http.sys; add opt-in support for kernel-mode response buffering

src/Servers/HttpSys/src/HttpSysOptions.cs

@@ -110,6 +110,15 @@ public string? RequestQueueName
     /// </summary>
     public bool ThrowWriteExceptions { get; set; }
 
+    /// <summary>
+    /// Enable buffering of response data in the Kernel. The default value is <code>false</code>.
+    /// It should be used by an application doing synchronous I/O or by an application doing asynchronous I/O with
+    /// no more than one outstanding write at a time, and can significantly improve throughput over high-latency connections.
+    /// Applications that use asynchronous I/O and that may have more than one send outstanding at a time should not use this flag.
+    /// Enabling this can results in higher CPU and memory usage by Http.Sys.
+    /// </summary>
+    public bool EnableKernelResponseBuffering { get; set; }
+
     /// <summary>
     /// Gets or sets the maximum number of concurrent connections to accept. Set `-1` for infinite.
     /// Set to `null` to use the registry's machine-wide setting.

src/Servers/HttpSys/src/PublicAPI.Unshipped.txt

@@ -1,4 +1,6 @@
 #nullable enable
+Microsoft.AspNetCore.Server.HttpSys.HttpSysOptions.EnableKernelResponseBuffering.get -> bool
+Microsoft.AspNetCore.Server.HttpSys.HttpSysOptions.EnableKernelResponseBuffering.set -> void
 Microsoft.AspNetCore.Server.HttpSys.HttpSysRequestTimingType
 Microsoft.AspNetCore.Server.HttpSys.HttpSysRequestTimingType.ConnectionStart = 0 -> Microsoft.AspNetCore.Server.HttpSys.HttpSysRequestTimingType
 Microsoft.AspNetCore.Server.HttpSys.HttpSysRequestTimingType.DataStart = 1 -> Microsoft.AspNetCore.Server.HttpSys.HttpSysRequestTimingType

src/Servers/HttpSys/src/RequestProcessing/Response.cs

@@ -275,8 +275,6 @@ actual HTTP header will be generated by the application and sent as entity body.
     // This will give us more control of the bytes that hit the wire, including encodings, HTTP 1.0, etc..
     // It may also be faster to do this work in managed code and then pass down only one buffer.
     // What would we loose by bypassing HttpSendHttpResponse?
-    //
-    // TODO: Consider using the HTTP_SEND_RESPONSE_FLAG_BUFFER_DATA flag for most/all responses rather than just Opaque.
     internal unsafe uint SendHeaders(ref UnmanagedBufferAllocator allocator,
         Span<HttpApiTypes.HTTP_DATA_CHUNK> dataChunks,
         ResponseStreamAsyncResult? asyncResult,

src/Servers/HttpSys/src/RequestProcessing/ResponseBody.cs

@@ -39,6 +39,8 @@ internal RequestContext RequestContext
 
     internal bool ThrowWriteExceptions => RequestContext.Server.Options.ThrowWriteExceptions;
 
+    internal bool EnableKernelResponseBuffering => RequestContext.Server.Options.EnableKernelResponseBuffering;
+
     internal bool IsDisposed => _disposed;
 
     public override bool CanSeek
@@ -496,6 +498,13 @@ private HttpApiTypes.HTTP_FLAGS ComputeLeftToWrite(long writeCount, bool endOfRe
         {
             flags |= HttpApiTypes.HTTP_FLAGS.HTTP_SEND_RESPONSE_FLAG_MORE_DATA;
         }
+        if (EnableKernelResponseBuffering)
+        {
+            // "When this flag is set, it should also be used consistently in calls to the HttpSendResponseEntityBody function."
+            // so: make sure we add it in *all* scenarios where it applies - our "close" could be at the end of a bunch
+            // of buffered chunks
+            flags |= HttpApiTypes.HTTP_FLAGS.HTTP_SEND_RESPONSE_FLAG_BUFFER_DATA;
+        }
 
         // Update _leftToWrite now so we can queue up additional async writes.
         if (_leftToWrite > 0)

src/Servers/HttpSys/test/FunctionalTests/ResponseBodyTests.cs

@@ -1,18 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Linq;
 using System.Net.Http;
 using System.Text;
-using System.Threading;
-using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.Testing;
-using Xunit;
 
 namespace Microsoft.AspNetCore.Server.HttpSys;
 
@@ -133,6 +126,41 @@ public async Task ResponseBody_WriteNoHeaders_SetsChunked()
         }
     }
 
+    [ConditionalTheory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task ResponseBody_WriteNoHeaders_SetsChunked_LargeBody(bool enableKernelBuffering)
+    {
+        const int WriteSize = 1024 * 1024;
+        const int NumWrites = 32;
+
+        string address;
+        using (Utilities.CreateHttpServer(
+            baseAddress: out address,
+            configureOptions: options => { options.EnableKernelResponseBuffering = enableKernelBuffering; },
+            app: async httpContext =>
+            {
+                httpContext.Features.Get<IHttpBodyControlFeature>().AllowSynchronousIO = true;
+                for (int i = 0; i < NumWrites - 1; i++)
+                {
+                    httpContext.Response.Body.Write(new byte[WriteSize], 0, WriteSize);
+                }
+                await httpContext.Response.Body.WriteAsync(new byte[WriteSize], 0, WriteSize);
+            }))
+        {
+            var response = await SendRequestAsync(address);
+            Assert.Equal(200, (int)response.StatusCode);
+            Assert.Equal(new Version(1, 1), response.Version);
+            IEnumerable<string> ignored;
+            Assert.False(response.Content.Headers.TryGetValues("content-length", out ignored), "Content-Length");
+            Assert.True(response.Headers.TransferEncodingChunked.HasValue, "Chunked");
+
+            var bytes = await response.Content.ReadAsByteArrayAsync();
+            Assert.Equal(WriteSize * NumWrites, bytes.Length);
+            Assert.True(bytes.All(b => b == 0));
+        }
+    }
+
     [ConditionalFact]
     public async Task ResponseBody_WriteNoHeadersAndFlush_DefaultsToChunked()
     {