Enhance HTTP/3 request body detection and testing

- Updated `IHttpRequestBodyDetectionFeature` to include HTTP/3 descriptions for the `END_STREAM` flag.
- Modified request body handling logic to return zero length content body instance when there endstream flag is set.
  It also completes the message body for empty requests, so that the RequestBodyPipe.Reader is closed (needed when
  stream is reused by the pool and the Pipe is reset).
- Added unit test `CanHaveBody_ReturnsFalseWithoutRequestBody`
to validate behavior when no request body is present.

Author: ladeak

src/Http/Http.Features/src/IHttpRequestBodyDetectionFeature.cs

@@ -20,6 +20,9 @@ public interface IHttpRequestBodyDetectionFeature
     /// <item><description>
     /// It's an HTTP/2 request that did not set the END_STREAM flag on the initial headers frame.
     /// </description></item>
+    /// <item><description>
+    /// It's an HTTP/3 request that did not set the END_STREAM flag on the initial headers frame.
+    /// </description></item>
     /// </list>
     /// The final request body length may still be zero for the chunked or HTTP/2 scenarios.
     /// <para>
@@ -35,6 +38,9 @@ public interface IHttpRequestBodyDetectionFeature
     /// <item><description>
     /// It's an HTTP/2 request that set END_STREAM on the initial headers frame.
     /// </description></item>
+    /// <item><description>
+    /// It's an HTTP/3 request that set END_STREAM on the initial headers frame.
+    /// </description></item>
     /// </list>
     /// </para>
     /// When false, the request body should never return data.

src/Servers/Kestrel/Core/src/Internal/Http3/Http3Stream.cs

@@ -54,6 +54,7 @@ internal abstract partial class Http3Stream : HttpProtocol, IHttp3Stream, IHttpS
     private bool _isMethodConnect;
     private bool _isWebTransportSessionAccepted;
     private Http3MessageBody? _messageBody;
+    private bool _requestBodyStarted;
 
     private readonly ManualResetValueTaskSource<object?> _appCompletedTaskSource = new();
     private readonly Lock _completionLock = new();
@@ -65,6 +66,17 @@ internal abstract partial class Http3Stream : HttpProtocol, IHttp3Stream, IHttpS
     private bool IsAbortedRead => (_completionState & StreamCompletionFlags.AbortedRead) == StreamCompletionFlags.AbortedRead;
     public bool IsCompleted => (_completionState & StreamCompletionFlags.Completed) == StreamCompletionFlags.Completed;
 
+    public bool ReceivedEmptyRequestBody
+    {
+        get
+        {
+            lock (_completionLock)
+            {
+                return EndStreamReceived && !_requestBodyStarted;
+            }
+        }
+    }
+
     public Pipe RequestBodyPipe { get; private set; } = default!;
     public long? InputRemaining { get; internal set; }
     public QPackDecoder QPackDecoder { get; private set; } = default!;
@@ -928,7 +940,7 @@ private Task ProcessDataFrameAsync(in ReadOnlySequence<byte> payload)
             {
                 return Task.CompletedTask;
             }
-
+            _requestBodyStarted = true;
             foreach (var segment in payload)
             {
                 RequestBodyPipe.Writer.Write(segment.Span);
@@ -975,6 +987,12 @@ protected override MessageBody CreateMessageBody()
             _messageBody = new Http3MessageBody(this);
         }
 
+        if (ReceivedEmptyRequestBody)
+        {
+            _messageBody.Complete(exception: null);
+            return MessageBody.ZeroContentLengthClose;
+        }
+
         return _messageBody;
     }
 

src/Servers/Kestrel/test/InMemory.FunctionalTests/Http3/Http3StreamTests.cs

@@ -3291,6 +3291,32 @@ public async Task Control_FrameParsingSingleByteAtATimeWorks()
         await outboundcontrolStream.ReceiveEndAsync();
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task CanHaveBody_ReturnsFalseWithoutRequestBody(bool endstream)
+    {
+        var headers = new[]
+        {
+            new KeyValuePair<string, string>(InternalHeaderNames.Method, "GET"),
+            new KeyValuePair<string, string>(InternalHeaderNames.Path, "/"),
+            new KeyValuePair<string, string>(InternalHeaderNames.Scheme, "http"),
+            new KeyValuePair<string, string>(InternalHeaderNames.Authority, "localhost:80"),
+        };
+
+        var requestStream = await Http3Api.InitializeConnectionAndStreamsAsync(context =>
+        {
+            Assert.Equal(!endstream, context.Features.Get<IHttpRequestBodyDetectionFeature>().CanHaveBody);
+            context.Response.StatusCode = 200;
+            return Task.CompletedTask;
+        }, headers, endStream: endstream);
+
+        var responseHeaders = await requestStream.ExpectHeadersAsync();
+        Assert.Equal("200", responseHeaders[InternalHeaderNames.Status]);
+
+        await requestStream.ExpectReceiveEndOfStream();
+    }
+
     private async Task WriteOneByteAtATime(PipeReader reader, PipeWriter writer)
     {
         var res = await reader.ReadAsync();