http.sys (6.0 backport): new option for response buffering

src/Servers/HttpSys/src/HttpSysOptions.cs

@@ -35,6 +35,8 @@ public class HttpSysOptions
         /// </summary>
         public HttpSysOptions()
         {
+            const string EnableKernelResponseBufferingSwitch = "Microsoft.AspNetCore.Server.HttpSys.EnableKernelResponseBuffering";
+            EnableKernelResponseBuffering = AppContext.TryGetSwitch(EnableKernelResponseBufferingSwitch, out var enabled) && enabled;
         }
 
         /// <summary>
@@ -109,6 +111,15 @@ public string? RequestQueueName
         /// </summary>
         public bool ThrowWriteExceptions { get; set; }
 
+        /// <summary>
+        /// Enable buffering of response data in the Kernel.
+        /// 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>
+        internal bool EnableKernelResponseBuffering { get; set; } // internal via app-context for non-public release
+
         /// <summary>
         /// Gets or sets the maximum number of concurrent connections to accept, -1 for infinite, or null to
         /// use the machine wide setting from the registry. The default value is null.

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

@@ -280,8 +280,6 @@ 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(HttpApiTypes.HTTP_DATA_CHUNK[]? dataChunks,
             ResponseStreamAsyncResult? asyncResult,
             HttpApiTypes.HTTP_FLAGS flags,

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

@@ -42,6 +42,8 @@ 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
@@ -466,6 +468,13 @@ 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

@@ -133,6 +133,41 @@ 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()
         {