#Backport SixLabors#2926 to release/2.1.x

Author: mus65

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -114,6 +114,11 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
         /// </summary>
         private PngChunk? nextChunk;
 
+        /// <summary>
+        /// A value indicating whether the image data has been read.
+        /// </summary>
+        private bool hasImageData;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="PngDecoderCore"/> class.
         /// </summary>
@@ -565,7 +570,11 @@ private void ReadScanlines<TPixel>(PngChunk chunk, ImageFrame<TPixel> image, Png
             where TPixel : unmanaged, IPixel<TPixel>
         {
             using var deframeStream = new ZlibInflateStream(this.currentStream, this.ReadNextDataChunk);
-            deframeStream.AllocateNewBytes(chunk.Length, true);
+            if (!deframeStream.AllocateNewBytes(chunk.Length, !this.hasImageData))
+            {
+                return;
+            }
+
             DeflateStream dataStream = deframeStream.CompressedStream;
 
             if (this.header.InterlaceMethod == PngInterlaceMode.Adam7)
@@ -596,7 +605,7 @@ private void DecodePixelData<TPixel>(DeflateStream compressedStream, ImageFrame<
                     int bytesRead = compressedStream.Read(scanlineSpan, this.currentRowBytesRead, this.bytesPerScanline - this.currentRowBytesRead);
                     if (bytesRead <= 0)
                     {
-                        return;
+                        goto EXIT;
                     }
 
                     this.currentRowBytesRead += bytesRead;
@@ -635,6 +644,9 @@ private void DecodePixelData<TPixel>(DeflateStream compressedStream, ImageFrame<
                 this.SwapScanlineBuffers();
                 this.currentRow++;
             }
+
+            EXIT:
+            this.hasImageData = true;
         }
 
         /// <summary>
@@ -672,7 +684,7 @@ private void DecodeInterlacedPixelData<TPixel>(DeflateStream compressedStream, I
                         int bytesRead = compressedStream.Read(this.scanline.GetSpan(), this.currentRowBytesRead, bytesPerInterlaceScanline - this.currentRowBytesRead);
                         if (bytesRead <= 0)
                         {
-                            return;
+                            goto EXIT;
                         }
 
                         this.currentRowBytesRead += bytesRead;
@@ -729,6 +741,9 @@ private void DecodeInterlacedPixelData<TPixel>(DeflateStream compressedStream, I
                     pass = 0;
                     break;
                 }
+
+                EXIT:
+                this.hasImageData = true;
             }
         }
 

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -534,5 +534,15 @@ public void Decode_BadPalette(string file)
             string path = Path.GetFullPath(Path.Combine(TestEnvironment.InputImagesDirectoryFullPath, file));
             using Image image = Image.Load(path);
         }
+
+        [Theory]
+        [WithFile(TestImages.Png.Issue2924, PixelTypes.Rgba32)]
+        public void CanDecode_Issue2924<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage(PngDecoder);
+            image.DebugSave(provider);
+            image.CompareToReferenceOutput(provider);
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -125,6 +125,9 @@ public static class Png
             // Discussion 1875: https://github.com/SixLabors/ImageSharp/discussions/1875
             public const string Issue1875 = "Png/raw-profile-type-exif.png";
 
+             // Issue 2924: https://github.com/SixLabors/ImageSharp/issues/2924
+            public const string Issue2924 = "Png/issues/Issue_2924.png";
+
             public static class Bad
             {
                 public const string MissingDataChunk = "Png/xdtn0g01.png";