better cancellation in jpeg

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -268,6 +268,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             while (fileMarker.Marker != JpegConstants.Markers.EOI
                 || (fileMarker.Marker == JpegConstants.Markers.EOI && fileMarker.Invalid))
             {
+                cancellationToken.ThrowIfCancellationRequested();
+
                 if (!fileMarker.Invalid)
                 {
                     // Get the marker length

src/ImageSharp/Image.FromFile.cs

@@ -131,7 +131,8 @@ namespace SixLabors.ImageSharp
         /// </returns>
         public static async Task<IImageInfo> IdentifyAsync(Configuration configuration, string filePath, CancellationToken cancellationToken)
         {
-            (IImageInfo ImageInfo, IImageFormat Format) res = await IdentifyWithFormatAsync(configuration, filePath, cancellationToken);
+            (IImageInfo ImageInfo, IImageFormat Format) res = await IdentifyWithFormatAsync(configuration, filePath, cancellationToken)
+                    .ConfigureAwait(false);
             return res.ImageInfo;
         }
 

tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.cs

@@ -127,34 +127,44 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
         }
 
         [Theory]
+        [InlineData(TestImages.Jpeg.Baseline.Jpeg420Small, 0)]
         [InlineData(TestImages.Jpeg.Issues.ExifGetString750Transform, 1)]
-        [InlineData(TestImages.Jpeg.Issues.ExifGetString750Transform, 3)]
+        [InlineData(TestImages.Jpeg.Issues.ExifGetString750Transform, 10)]
+        [InlineData(TestImages.Jpeg.Issues.ExifGetString750Transform, 30)]
         [InlineData(TestImages.Jpeg.Issues.BadRstProgressive518, 1)]
-        [InlineData(TestImages.Jpeg.Issues.BadRstProgressive518, 3)]
-        public async Task Decode_IsCancellable(string fileName, int waitMilliseconds)
+        [InlineData(TestImages.Jpeg.Issues.BadRstProgressive518, 10)]
+        [InlineData(TestImages.Jpeg.Issues.BadRstProgressive518, 30)]
+        public async Task Decode_IsCancellable(string fileName, int cancellationDelayMs)
         {
+            // Decoding these huge files took 300ms on i7-8650U in 2020. 30ms should be safe for cancellation delay.
             string hugeFile = Path.Combine(
                 TestEnvironment.InputImagesDirectoryFullPath,
                 fileName);
 
             var cts = new CancellationTokenSource();
-            cts.CancelAfter(waitMilliseconds);
+            if (cancellationDelayMs == 0)
+            {
+                cts.Cancel();
+            }
+            else
+            {
+                cts.CancelAfter(cancellationDelayMs);
+            }
+
             await Assert.ThrowsAsync<TaskCanceledException>(() => Image.LoadAsync(hugeFile, cts.Token));
         }
 
         [Theory]
-        [InlineData(TestImages.Jpeg.Issues.ExifGetString750Transform, 1)]
-        [InlineData(TestImages.Jpeg.Issues.ExifGetString750Transform, 3)]
-        [InlineData(TestImages.Jpeg.Issues.BadRstProgressive518, 1)]
-        [InlineData(TestImages.Jpeg.Issues.BadRstProgressive518, 3)]
-        public async Task Identify_IsCancellable(string fileName, int waitMilliseconds)
+        [InlineData(TestImages.Jpeg.Issues.ExifGetString750Transform)]
+        [InlineData(TestImages.Jpeg.Issues.BadRstProgressive518)]
+        public async Task Identify_IsCancellable(string fileName)
         {
             string hugeFile = Path.Combine(
                 TestEnvironment.InputImagesDirectoryFullPath,
                 fileName);
 
             var cts = new CancellationTokenSource();
-            cts.CancelAfter(waitMilliseconds);
+            cts.CancelAfter(TimeSpan.FromTicks(1));
             await Assert.ThrowsAsync<TaskCanceledException>(() => Image.IdentifyAsync(hugeFile, cts.Token));
         }