Send window updates based on examined rather than consumed.

src/Servers/Kestrel/Core/src/Internal/Http2/Http2MessageBody.cs

@@ -14,6 +14,7 @@ public class Http2MessageBody : MessageBody
     {
         private readonly Http2Stream _context;
         private ReadResult _readResult;
+        private long _alreadyExaminedInNextReadResult;
 
         private Http2MessageBody(Http2Stream context, MinDataRate minRequestBodyDataRate)
             : base(context, minRequestBodyDataRate)
@@ -63,14 +64,57 @@ public override void AdvanceTo(SequencePosition consumed)
 
         public override void AdvanceTo(SequencePosition consumed, SequencePosition examined)
         {
-            var dataLength = _readResult.Buffer.Slice(_readResult.Buffer.Start, consumed).Length;
+            // This code path is fairly hard to understand so let's break it down with an example
+            // ReadAsync returns a ReadResult of length 50.
+            // Advance(25, 40). The examined length would be 40 and consumed length would be 25.
+            // _totalExaminedInPreviousReadResult starts at 0. newlyExamined is 40.
+            // OnDataRead is called with length 40.
+            // _totalExaminedInPreviousReadResult is now 40 - 25 = 15.
+
+            // The next call to ReadAsync returns 50 again
+            // Advance(5, 5) is called
+            // newlyExamined is 5 - 15, or -10.
+            // Update _totalExaminedInPreviousReadResult to 10 as we consumed 5.
+
+            // The next call to ReadAsync returns 50 again
+            // _totalExaminedInPreviousReadResult is 10
+            // Advance(50, 50) is called
+            // newlyExamined = 50 - 10 = 40
+            // _totalExaminedInPreviousReadResult is now 50
+            // _totalExaminedInPreviousReadResult is finally 0 after subtracting consumedLength.
+
+            long examinedLength;
+            long consumedLength;
+            if (consumed.Equals(examined))
+            {
+                examinedLength = _readResult.Buffer.Slice(_readResult.Buffer.Start, examined).Length;
+                consumedLength = examinedLength;
+            }
+            else 
+            {
+                consumedLength = _readResult.Buffer.Slice(_readResult.Buffer.Start, consumed).Length;
+                examinedLength = consumedLength + _readResult.Buffer.Slice(consumed, examined).Length;
+            }
+
             _context.RequestBodyPipe.Reader.AdvanceTo(consumed, examined);
-            OnDataRead(dataLength);
+
+            var newlyExamined = examinedLength - _alreadyExaminedInNextReadResult;
+
+            if (newlyExamined > 0)
+            {
+                OnDataRead(newlyExamined);
+                _alreadyExaminedInNextReadResult += newlyExamined;
+            }
+
+            _alreadyExaminedInNextReadResult -= consumedLength;
         }
 
         public override bool TryRead(out ReadResult readResult)
         {
-            return _context.RequestBodyPipe.Reader.TryRead(out readResult);
+            var result = _context.RequestBodyPipe.Reader.TryRead(out readResult);
+            _readResult = readResult;
+
+            return result;
         }
 
         public override async ValueTask<ReadResult> ReadAsync(CancellationToken cancellationToken = default)

src/Servers/Kestrel/test/InMemory.FunctionalTests/Http2/Http2ConnectionTests.cs

@@ -10,6 +10,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Connections;
+using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Features;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.HPack;
@@ -682,6 +683,100 @@ await ExpectAsync(Http2FrameType.DATA,
             Assert.Equal(updateSize, connectionWindowUpdateFrame.WindowUpdateSizeIncrement);
         }
 
