Clean up scanline decoding code.

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -5,6 +5,7 @@ using System;
 using System.Buffers.Binary;
 using System.Collections.Generic;
 using System.IO;
+using System.IO.Compression;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
@@ -175,18 +176,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                                     this.InitializeImage(metadata, out image);
                                 }
 
-                                var deframeStream = new ZlibInflateStream(this.currentStream, this.ReadNextDataChunk);
-                                try
-                                {
-                                    deframeStream.AllocateNewBytes(chunk.Length, true);
-                                    this.ReadScanlines(deframeStream.CompressedStream, image.Frames.RootFrame, pngMetadata);
-                                }
-                                finally
-                                {
-                                    // If an invalid Zlib stream is discovered the decoder will throw an exception
-                                    // due to the critical nature of the data chunk.
-                                    deframeStream.Dispose();
-                                }
+                                this.ReadScanlines(chunk, image.Frames.RootFrame, pngMetadata);
 
                                 break;
                             case PngChunkType.Palette:
@@ -472,19 +462,25 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// Reads the scanlines within the image.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
-        /// <param name="dataStream">The <see cref="MemoryStream"/> containing data.</param>
+        /// <param name="chunk">The png chunk containing the compressed scanline data.</param>
         /// <param name="image"> The pixel data.</param>
         /// <param name="pngMetadata">The png metadata</param>
-        private void ReadScanlines<TPixel>(Stream dataStream, ImageFrame<TPixel> image, PngMetadata pngMetadata)
+        private void ReadScanlines<TPixel>(PngChunk chunk, ImageFrame<TPixel> image, PngMetadata pngMetadata)
             where TPixel : struct, IPixel<TPixel>
         {
-            if (this.header.InterlaceMethod == PngInterlaceMode.Adam7)
-            {
-                this.DecodeInterlacedPixelData(dataStream, image, pngMetadata);
-            }
-            else
+            using (var deframeStream = new ZlibInflateStream(this.currentStream, this.ReadNextDataChunk))
             {
-                this.DecodePixelData(dataStream, image, pngMetadata);
+                deframeStream.AllocateNewBytes(chunk.Length, true);
+                DeflateStream dataStream = deframeStream.CompressedStream;
+
+                if (this.header.InterlaceMethod == PngInterlaceMode.Adam7)
+                {
+                    this.DecodeInterlacedPixelData(dataStream, image, pngMetadata);
+                }
+                else
+                {
+                    this.DecodePixelData(dataStream, image, pngMetadata);
+                }
             }
         }
 
@@ -1021,7 +1017,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         private bool TryUncompressTextData(ReadOnlySpan<byte> compressedData, Encoding encoding, out string value)
         {
             using (var memoryStream = new MemoryStream(compressedData.ToArray()))
-            using (var inflateStream = new ZlibInflateStream(memoryStream, () => 0))
+            using (var inflateStream = new ZlibInflateStream(memoryStream))
             {
                 if (!inflateStream.AllocateNewBytes(compressedData.Length, false))
                 {

src/ImageSharp/Formats/Png/Zlib/ZlibInflateStream.cs

@@ -20,14 +20,14 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         private static readonly byte[] ChecksumBuffer = new byte[4];
 
         /// <summary>
-        /// The inner raw memory stream
+        /// A default delegate to get more data from the inner stream.
         /// </summary>
-        private readonly Stream innerStream;
+        private static readonly Func<int> GetDataNoOp = () => 0;
 
         /// <summary>
-        /// The compressed stream sitting over the top of the deframer
+        /// The inner raw memory stream
         /// </summary>
-        private DeflateStream compressedStream;
+        private readonly Stream innerStream;
 
         /// <summary>
         /// A value indicating whether this instance of the given entity has been disposed.
@@ -55,8 +55,17 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         /// <summary>
         /// Initializes a new instance of the <see cref="ZlibInflateStream"/> class.
         /// </summary>
-        /// <param name="innerStream">The inner raw stream</param>
-        /// <param name="getData">A delegate to get more data from the inner stream</param>
+        /// <param name="innerStream">The inner raw stream.</param>
+        public ZlibInflateStream(Stream innerStream)
+            : this(innerStream, GetDataNoOp)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ZlibInflateStream"/> class.
+        /// </summary>
+        /// <param name="innerStream">The inner raw stream.</param>
+        /// <param name="getData">A delegate to get more data from the inner stream.</param>
         public ZlibInflateStream(Stream innerStream, Func<int> getData)
         {
             this.innerStream = innerStream;
@@ -76,12 +85,12 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         public override long Length => throw new NotSupportedException();
 
         /// <inheritdoc/>
-        public override long Position { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
+        public override long Position { get => throw new NotSupportedException(); set => throw new NotSupportedException(); }
 
         /// <summary>
         /// Gets the compressed stream over the deframed inner stream
         /// </summary>
-        public DeflateStream CompressedStream => this.compressedStream;
+        public DeflateStream CompressedStream { get; private set; }
 
         /// <summary>
         /// Adds new bytes from a frame found in the original stream
@@ -92,7 +101,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         public bool AllocateNewBytes(int bytes, bool isCriticalChunk)
         {
             this.currentDataRemaining = bytes;
-            if (this.compressedStream is null)
+            if (this.CompressedStream is null)
             {
                 return this.InitializeInflateStream(isCriticalChunk);
             }
@@ -101,10 +110,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         }
 
         /// <inheritdoc/>
-        public override void Flush()
-        {
-            throw new NotSupportedException();
-        }
+        public override void Flush() => throw new NotSupportedException();
 
         /// <inheritdoc/>
         public override int ReadByte()
@@ -186,10 +192,10 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
             if (disposing)
             {
                 // dispose managed resources
-                if (this.compressedStream != null)
+                if (this.CompressedStream != null)
                 {
-                    this.compressedStream.Dispose();
-                    this.compressedStream = null;
+                    this.CompressedStream.Dispose();
+                    this.CompressedStream = null;
                 }
             }
 
@@ -264,7 +270,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
             }
 
             // Initialize the deflate Stream.
-            this.compressedStream = new DeflateStream(this, CompressionMode.Decompress, true);
+            this.CompressedStream = new DeflateStream(this, CompressionMode.Decompress, true);
 
             return true;
         }