Added fast path for single memory ReadableBuffer (#1512)

* Special case single buffer
* Added fast path for single span buffers in UvWriteReq

src/Microsoft.AspNetCore.Server.Kestrel/Adapter/Internal/StreamSocketOutput.cs

@@ -109,12 +109,20 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Adapter.Internal
                             await _outputStream.FlushAsync();
                         }
 
+                        if (buffer.IsSingleSpan)
+                        {
+                            var array = buffer.First.GetArray();
+                            await _outputStream.WriteAsync(array.Array, array.Offset, array.Count);
+                        }
+                        else
+                        {
                             foreach (var memory in buffer)
                             {
                                 var array = memory.GetArray();
                                 await _outputStream.WriteAsync(array.Array, array.Offset, array.Count);
                             }
                         }
+                    }
                     finally
                     {
                         _pipe.Reader.Advance(readResult.Buffer.End);

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Networking/UvWriteReq.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Buffers;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.IO.Pipelines;
 using System.Runtime.InteropServices;
 using Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure;
@@ -68,10 +67,17 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Networking
                 _pins.Add(GCHandle.Alloc(this, GCHandleType.Normal));
 
                 var nBuffers = 0;
+                if (buffer.IsSingleSpan)
+                {
+                    nBuffers = 1;
+                }
+                else
+                {
                     foreach (var _ in buffer)
                     {
                         nBuffers++;
                     }
+                }
 
                 var pBuffers = (Libuv.uv_buf_t*)_bufs;
                 if (nBuffers > BUFFER_COUNT)
@@ -82,11 +88,24 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Networking
                     _pins.Add(gcHandle);
                     pBuffers = (Libuv.uv_buf_t*)gcHandle.AddrOfPinnedObject();
                 }
+
+                if (nBuffers == 1)
+                {
+                    var memory = buffer.First;
+                    var memoryHandle = memory.Pin();
+                    _handles.Add(memoryHandle);
+
+                    // Fast path for single buffer
+                    pBuffers[0] = Libuv.buf_init(
+                            (IntPtr)memoryHandle.PinnedPointer,
+                            memory.Length);
+                }
+                else
+                {
                     var index = 0;
                     foreach (var memory in buffer)
                     {
-                    // REVIEW: This isn't necessary for our default pool since the memory is 
+                        // This won't actually pin the buffer since we're already using pinned memory
-                    // already pinned but it also makes tests pass
                         var memoryHandle = memory.Pin();
                         _handles.Add(memoryHandle);
 
@@ -96,6 +115,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Networking
                             memory.Length);
                         index++;
                     }
+                }
 
                 _callback = callback;
                 _state = state;