Ignore zero length writes when automatically chunking responses

- Zero length writes would previously be interpreted as the end of response
- Optimize writing the chunked response suffix
- Add tests for automatic response chunking

src/Microsoft.AspNet.Server.Kestrel/Http/Frame.cs

@@ -257,8 +257,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
                 if (_autoChunk)
                 {
-                    WriteChunkPrefix(numOctets: 0);
+                    WriteChunkedResponseSuffix();
-                    WriteChunkSuffix();
                 }
             }
             catch (Exception ex)
@@ -278,6 +277,12 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
             if (_autoChunk)
             {
+                if (data.Count == 0)
+                {
+                    callback(null, state);
+                    return;
+                }
+
                 WriteChunkPrefix(data.Count);
             }
 
@@ -306,6 +311,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
         }
 
         private static readonly ArraySegment<byte> _endChunkBytes = CreateAsciiByteArraySegment("\r\n");
+        private static readonly ArraySegment<byte> _endChunkedResponseBytes = CreateAsciiByteArraySegment("0\r\n\r\n");
 
         private void WriteChunkSuffix()
         {
@@ -321,6 +327,20 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
                 immediate: true);
         }
 
+        private void WriteChunkedResponseSuffix()
+        {
+            SocketOutput.Write(_endChunkedResponseBytes,
+                (error, _) =>
+                {
+                    if (error != null)
+                    {
+                        Trace.WriteLine("WriteChunkedResponseSuffix" + error.ToString());
+                    }
+                },
+                null,
+                immediate: true);
+        }
+
         public void Upgrade(IDictionary<string, object> options, Func<object, Task> callback)
         {
             _keepAlive = false;

test/Microsoft.AspNet.Server.KestrelTests/ChunkedResponseTests.cs

@@ -0,0 +1,75 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Text;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Microsoft.AspNet.Server.KestrelTests
+{
+    public class ChunkedResponseTests
+    {
+        [Fact]
+        public async Task ResponsesAreChunkedAutomatically()
+        {
+            using (var server = new TestServer(async frame =>
+            {
+                frame.ResponseHeaders.Clear();
+                await frame.ResponseBody.WriteAsync(Encoding.ASCII.GetBytes("Hello "), 0, 6);
+                await frame.ResponseBody.WriteAsync(Encoding.ASCII.GetBytes("World!"), 0, 6);
+            }))
+            {
+                using (var connection = new TestConnection())
+                {
+                    await connection.SendEnd(
+                        "GET / HTTP/1.1",
+                        "",
+                        "");
+                    await connection.ReceiveEnd(
+                        "HTTP/1.1 200 OK",
+                        "Transfer-Encoding: chunked",
+                        "",
+                        "6",
+                        "Hello ",
+                        "6",
+                        "World!",
+                        "0",
+                        "",
+                        "");
+                }
+            }
+        }
+
+        [Fact]
+        public async Task ZeroLengthWritesAreIgnored()
+        {
+            using (var server = new TestServer(async frame =>
+            {
+                frame.ResponseHeaders.Clear();
+                await frame.ResponseBody.WriteAsync(Encoding.ASCII.GetBytes("Hello "), 0, 6);
+                await frame.ResponseBody.WriteAsync(new byte[0], 0, 0);
+                await frame.ResponseBody.WriteAsync(Encoding.ASCII.GetBytes("World!"), 0, 6);
+            }))
+            {
+                using (var connection = new TestConnection())
+                {
+                    await connection.SendEnd(
+                        "GET / HTTP/1.1",
+                        "",
+                        "");
+                    await connection.ReceiveEnd(
+                        "HTTP/1.1 200 OK",
+                        "Transfer-Encoding: chunked",
+                        "",
+                        "6",
+                        "Hello ",
+                        "6",
+                        "World!",
+                        "0",
+                        "",
+                        "");
+                }
+            }
+        }
+    }
+}