Small optimizations (#1617)

- Return ValueTask instead of Task from WriteAsync helpers
- Use TryGet instead of foreach to avoid enumerator (though it's just a stack allocation here)

src/Common/PipeWriterStream.cs

@@ -67,7 +67,7 @@ namespace System.IO.Pipelines
         {
             _pipeWriter.Write(source.Span);
             _length += source.Length;
-            return new ValueTask(Task.CompletedTask);
+            return default;
         }
 #endif
     }

src/Common/StreamExtensions.cs

@@ -11,10 +11,27 @@ namespace System.IO
 {
     internal static class StreamExtensions
     {
-        public static async Task WriteAsync(this Stream stream, ReadOnlySequence<byte> buffer, CancellationToken cancellationToken = default)
+        public static ValueTask WriteAsync(this Stream stream, ReadOnlySequence<byte> buffer, CancellationToken cancellationToken = default)
         {
-            // REVIEW: Should we special case IsSingleSegment here?
-            foreach (var segment in buffer)
+            if (buffer.IsSingleSegment)
+            {
+#if NETCOREAPP2_1
+                return stream.WriteAsync(buffer.First, cancellationToken);
+#else
+                var isArray = MemoryMarshal.TryGetArray(buffer.First, out var arraySegment);
+                // We're using the managed memory pool which is backed by managed buffers
+                Debug.Assert(isArray);
+                return new ValueTask(stream.WriteAsync(arraySegment.Array, arraySegment.Offset, arraySegment.Count, cancellationToken));
+#endif
+            }
+
+            return WriteMultiSegmentAsync(stream, buffer, cancellationToken);
+        }
+
+        private static async ValueTask WriteMultiSegmentAsync(Stream stream, ReadOnlySequence<byte> buffer, CancellationToken cancellationToken)
+        {
+            var position = buffer.Start;
+            while (buffer.TryGet(ref position, out var segment))
             {
 #if NETCOREAPP2_1
                 await stream.WriteAsync(segment, cancellationToken);

src/Common/WebSocketExtensions.cs

@@ -1,12 +1,9 @@
 // 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;
 using System.Buffers;
-using System.Collections.Generic;
 using System.Diagnostics;
 using System.Runtime.InteropServices;
-using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -14,29 +11,50 @@ namespace System.Net.WebSockets
 {
     internal static class WebSocketExtensions
     {
-        public static Task SendAsync(this WebSocket webSocket, ReadOnlySequence<byte> buffer, WebSocketMessageType webSocketMessageType, CancellationToken cancellationToken = default)
+        public static ValueTask SendAsync(this WebSocket webSocket, ReadOnlySequence<byte> buffer, WebSocketMessageType webSocketMessageType, CancellationToken cancellationToken = default)
         {
-            // TODO: Consider chunking writes here if we get a multi segment buffer
 #if NETCOREAPP2_1
             if (buffer.IsSingleSegment)
             {
-                return webSocket.SendAsync(buffer.First, webSocketMessageType, endOfMessage: true, cancellationToken).AsTask();
+                return webSocket.SendAsync(buffer.First, webSocketMessageType, endOfMessage: true, cancellationToken);
             }
             else
             {
-                return webSocket.SendAsync(buffer.ToArray(), webSocketMessageType, endOfMessage: true, cancellationToken);
+                return SendMultiSegmentAsync(webSocket, buffer, webSocketMessageType, cancellationToken);
             }
 #else
             if (buffer.IsSingleSegment)
             {
                 var isArray = MemoryMarshal.TryGetArray(buffer.First, out var segment);
                 Debug.Assert(isArray);
-                return webSocket.SendAsync(segment, webSocketMessageType, endOfMessage: true, cancellationToken);
+                return new ValueTask(webSocket.SendAsync(segment, webSocketMessageType, endOfMessage: true, cancellationToken));
             }
             else
             {
-                return webSocket.SendAsync(new ArraySegment<byte>(buffer.ToArray()), webSocketMessageType, true, cancellationToken);
+                return SendMultiSegmentAsync(webSocket, buffer, webSocketMessageType, cancellationToken);
             }
+#endif
+        }
+
+        private static async ValueTask SendMultiSegmentAsync(WebSocket webSocket, ReadOnlySequence<byte> buffer, WebSocketMessageType webSocketMessageType, CancellationToken cancellationToken = default)
+        {
+            var position = buffer.Start;
+            while (buffer.TryGet(ref position, out var segment))
+            {
+#if NETCOREAPP2_1
+                await webSocket.SendAsync(segment, webSocketMessageType, endOfMessage: false, cancellationToken);
+#else
+                var isArray = MemoryMarshal.TryGetArray(segment, out var arraySegment);
+                Debug.Assert(isArray);
+                await webSocket.SendAsync(arraySegment, webSocketMessageType, endOfMessage: false, cancellationToken);
+#endif
+            }
+
+            // Empty end of message frame
+#if NETCOREAPP2_1
+            await webSocket.SendAsync(Memory<byte>.Empty, webSocketMessageType, endOfMessage: true, cancellationToken);
+#else
+            await webSocket.SendAsync(new ArraySegment<byte>(Array.Empty<byte>()), webSocketMessageType, endOfMessage: true, cancellationToken);
 #endif
         }
     }

src/Microsoft.AspNetCore.Sockets.Client.Http/Internal/StreamExtensions.cs

@@ -12,7 +12,9 @@ namespace System.IO.Pipelines
         {
             try
             {
-                await stream.CopyToAsync(writer, cancellationToken);
+                // REVIEW: Should we use the default buffer size here?
+                // 81920 is the default bufferSize, there is no stream.CopyToAsync overload that takes only a cancellationToken
+                await stream.CopyToAsync(new PipelineWriterStream(writer), bufferSize: 81920, cancellationToken: cancellationToken);
             }
             catch (Exception ex)
             {
@@ -22,19 +24,6 @@ namespace System.IO.Pipelines
             writer.Complete();
         }
 
-        /// <summary>
-        /// Copies the content of a <see cref="Stream"/> into a <see cref="PipeWriter"/>.
-        /// </summary>
-        /// <param name="stream"></param>
-        /// <param name="writer"></param>
-        /// <param name="cancellationToken"></param>
-        /// <returns></returns>
-        private static Task CopyToAsync(this Stream stream, PipeWriter writer, CancellationToken cancellationToken = default)
-        {
-            // 81920 is the default bufferSize, there is not stream.CopyToAsync overload that takes only a cancellationToken
-            return stream.CopyToAsync(new PipelineWriterStream(writer), bufferSize: 81920, cancellationToken: cancellationToken);
-        }
-
         private class PipelineWriterStream : Stream
         {
             private readonly PipeWriter _writer;

src/Microsoft.AspNetCore.Sockets.Client.Http/SendUtils.cs

@@ -111,7 +111,7 @@ namespace Microsoft.AspNetCore.Sockets.Client
 
             protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
             {
-                return stream.WriteAsync(_buffer);
+                return stream.WriteAsync(_buffer).AsTask();
             }
 
             protected override bool TryComputeLength(out long length)