequestTransform(async requestContext =>
                {
                    using var reader = new StreamReader(requestContext.HttpContext.Request.Body);
                    // TODO: size limits, timeouts
                    var body = await reader.ReadToEndAsync();
                    //if (!string.IsNullOrEmpty(body))
                    {
                        body = "test=test";
                        var bytes = Encoding.UTF8.GetBytes(body);
                        // Change Content-Length to match the modified body, or remove it.
                        requestContext.HttpContext.Request.Body = new MemoryStream(bytes);
                    }
                });

Is there somewhere else in the pipeline that I can add a body to the request in a YARP way?
I can do this in the middleware before Yarp, but then it feels like overkill for a single route to take over?
Thank you for any suggestions/advice

PS. Reporting it as bug either documentation (to describe this only applicable to requests that already come in with a body) or if it's not intended to be stopped like it is right now?

[MihaZupan]

You ran into #2617.

If you want to add a body to a request that previously had none, you'll have to do it in a middleware right now, including updating the IHttpRequestBodyDetectionFeature as described in #2617.
Note that you can still filter based on context.GetRouteModel().Config.RouteId there.

Submitting a POST with no body results in the: Replacing the YARP outgoing request HttpContent is not supported.

You'll see that exception if you try to replace the proxyRequest.Content property (never supported), not if you change the HttpContext.Request.Body.

[MihaZupan]

We've had a few users hit this, so this'd be worth documenting. Keeping open to track docs improvements.

[MihaZupan]

Documentation has been improved in dotnet/AspNetCore.Docs#35422