fix for loading multiple sub ifds

Fixes #2857

src/ImageSharp/Metadata/Profiles/Exif/ExifReader.cs

@@ -189,10 +189,19 @@ internal abstract class BaseExifReader
     {
         if (this.subIfds is not null)
         {
-            foreach (ulong subIfdOffset in this.subIfds)
+            do
             {
-                this.ReadValues(values, (uint)subIfdOffset);
+                int sz = this.subIfds.Count;
+                Span<ulong> buf = sz <= 256 ? stackalloc ulong[sz] : new ulong[sz];
+
+                this.subIfds.CopyTo(buf);
+                this.subIfds.Clear();
+                foreach (ulong subIfdOffset in buf)
+                {
+                    this.ReadValues(values, (uint)subIfdOffset);
+                }
             }
+            while (this.subIfds.Count > 0);
         }
     }
 

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

@@ -10,6 +10,7 @@ using SixLabors.ImageSharp.Metadata.Profiles.Icc;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
 using SixLabors.ImageSharp.Tests.TestUtilities;
+using static SixLabors.ImageSharp.Metadata.Profiles.Exif.EncodedString;
 
 // ReSharper disable InconsistentNaming
 namespace SixLabors.ImageSharp.Tests.Formats.Jpg;
@@ -439,9 +440,8 @@ public partial class JpegDecoderTests
         Assert.Equal(expectedComment, metadata.Comments.ElementAtOrDefault(0).ToString());
         image.DebugSave(provider);
         image.CompareToOriginal(provider);
-
     }
-    
+
     // https://github.com/SixLabors/ImageSharp/issues/2758
     [Theory]
     [WithFile(TestImages.Jpeg.Issues.Issue2758, PixelTypes.L8)]
@@ -468,6 +468,47 @@ public partial class JpegDecoderTests
         image.Save(ms, new JpegEncoder());
     }
 
+    // https://github.com/SixLabors/ImageSharp/issues/2857
+    [Theory]
+    [WithFile(TestImages.Jpeg.Issues.Issue2857, PixelTypes.Rgb24)]
+    public void Issue2857_SubSubIfds<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(JpegDecoder.Instance);
+
+        Assert.Equal(5616, image.Width);
+        Assert.Equal(3744, image.Height);
+
+        JpegMetadata meta = image.Metadata.GetJpegMetadata();
+        Assert.Equal(92, meta.LuminanceQuality);
+        Assert.Equal(93, meta.ChrominanceQuality);
+
+        ExifProfile exifProfile = image.Metadata.ExifProfile;
+        Assert.NotNull(exifProfile);
+
+        using MemoryStream ms = new();
+        bool hasThumbnail = exifProfile.TryCreateThumbnail(out _);
+        Assert.False(hasThumbnail);
+
+        Assert.Equal("BilderBox - Erwin Wodicka / wodicka@aon.at", exifProfile.GetValue(ExifTag.Copyright).Value);
+        Assert.Equal("Adobe Photoshop CS3 Windows", exifProfile.GetValue(ExifTag.Software).Value);
+
+        Assert.Equal("Carers; seniors; caregiver; senior care; retirement home; hands; old; elderly; elderly caregiver; elder care; elderly care; geriatric care; nursing home; age; old age care; outpatient; needy; health care; home nurse; home care; sick; retirement; medical; mobile; the elderly; nursing department; nursing treatment; nursing; care services; nursing services; nursing care; nursing allowance; nursing homes; home nursing; care category; nursing class; care; nursing shortage; nursing patient care staff\0", exifProfile.GetValue(ExifTag.XPKeywords).Value);
+
+        Assert.Equal(
+            new EncodedString(CharacterCode.ASCII, "StockSubmitter|Miscellaneous||Miscellaneous$|00|0000330000000110000000000000000|22$@NA_1005010.460@145$$@Miscellaneous.Miscellaneous$$@$@26$$@$@$@$@205$@$@$@$@$@$@$@$@$@43$@$@$@$$@Miscellaneous.Miscellaneous$$@90$$@22$@$@$@$@$@$@$|||"),
+            exifProfile.GetValue(ExifTag.UserComment).Value);
+
+        image.Mutate(x => x.Crop(new(0, 0, 100, 100)));
+
+        image.Save(ms, new JpegEncoder());
+
+        foreach (IExifValue val in image.Metadata.ExifProfile.Values)
+        {
+            this.Output.WriteLine($"{val.Tag}={val.GetValue()}");
+        }
+    }
+
     private static void VerifyEncodedStrings(ExifProfile exif)
     {
         Assert.NotNull(exif);

tests/ImageSharp.Tests/TestImages.cs

@@ -325,6 +325,7 @@ public static class TestImages
             public const string Issue2067_CommentMarker = "Jpg/issues/issue-2067-comment.jpg";
             public const string Issue2638 = "Jpg/issues/Issue2638.jpg";
             public const string Issue2758 = "Jpg/issues/issue-2758.jpg";
+            public const string Issue2857 = "Jpg/issues/issue-2857-subsub-ifds.jpg";
 
             public static class Fuzz
             {

tests/Images/Input/Jpg/issues/issue-2857-subsub-ifds.jpg

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:ab45137ded01a42658aa94421165a358b184a536b6ab64427d8255e8d78c25b9
+size 2829977