Merge pull request #1812 from SixLabors/af/InternalDetectFormat-DoNotAllocate

stackalloc header buffer in InternalDetectFormat

src/ImageSharp/Image.Decode.cs

@@ -58,9 +58,10 @@ namespace SixLabors.ImageSharp
                 return null;
             }
 
-            using (IMemoryOwner<byte> buffer = config.MemoryAllocator.Allocate<byte>(headerSize, AllocationOptions.Clean))
+            // Header sizes are so small, that headersBuffer will be always stackalloc-ed in practice,
-            {
+            // and heap allocation will never happen, there is no need for the usual try-finally ArrayPool dance.
-                Span<byte> bufferSpan = buffer.GetSpan();
+            // The array case is only a safety mechanism following stackalloc best practices.
+            Span<byte> headersBuffer = headerSize > 512 ? new byte[headerSize] : stackalloc byte[headerSize];
             long startPosition = stream.Position;
 
             // Read doesn't always guarantee the full returned length so read a byte
@@ -69,7 +70,7 @@ namespace SixLabors.ImageSharp
             int i;
             do
             {
-                    i = stream.Read(bufferSpan, n, headerSize - n);
+                i = stream.Read(headersBuffer, n, headerSize - n);
                 n += i;
             }
             while (n < headerSize && i > 0);
@@ -79,11 +80,21 @@ namespace SixLabors.ImageSharp
             // Does the given stream contain enough data to fit in the header for the format
             // and does that data match the format specification?
             // Individual formats should still check since they are public.
-                return config.ImageFormatsManager.FormatDetectors
+            IImageFormat format = null;
-                    .Where(x => x.HeaderSize <= headerSize)
+            foreach (IImageFormatDetector formatDetector in config.ImageFormatsManager.FormatDetectors)
-                    .Select(x => x.DetectFormat(buffer.GetSpan())).LastOrDefault(x => x != null);
+            {
+                if (formatDetector.HeaderSize <= headerSize)
+                {
+                    IImageFormat attemptFormat = formatDetector.DetectFormat(headersBuffer);
+                    if (attemptFormat != null)
+                    {
+                        format = attemptFormat;
                     }
                 }
+            }
+
+            return format;
+        }
 
         /// <summary>
         /// By reading the header on the provided stream this calculates the images format.