HttpRequest.BodyReader.TryRead returns false even though request has a body

[Ugochukwu Mmaduekwe]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

HttpRequest.BodyReader.TryRead returns false (and ReadResult isn't populated) even though the request has a body and the readResult isn't completed or cancelled.
When using HttpRequest.BodyReader.ReadAsync, the ReadResult response is populated successfully.

Here is a Demo that reproduces the issue.

Expected Behavior

HttpRequest.BodyReader.TryRead should not return false when a request body exists and the readResult isn't cancelled or completed.

Steps To Reproduce

1. Clone this Repo, build and run.

2. Fire a POST request to https://localhost:7244/bodyreaderbugdemo/working with the request body below

{
     "name": "Chucky",
     "description": "Child's Play is an American slasher media franchise created by Don Mancini."
}

Notice that the response is a 200 Ok

3. Now, fire another POST request to https://localhost:7244/bodyreaderbugdemo/buggy with the same request body as above.

Notice that we get a 400 Bad Request

Key Notes

1. Both endpoints employ filters that shortcircuits if the read request body is null.

2. The first endpoint uses a filter ExtractRequestBodyWorking which uses HttpRequest.BodyReader.ReadAsync to read request body. This works as expected.

3. However, the second endpoint uses another filter ExtractRequestBodyBuggy which uses HttpRequest.BodyReader.TryRead to read request body. in this case, HttpRequest.BodyReader.TryRead returns false even though a request body exists.

Exceptions (if any)

No response

.NET Version

8.0.200

Anything else?

No response

[Chris Ross]

This is a misunderstanding. TryRead returns false if data isn't immediately available. TryRead or ReadAsync may still return data in the future. The request may have a large body, but it isn't available yet.

I would also suggest trying this without request body buffering, that construct can skew your results since it involves pipe<->stream adapters, you don't have direct access to the server buffers.
https://github.com/dotnet/runtime/blob/52638488c45ef62d1e9273902f57c010c6310f1d/src/libraries/System.IO.Pipelines/src/System/IO/Pipelines/StreamPipeReader.cs#L465-L468

[Ugochukwu Mmaduekwe]

I would also suggest trying this without request body buffering, that construct can skew your results since it involves pipe<->stream adapters, you don't have direct access to the server buffers. https://github.com/dotnet/runtime/blob/52638488c45ef62d1e9273902f57c010c6310f1d/src/libraries/System.IO.Pipelines/src/System/IO/Pipelines/StreamPipeReader.cs#L465-L468

Thanks for replying @Tratcher .
I read some where some months back that Asp.Net Core does not buffer request body by default so if we need to read the body multiple times, we have to enable request.EnableBuffering.

To clear all doubts, I tried your suggestion, I removed request.EnableBuffering and now when trying to read using either ReadAsync or TryRead, they fail with an exception.
I then removed httpRequest.Body.Seek(0, SeekOrigin.Begin); and bodyReader.AdvanceTo(readResult.Buffer.Start, readResult.Buffer.Start); and everything seems to work Ok.
does this mean that I do not have to reset the bodyReader in the finally block after reading?

[Chris Ross]

You can put that all back, it was just removed to simplify the scenario. The real fix is to replace TryRead with ReadAsync.

[Ugochukwu Mmaduekwe]

You can put that all back, it was just removed to simplify the scenario. The real fix is to replace TryRead with ReadAsync.

Ok. but say I need to use TryRead, continue instead of break would be the proper approach right?
something like the code below.

        public static T? ReadBody<T>(this HttpRequest httpRequest)
        {
            var bodyReader = httpRequest.BodyReader;

            ReadResult readResult = default;
            
            do
            {
                if (!bodyReader.TryRead(out readResult))
                    continue; // continue instead of breaking here ??

                bodyReader.AdvanceTo(readResult.Buffer.Start, readResult.Buffer.End);

            } while (!readResult.IsCanceled && !readResult.IsCompleted);

            var buffer = readResult.Buffer;

            if (buffer.IsEmpty)
                return default;

            var reader = new Utf8JsonReader(buffer);

            return JsonSerializer.Deserialize<T>(ref reader, JsonExtensions.JsonOptions);
        }

[Chris Ross]

If TryRead returns false then you need to call ReadAsync next, not stop reading entirely.