Fast path pre-completed Input reads

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

@@ -116,13 +116,6 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
             Complete();
         }
 
-        public void AbortAwaiting()
-        {
-            _awaitableError = new ObjectDisposedException(nameof(SocketInput), "The request was aborted");
-
-            Complete();
-        }
-
         private void Complete()
         {
             var awaitableState = Interlocked.Exchange(
@@ -177,6 +170,13 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
             }
         }
 
+        public void AbortAwaiting()
+        {
+            _awaitableError = new ObjectDisposedException(nameof(SocketInput), "The request was aborted");
+
+            Complete();
+        }
+
         public SocketInput GetAwaiter()
         {
             return this;

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

@@ -1,8 +1,8 @@
 // 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.Threading.Tasks;
+using Microsoft.AspNet.Server.Kestrel.Infrastructure;
 
 namespace Microsoft.AspNet.Server.Kestrel.Http
 {
@@ -10,15 +10,9 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
     {
         public static ValueTask<int> ReadAsync(this SocketInput input, byte[] buffer, int offset, int count)
         {
-            while (true)
+            while (input.IsCompleted)
             {
-                if (!input.IsCompleted)
-                {
-                    return input.ReadAsyncAwaited(buffer, offset, count);
-                }
-
                 var begin = input.ConsumingStart();
-
                 int actual;
                 var end = begin.CopyTo(buffer, offset, count, out actual);
                 input.ConsumingComplete(end, end);
@@ -32,6 +26,8 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
                     return 0;
                 }
             }
+
+            return input.ReadAsyncAwaited(buffer, offset, count);
         }
 
         private static async Task<int> ReadAsyncAwaited(this SocketInput input, byte[] buffer, int offset, int count)

src/Microsoft.AspNet.Server.Kestrel/Infrastructure/TaskUtilities.cs

@@ -12,5 +12,6 @@ namespace Microsoft.AspNet.Server.Kestrel.Infrastructure
 #else
         public static Task CompletedTask = Task.FromResult<object>(null);
 #endif
+        public static Task<int> ZeroTask = Task.FromResult(0);
     }
 }