[release/6.0] HTTP/3: Use new QuicStream.ReadsCompleted property in transport

src/Servers/Kestrel/Transport.Quic/src/Internal/QuicStreamContext.cs

@@ -187,7 +187,30 @@ private async Task DoReceive()
 
                     input.Advance(bytesReceived);
 
-                    var flushTask = input.FlushAsync();
+                    ValueTask<FlushResult> flushTask;
+
+                    if (_stream.ReadsCompleted)
+                    {
+                        // If the data returned from ReadAsync is the final chunk on the stream then
+                        // flush data and end pipe together with CompleteAsync.
+                        //
+                        // Getting data and complete together is important for HTTP/3 when parsing headers.
+                        // It is important that it knows that there is no body after the headers.
+                        var completeTask = input.CompleteAsync(ResolveCompleteReceiveException(error));
+                        if (completeTask.IsCompletedSuccessfully)
+                        {
+                            // Fast path. CompleteAsync completed immediately.
+                            flushTask = ValueTask.FromResult(new FlushResult(isCanceled: false, isCompleted: true));
+                        }
+                        else
+                        {
+                            flushTask = AwaitCompleteTaskAsync(completeTask);
+                        }
+                    }
+                    else
+                    {
+                        flushTask = input.FlushAsync();
+                    }
 
                     var paused = !flushTask.IsCompleted;
 
@@ -240,12 +263,23 @@ private async Task DoReceive()
             finally
             {
                 // If Shutdown() has already bee called, assume that was the reason ProcessReceives() exited.
-                Input.Complete(_shutdownReadReason ?? _shutdownReason ?? error);
+                Input.Complete(ResolveCompleteReceiveException(error));
 
                 FireStreamClosed();
 
                 await _waitForConnectionClosedTcs.Task;
             }
+
+            async static ValueTask<FlushResult> AwaitCompleteTaskAsync(ValueTask completeTask)
+            {
+                await completeTask;
+                return new FlushResult(isCanceled: false, isCompleted: true);
+            }
+        }
+
+        private Exception? ResolveCompleteReceiveException(Exception? error)
+        {
+            return _shutdownReadReason ?? _shutdownReason ?? error;
         }
 
         private void FireStreamClosed()

src/Servers/Kestrel/Transport.Quic/test/QuicStreamContextTests.cs

@@ -255,6 +255,38 @@ public async Task ClientToServerUnidirectionalStream_ClientAbort_ServerReceivesA
             await closedTcs.Task.DefaultTimeout();
         }
 
+        [ConditionalFact]
+        [MsQuicSupported]
+        public async Task ClientToServerUnidirectionalStream_CompleteWrites_PipeProvidesDataAndCompleteTogether()
+        {
+            // Arrange
+            await using var connectionListener = await QuicTestHelpers.CreateConnectionListenerFactory(LoggerFactory);
+
+            var options = QuicTestHelpers.CreateClientConnectionOptions(connectionListener.EndPoint);
+            using var quicConnection = new QuicConnection(QuicImplementationProviders.MsQuic, options);
+            await quicConnection.ConnectAsync().DefaultTimeout();
+
+            await using var serverConnection = await connectionListener.AcceptAndAddFeatureAsync().DefaultTimeout();
+
+            // Act
+            await using var clientStream = quicConnection.OpenUnidirectionalStream();
+            await clientStream.WriteAsync(TestData).DefaultTimeout();
+
+            await using var serverStream = await serverConnection.AcceptAsync().DefaultTimeout();
+            var readResult = await serverStream.Transport.Input.ReadAtLeastAsync(TestData.Length).DefaultTimeout();
+            serverStream.Transport.Input.AdvanceTo(readResult.Buffer.End);
+
+            var readResultTask = serverStream.Transport.Input.ReadAsync();
+
+            await clientStream.WriteAsync(TestData, endStream: true).DefaultTimeout();
+
+            // Assert
+            var completeReadResult = await readResultTask.DefaultTimeout();
+
+            Assert.Equal(TestData, completeReadResult.Buffer.ToArray());
+            Assert.True(completeReadResult.IsCompleted);
+        }
+
         [ConditionalFact]
         [MsQuicSupported]
         public async Task ServerToClientUnidirectionalStream_ServerWritesDataAndCompletes_GracefullyClosed()