+        [Fact]
+        public async Task DATA_BufferRequestBodyLargerThanStreamSizeSmallerThanConnectionPipe_Works()
+        {
+            var initialStreamWindowSize = _serviceContext.ServerOptions.Limits.Http2.InitialStreamWindowSize;
+            var framesInStreamWindow = initialStreamWindowSize / Http2PeerSettings.DefaultMaxFrameSize;
+            var initialConnectionWindowSize = _serviceContext.ServerOptions.Limits.Http2.InitialConnectionWindowSize;
+            var framesInConnectionWindow = initialConnectionWindowSize / Http2PeerSettings.DefaultMaxFrameSize;
+
+            // Grow the client stream windows so no stream WINDOW_UPDATEs need to be sent.
+            _clientSettings.InitialWindowSize = int.MaxValue;
+
+            await InitializeConnectionAsync(async context =>
+            {
+                await context.Response.BodyWriter.FlushAsync();
+                var readResult = await context.Request.BodyReader.ReadAsync();
+                while (readResult.Buffer.Length != _maxData.Length * 4)
+                {
+                    context.Request.BodyReader.AdvanceTo(readResult.Buffer.Start, readResult.Buffer.End);
+                    readResult = await context.Request.BodyReader.ReadAsync();
+                }
+
+                context.Request.BodyReader.AdvanceTo(readResult.Buffer.Start, readResult.Buffer.End);
+
+                readResult = await context.Request.BodyReader.ReadAsync();
+                Assert.Equal(readResult.Buffer.Length, _maxData.Length * 5);
+
+                await context.Response.BodyWriter.WriteAsync(readResult.Buffer.ToArray());
+
+                context.Request.BodyReader.AdvanceTo(readResult.Buffer.End);
+            });
+
+            // Grow the client connection windows so no connection WINDOW_UPDATEs need to be sent.
+            await SendWindowUpdateAsync(0, int.MaxValue - (int)Http2PeerSettings.DefaultInitialWindowSize);
+
+            await StartStreamAsync(1, _browserRequestHeaders, endStream: false);
+
+            // Rounds down so we don't go over the half window size and trigger an update
+            for (var i = 0; i < framesInStreamWindow / 2; i++)
+            {
+                await SendDataAsync(1, _maxData, endStream: false);
+            }
+
+            // trip over the update size.
+            await SendDataAsync(1, _maxData, endStream: false);
+
+            await ExpectAsync(Http2FrameType.HEADERS,
+                withLength: 37,
+                withFlags: (byte)Http2HeadersFrameFlags.END_HEADERS,
+                withStreamId: 1);
+
+            var dataFrames = new List<Http2FrameWithPayload>();
+
+            var streamWindowUpdateFrame1 = await ExpectAsync(Http2FrameType.WINDOW_UPDATE,
+                withLength: 4,
+                withFlags: (byte)Http2DataFrameFlags.NONE,
+                withStreamId: 1);
+
+            // Writing over half the initial window size induces both a connection-level and stream-level window update.
+
+            await SendDataAsync(1, _maxData, endStream: true);
+
+            for (var i = 0; i < framesInStreamWindow / 2 + 2; i++)
+            {
+                var dataFrame3 = await ExpectAsync(Http2FrameType.DATA,
+                    withLength: _maxData.Length,
+                    withFlags: (byte)Http2DataFrameFlags.NONE,
+                    withStreamId: 1);
+                dataFrames.Add(dataFrame3);
+            }
+
+            var connectionWindowUpdateFrame = await ExpectAsync(Http2FrameType.WINDOW_UPDATE,
+                withLength: 4,
+                withFlags: (byte)Http2DataFrameFlags.NONE,
+                withStreamId: 0);
+            // End
+
+            await ExpectAsync(Http2FrameType.DATA,
+                withLength: 0,
+                withFlags: (byte)Http2DataFrameFlags.END_STREAM,
+                withStreamId: 1);
+
+            await StopConnectionAsync(expectedLastStreamId: 1, ignoreNonGoAwayFrames: false);
+
+            foreach (var frame in dataFrames)
+            {
+                Assert.True(_maxData.AsSpan().SequenceEqual(frame.PayloadSequence.ToArray()));
+            }
+
+            var updateSize = ((framesInStreamWindow / 2) + 1) * _maxData.Length;
+            Assert.Equal(updateSize, streamWindowUpdateFrame1.WindowUpdateSizeIncrement);
+            updateSize = ((framesInConnectionWindow / 2) + 1) * _maxData.Length;
+            Assert.Equal(updateSize, connectionWindowUpdateFrame.WindowUpdateSizeIncrement);
+        }
+
         [Fact]
         public async Task DATA_Received_StreamIdZero_ConnectionError()
         {