Don't backpeddle when reading chunk lengths. Fix #1127

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -1101,10 +1101,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
             while (length < 0 || length > (this.currentStream.Length - this.currentStream.Position))
             {
-                // Not a valid chunk so we skip back all but one of the four bytes we have just read.
+                // Not a valid chunk so try again until we reach a known chunk.
-                // That lets us read one byte at a time until we reach a known chunk.
-                this.currentStream.Position -= 3;
-
                 if (!this.TryReadChunkLength(out length))
                 {
                     chunk = default;

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

@@ -243,6 +243,23 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             Assert.Null(ex);
         }
 
+        [Theory]
+        [WithFile(TestImages.Png.Issue1127, PixelTypes.Rgba32)]
+        public void Issue1127<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            System.Exception ex = Record.Exception(
+                () =>
+                {
+                    using (Image<TPixel> image = provider.GetImage(PngDecoder))
+                    {
+                        image.DebugSave(provider);
+                        image.CompareToOriginal(provider, ImageComparer.Exact);
+                    }
+                });
+            Assert.Null(ex);
+        }
+
         [Theory]
         [WithFile(TestImages.Png.Splash, PixelTypes.Rgba32)]
         [WithFile(TestImages.Png.Bike, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -90,6 +90,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Issue1014_4 = "Png/issues/Issue_1014_4.png";
             public const string Issue1014_5 = "Png/issues/Issue_1014_5.png";
             public const string Issue1014_6 = "Png/issues/Issue_1014_6.png";
+            public const string Issue1127 = "Png/issues/Issue_1127.png";
 
             public static class Bad
             {

tests/Images/Input/Png/issues/Issue_1127.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:9913e68387bb596198089315ffd8e01d27356493f78b26add68b5cf37183b239
+size 13855