Handle WebP Exif with preamble

Some WebP files seem to have the JPEG Exif preamble. It may be an error,
but it's safe to recover from it.

Fixes the .NET side of drewnoakes/metadata-extractor#473.

Author: drewnoakes

MetadataExtractor/Formats/Exif/ExifReader.cs

@@ -24,19 +24,27 @@ namespace MetadataExtractor.Formats.Exif
     /// <author>Drew Noakes https://drewnoakes.com</author>
     public sealed class ExifReader : IJpegSegmentMetadataReader
     {
-        /// <summary>Exif data stored in JPEG files' APP1 segment are preceded by this six character preamble.</summary>
+        /// <summary>Exif data stored in JPEG files' APP1 segment are preceded by this six character preamble "Exif\0\0".</summary>
         public const string JpegSegmentPreamble = "Exif\x0\x0";
 
         ICollection<JpegSegmentType> IJpegSegmentMetadataReader.SegmentTypes => new[] { JpegSegmentType.App1 };
 
         public DirectoryList ReadJpegSegments(IEnumerable<JpegSegment> segments)
         {
             return segments
-                .Where(segment => segment.Bytes.Length >= JpegSegmentPreamble.Length && Encoding.UTF8.GetString(segment.Bytes, 0, JpegSegmentPreamble.Length) == JpegSegmentPreamble)
+                .Where(segment => StartsWithJpegExifPreamble(segment.Bytes))
                 .SelectMany(segment => Extract(new ByteArrayReader(segment.Bytes, baseOffset: JpegSegmentPreamble.Length)))
                 .ToList();
         }
 
+        /// <summary>
+        /// Indicates whether <paramref name="bytes"/> starts with <see cref="JpegSegmentPreamble"/>.
+        /// </summary>
+        public static bool StartsWithJpegExifPreamble(byte[] bytes)
+        {
+            return bytes.Length >= JpegSegmentPreamble.Length && Encoding.UTF8.GetString(bytes, 0, JpegSegmentPreamble.Length) == JpegSegmentPreamble;
+        }
+
         /// <summary>
         /// Reads TIFF formatted Exif data a specified offset within a <see cref="IndexedReader"/>.
         /// </summary>

MetadataExtractor/Formats/WebP/WebpRiffHandler.cs

@@ -50,7 +50,12 @@ public void ProcessChunk(string fourCc, byte[] payload)
             {
                 case "EXIF":
                 {
-                    _directories.AddRange(new ExifReader().Extract(new ByteArrayReader(payload)));
+                    // We have seen WebP images with and without the preamble here. It's likely that some software incorrectly
+                    // copied an entire JPEG segment into the WebP image. Regardless, we can handle it here.
+                    var reader = ExifReader.StartsWithJpegExifPreamble(payload)
+                        ? new ByteArrayReader(payload, ExifReader.JpegSegmentPreamble.Length)
+                        : new ByteArrayReader(payload);
+                    _directories.AddRange(new ExifReader().Extract(reader));
                     break;
                 }
                 case "ICCP":

MetadataExtractor/PublicAPI/net45/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static MetadataExtractor.Formats.Exif.ExifReader.StartsWithJpegExifPreamble(byte[]! bytes) -> bool