Change SendFileAsync to use a fallback implementation instead of throwing

- If the user wants to use the SendFile API directly then they can access the feature
explicitly.

#496

Author: davidfowl

src/Microsoft.AspNet.Http.Abstractions/project.json

@@ -36,6 +36,7 @@
         "System.Net.Primitives": "4.0.11-*",
         "System.Net.WebSockets": "4.0.0-*",
         "System.Reflection.TypeExtensions": "4.0.1-*",
+        "System.IO": "4.0.11-*",
         "System.Runtime": "4.0.21-*",
         "System.Runtime.InteropServices": "4.0.21-*",
         "System.Security.Claims": "4.0.1-*",

src/Microsoft.AspNet.Http.Extensions/Properties/Resources.Designer.cs

@@ -2,9 +2,9 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
-using Microsoft.AspNet.Http.Extensions;
 using Microsoft.AspNet.Http.Features;
 
 namespace Microsoft.AspNet.Http
@@ -33,7 +33,7 @@ public static bool SupportsSendFile(this HttpResponse response)
         /// Sends the given file using the SendFile extension.
         /// </summary>
         /// <param name="response"></param>
-        /// <param name="fileName"></param>
+        /// <param name="fileName">The full or relative path to the file.</param>
         /// <returns></returns>
         public static Task SendFileAsync(this HttpResponse response, string fileName)
         {
@@ -74,10 +74,56 @@ public static Task SendFileAsync(this HttpResponse response, string fileName, lo
             var sendFile = response.HttpContext.Features.Get<IHttpSendFileFeature>();
             if (sendFile == null)
             {
-                throw new NotSupportedException(Resources.Exception_SendFileNotSupported);
+                return SendFileAsync(response.Body, fileName, offset, count, cancellationToken);
             }
 
             return sendFile.SendFileAsync(fileName, offset, count, cancellationToken);
         }
+
+        // Not safe for overlapped writes.
+        private static async Task SendFileAsync(Stream outputStream, string fileName, long offset, long? length, CancellationToken cancel)
+        {
+            cancel.ThrowIfCancellationRequested();
+
+            if (string.IsNullOrWhiteSpace(fileName))
+            {
+                throw new ArgumentNullException(nameof(fileName));
+            }
+            if (!File.Exists(fileName))
+            {
+                throw new FileNotFoundException(string.Empty, fileName);
+            }
+
+            var fileInfo = new FileInfo(fileName);
+            if (offset < 0 || offset > fileInfo.Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(offset), offset, string.Empty);
+            }
+
+            if (length.HasValue &&
+                (length.Value < 0 || length.Value > fileInfo.Length - offset))
+            {
+                throw new ArgumentOutOfRangeException(nameof(length), length, string.Empty);
+            }
+
+            var fileStream = new FileStream(
+                fileName,
+                FileMode.Open,
+                FileAccess.Read,
+                FileShare.ReadWrite,
+                bufferSize: 1024 * 64,
+                options: FileOptions.Asynchronous | FileOptions.SequentialScan);
+
+            try
+            {
+                fileStream.Seek(offset, SeekOrigin.Begin);
+
+                await StreamCopyOperation.CopyToAsync(fileStream, outputStream, length, cancel);
+            }
+            finally
+            {
+                fileStream.Dispose();
+            }
+        }
     }
 }

src/Microsoft.AspNet.Http.Extensions/Resources.resx

@@ -0,0 +1,61 @@
+// 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.Diagnostics;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNet.Http
+{
+    // FYI: In most cases the source will be a FileStream and the destination will be to the network.
+    internal static class StreamCopyOperation
+    {
+        private const int DefaultBufferSize = 1024 * 16;
+
+        internal static async Task CopyToAsync(Stream source, Stream destination, long? length, CancellationToken cancel)
+        {
+            long? bytesRemaining = length;
+            byte[] buffer = new byte[DefaultBufferSize];
+
+            Debug.Assert(source != null);
+            Debug.Assert(destination != null);
+            Debug.Assert(!bytesRemaining.HasValue || bytesRemaining.Value >= 0);
+            Debug.Assert(buffer != null);
+
+            while (true)
+            {
+                // The natural end of the range.
+                if (bytesRemaining.HasValue && bytesRemaining.Value <= 0)
+                {
+                    return;
+                }
+
+                cancel.ThrowIfCancellationRequested();
+
+                int readLength = buffer.Length;
+                if (bytesRemaining.HasValue)
+                {
+                    readLength = (int)Math.Min(bytesRemaining.Value, (long)readLength);
+                }
+                int count = await source.ReadAsync(buffer, 0, readLength, cancel);
+
+                if (bytesRemaining.HasValue)
+                {
+                    bytesRemaining -= count;
+                }
+
+                // End of the source stream.
+                if (count == 0)
+                {
+                    return;
+                }
+
+                cancel.ThrowIfCancellationRequested();
+
+                await destination.WriteAsync(buffer, 0, count, cancel);
+            }
+        }
+    }
+}

src/Microsoft.AspNet.Http.Extensions/SendFileResponseExtensions.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved. See License.txt in the project root for license information.
 
 using System;
+using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http.Features;
@@ -22,10 +23,10 @@ public void SendFileSupport()
         }
 
         [Fact]
-        public Task SendFileWhenNotSupported()
+        public Task SendFileWhenFileNotFoundThrows()
         {
             var response = new DefaultHttpContext().Response;
-            return Assert.ThrowsAsync<NotSupportedException>(() => response.SendFileAsync("foo"));
+            return Assert.ThrowsAsync<FileNotFoundException>(() => response.SendFileAsync("foo"));
         }
 
         [Fact]