Use pooled stream for async decode/identify

src/ImageSharp/Formats/Bmp/BmpDecoder.cs

@@ -83,11 +83,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         }
 
         /// <inheritdoc/>
-        public async Task<IImageInfo> IdentifyAsync(Configuration configuration, Stream stream)
+        public Task<IImageInfo> IdentifyAsync(Configuration configuration, Stream stream)
         {
             Guard.NotNull(stream, nameof(stream));
 
-            return await new BmpDecoderCore(configuration, this).IdentifyAsync(stream).ConfigureAwait(false);
+            return new BmpDecoderCore(configuration, this).IdentifyAsync(stream);
         }
     }
 }

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -9,6 +9,7 @@ using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 using SixLabors.ImageSharp.Common.Helpers;
+using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
@@ -257,7 +258,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
         public async Task<IImageInfo> IdentifyAsync(Stream stream)
         {
-            using (var ms = new MemoryStream())
+            using (var ms = new FixedCapacityPooledMemoryStream(stream.Length))
             {
                 await stream.CopyToAsync(ms).ConfigureAwait(false);
                 ms.Position = 0;

src/ImageSharp/Formats/Gif/GifDecoder.cs

@@ -77,12 +77,12 @@ namespace SixLabors.ImageSharp.Formats.Gif
         }
 
         /// <inheritdoc/>
-        public async Task<IImageInfo> IdentifyAsync(Configuration configuration, Stream stream)
+        public Task<IImageInfo> IdentifyAsync(Configuration configuration, Stream stream)
         {
             Guard.NotNull(stream, nameof(stream));
 
             var decoder = new GifDecoderCore(configuration, this);
-            return await decoder.IdentifyAsync(stream).ConfigureAwait(false);
+            return decoder.IdentifyAsync(stream);
         }
 
         /// <inheritdoc />

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -7,6 +7,7 @@ using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading.Tasks;
+using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
@@ -199,7 +200,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
             }
             else
             {
-                using (var ms = new MemoryStream())
+                using (var ms = new FixedCapacityPooledMemoryStream(stream.Length))
                 {
                     await stream.CopyToAsync(ms).ConfigureAwait(false);
                     ms.Position = 0;

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -266,7 +266,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             }
             else
             {
-                using (var ms = new MemoryStream())
+                using (var ms = new FixedCapacityPooledMemoryStream(stream.Length))
                 {
                     await stream.CopyToAsync(ms).ConfigureAwait(false);
                     ms.Position = 0;

src/ImageSharp/Formats/Png/PngDecoder.cs

@@ -9,25 +9,8 @@ using SixLabors.ImageSharp.PixelFormats;
 namespace SixLabors.ImageSharp.Formats.Png
 {
     /// <summary>
-    /// Encoder for generating an image out of a png encoded stream.
+    /// Decoder for generating an image out of a png encoded stream.
     /// </summary>
-    /// <remarks>
-    /// At the moment the following features are supported:
-    /// <para>
-    /// <b>Filters:</b> all filters are supported.
-    /// </para>
-    /// <para>
-    /// <b>Pixel formats:</b>
-    /// <list type="bullet">
-    ///     <item>RGBA (True color) with alpha (8 bit).</item>
-    ///     <item>RGB (True color) without alpha (8 bit).</item>
-    ///     <item>grayscale with alpha (8 bit).</item>
-    ///     <item>grayscale without alpha (8 bit).</item>
-    ///     <item>Palette Index with alpha (8 bit).</item>
-    ///     <item>Palette Index without alpha (8 bit).</item>
-    /// </list>
-    /// </para>
-    /// </remarks>
     public sealed class PngDecoder : IImageDecoder, IPngDecoderOptions, IImageInfoDetector
     {
         /// <summary>
@@ -97,10 +80,10 @@ namespace SixLabors.ImageSharp.Formats.Png
         }
 
         /// <inheritdoc/>
-        public async Task<IImageInfo> IdentifyAsync(Configuration configuration, Stream stream)
+        public Task<IImageInfo> IdentifyAsync(Configuration configuration, Stream stream)
         {
             var decoder = new PngDecoderCore(configuration, this);
-            return await decoder.IdentifyAsync(stream).ConfigureAwait(false);
+            return decoder.IdentifyAsync(stream);
         }
 
