Merge pull request #708 from cwensley/curtis/png-decoder-mono

Fix issue on mono <= 5.14 when reading multiple png data chunks.

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -187,6 +187,11 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// </summary>
         private bool hasTrans;
 
+        /// <summary>
+        /// The next chunk of data to return
+        /// </summary>
+        private PngChunk? nextChunk;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="PngDecoderCore"/> class.
         /// </summary>
@@ -222,8 +227,6 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.currentStream.Skip(8);
             Image<TPixel> image = null;
             try
-            {
-                using (var deframeStream = new ZlibInflateStream(this.currentStream))
             {
                 while (!this.isEndChunkReached && this.TryReadChunk(out PngChunk chunk))
                 {
@@ -247,9 +250,12 @@ namespace SixLabors.ImageSharp.Formats.Png
                                     this.InitializeImage(metaData, out image);
                                 }
 
+                                using (var deframeStream = new ZlibInflateStream(this.currentStream, this.ReadNextDataChunk))
+                                {
                                     deframeStream.AllocateNewBytes(chunk.Length);
                                     this.ReadScanlines(deframeStream.CompressedStream, image.Frames.RootFrame);
-                                    this.currentStream.Read(this.crcBuffer, 0, 4);
+                                }
+
                                 break;
                             case PngChunkType.Palette:
                                 byte[] pal = new byte[chunk.Length];
@@ -284,7 +290,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                         chunk.Data?.Dispose(); // Data is rented in ReadChunkData()
                     }
                 }
-                }
 
                 if (image is null)
                 {
@@ -1366,6 +1371,32 @@ namespace SixLabors.ImageSharp.Formats.Png
             metadata.Properties.Add(new ImageProperty(name, value));
         }
 
+        /// <summary>
+        /// Reads the next data chunk.
+        /// </summary>
+        /// <returns>Count of bytes in the next data chunk, or 0 if there are no more data chunks left.</returns>
+        private int ReadNextDataChunk()
+        {
+            if (this.nextChunk != null)
+            {
+                return 0;
+            }
+
+            this.currentStream.Read(this.crcBuffer, 0, 4);
+
+            if (this.TryReadChunk(out PngChunk chunk))
+            {
+                if (chunk.Type == PngChunkType.Data)
+                {
+                    return chunk.Length;
+                }
+
+                this.nextChunk = chunk;
+            }
+
+            return 0;
+        }
+
         /// <summary>
         /// Reads a chunk from the stream.
         /// </summary>
@@ -1375,6 +1406,15 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// </returns>
         private bool TryReadChunk(out PngChunk chunk)
         {
+            if (this.nextChunk != null)
+            {
+                chunk = this.nextChunk.Value;
+
+                this.nextChunk = null;
+
+                return true;
+            }
+
             int length = this.ReadChunkLength();
 
             if (length == -1)

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

@@ -43,22 +43,24 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         private bool isDisposed;
 
         /// <summary>
-        /// Whether the crc value has been read.
+        /// The current data remaining to be read
         /// </summary>
-        private bool crcRead;
+        private int currentDataRemaining;
 
         /// <summary>
-        /// The current data remaining to be read
+        /// Delegate to get more data once we've exhausted the current data remaining
         /// </summary>
-        private int currentDataRemaining;
+        private Func<int> getData;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ZlibInflateStream"/> class.
         /// </summary>
         /// <param name="innerStream">The inner raw stream</param>
-        public ZlibInflateStream(Stream innerStream)
+        /// <param name="getData">A delegate to get more data from the inner stream</param>
+        public ZlibInflateStream(Stream innerStream, Func<int> getData)
         {
             this.innerStream = innerStream;
+            this.getData = getData;
         }
 
         /// <inheritdoc/>
@@ -110,14 +112,38 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         /// <inheritdoc/>
         public override int Read(byte[] buffer, int offset, int count)
         {
+            if (this.currentDataRemaining == 0)
+            {
+                // last buffer was read in its entirety, let's make sure we don't actually have more
+                this.currentDataRemaining = this.getData();
+
                 if (this.currentDataRemaining == 0)
                 {
                     return 0;
                 }
+            }
 
             int bytesToRead = Math.Min(count, this.currentDataRemaining);
             this.currentDataRemaining -= bytesToRead;
-            return this.innerStream.Read(buffer, offset, bytesToRead);
+            int bytesRead = this.innerStream.Read(buffer, offset, bytesToRead);
+
+            // keep reading data until we've reached the end of the stream or filled the buffer
+            while (this.currentDataRemaining == 0 && bytesRead < count)
+            {
+                this.currentDataRemaining = this.getData();
+
+                if (this.currentDataRemaining == 0)
+                {
+                    return bytesRead;
+                }
+
+                offset += bytesRead;
+                bytesToRead = Math.Min(count - bytesRead, this.currentDataRemaining);
+                this.currentDataRemaining -= bytesToRead;
+                bytesRead += this.innerStream.Read(buffer, offset, bytesToRead);
+            }
+
+            return bytesRead;
         }
 
         /// <inheritdoc/>
@@ -153,14 +179,6 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
                 {
                     this.compressedStream.Dispose();
                     this.compressedStream = null;
-
-                    if (!this.crcRead)
-                    {
-                        // Consume the trailing 4 bytes
-                        this.innerStream.Read(ChecksumBuffer, 0, 4);
-                        this.currentDataRemaining -= 4;
-                        this.crcRead = true;
-                    }
                 }
             }