         /// <inheritdoc />

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -13,6 +13,7 @@ using System.Threading.Tasks;
 using SixLabors.ImageSharp.Formats.Png.Chunks;
 using SixLabors.ImageSharp.Formats.Png.Filters;
 using SixLabors.ImageSharp.Formats.Png.Zlib;
+using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.Metadata.Profiles.Exif;
@@ -277,7 +278,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             }
             else
             {
-                using (var ms = new MemoryStream())
+                using (var ms = new FixedCapacityPooledMemoryStream(stream.Length))
                 {
                     await stream.CopyToAsync(ms).ConfigureAwait(false);
                     ms.Position = 0;

src/ImageSharp/Formats/Tga/TgaDecoderCore.cs

@@ -6,6 +6,7 @@ using System.Buffers;
 using System.IO;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
+using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
@@ -689,7 +690,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
             }
             else
             {
-                using (var ms = new MemoryStream())
+                using (var ms = new FixedCapacityPooledMemoryStream(stream.Length))
                 {
                     await stream.CopyToAsync(ms).ConfigureAwait(false);
                     ms.Position = 0;

src/ImageSharp/IO/FixedCapacityPooledMemoryStream.cs

@@ -0,0 +1,50 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Buffers;
+using System.IO;
+
+namespace SixLabors.ImageSharp.IO
+{
+    /// <summary>
+    /// A memory stream constructed from a pooled buffer of known length.
+    /// </summary>
+    internal sealed class FixedCapacityPooledMemoryStream : MemoryStream
+    {
+        private readonly byte[] buffer;
+        private bool isDisposed;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="FixedCapacityPooledMemoryStream"/> class.
+        /// </summary>
+        /// <param name="length">The length of the stream buffer to rent.</param>
+        public FixedCapacityPooledMemoryStream(long length)
+            : this(RentBuffer(length)) => this.Length = length;
+
+        private FixedCapacityPooledMemoryStream(byte[] buffer)
+            : base(buffer) => this.buffer = buffer;
+
+        /// <inheritdoc/>
+        public override long Length { get; }
+
+        /// <inheritdoc/>
+        protected override void Dispose(bool disposing)
+        {
+            if (!this.isDisposed)
+            {
+                this.isDisposed = true;
+
+                if (disposing)
+                {
+                    ArrayPool<byte>.Shared.Return(this.buffer);
+                }
+
+                base.Dispose(disposing);
+            }
+        }
+
+        // In the extrememly unlikely event someone ever gives us a stream
+        // with length longer than int.MaxValue then we'll use something else.
+        private static byte[] RentBuffer(long length) => ArrayPool<byte>.Shared.Rent((int)length);
+    }
+}

src/ImageSharp/Image.FromStream.cs

@@ -7,6 +7,7 @@ using System.IO;
 using System.Text;
 using System.Threading.Tasks;
 using SixLabors.ImageSharp.Formats;
+using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp
@@ -674,10 +675,7 @@ namespace SixLabors.ImageSharp
             }
 
             // We want to be able to load images from things like HttpContext.Request.Body
-            // TODO: Should really find a nice way to use a pool for these.
-            // Investigate readonly version of the linked implementation.
-            // https://github.com/mgravell/Pipelines.Sockets.Unofficial/compare/mgravell:24482d4...mgravell:6740ea4
-            using (var memoryStream = new MemoryStream())
+            using (var memoryStream = new FixedCapacityPooledMemoryStream(stream.Length))
             {
                 stream.CopyTo(memoryStream);
                 memoryStream.Position = 0;
@@ -713,8 +711,7 @@ namespace SixLabors.ImageSharp
                 return await action(stream).ConfigureAwait(false);
             }
 
-            // TODO: See above comment.
-            using (var memoryStream = new MemoryStream())
+            using (var memoryStream = new FixedCapacityPooledMemoryStream(stream.Length))
             {
                 await stream.CopyToAsync(memoryStream).ConfigureAwait(false);
                 memoryStream.Position = 0;

tests/ImageSharp.Tests/TestUtilities/AsyncStreamWrapper.cs

@@ -26,7 +26,7 @@ namespace SixLabors.ImageSharp.Tests.TestUtilities
 
         public override bool CanWrite => this.inner.CanWrite;
 
-        public override long Length => throw new NotSupportedException("The stream is not seekable.");
+        public override long Length => this.inner.Length;
 
         public override long Position
         {