Add Read(Span<byte>) override to MultipartReaderStream

src/Http/WebUtilities/src/MultipartReaderStream.cs

@@ -116,6 +116,11 @@ public override void Write(byte[] buffer, int offset, int count)
         throw new NotSupportedException();
     }
 
+    public override void Write(ReadOnlySpan<byte> buffer)
+    {
+        throw new NotSupportedException();
+    }
+
     public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
     {
         throw new NotSupportedException();
@@ -164,6 +169,9 @@ private int UpdatePosition(int read)
     }
 
     public override int Read(byte[] buffer, int offset, int count)
+        => Read(buffer.AsSpan(offset, count));
+
+    public override int Read(Span<byte> buffer)
     {
         if (_finished)
         {
@@ -184,7 +192,7 @@ public override int Read(byte[] buffer, int offset, int count)
             if (index != 0)
             {
                 // Sync, it's already buffered
-                var slice = buffer.AsSpan(offset, Math.Min(count, index));
+                var slice = buffer[..Math.Min(buffer.Length, index)];
 
                 var readAmount = _innerStream.Read(slice);
                 return UpdatePosition(readAmount);
@@ -204,7 +212,7 @@ public override int Read(byte[] buffer, int offset, int count)
             // We found a possible match, return any data before it.
             if (matchOffset > bufferedData.Offset)
             {
-                read = _innerStream.Read(buffer, offset, Math.Min(count, matchOffset - bufferedData.Offset));
+                read = _innerStream.Read(buffer[..Math.Min(buffer.Length, matchOffset - bufferedData.Offset)]);
                 return UpdatePosition(read);
             }
 
@@ -215,7 +223,7 @@ public override int Read(byte[] buffer, int offset, int count)
         }
 
         // No possible boundary match within the buffered data, return the data from the buffer.
-        read = _innerStream.Read(buffer, offset, Math.Min(count, bufferedData.Count));
+        read = _innerStream.Read(buffer[..Math.Min(buffer.Length, bufferedData.Count)]);
         return UpdatePosition(read);
 
         static int ReadBoundary(MultipartReaderStream stream, int length)

src/Http/WebUtilities/test/MultipartReaderTests.cs

@@ -93,6 +93,18 @@ public class MultipartReaderTests
 "\r\n" +
 "--9051914041544843365";
 
+    private static string ReadToEndUsingSpans(Stream stream, int spanLength)
+    {
+        var buffer = new byte[spanLength];
+        var result = new MemoryStream();
+        int read;
+        while ((read = stream.Read(buffer.AsSpan())) > 0)
+        {
+            result.Write(buffer, 0, read);
+        }
+        return Encoding.ASCII.GetString(result.ToArray());
+    }
+
     private static MemoryStream MakeStream(string text)
     {
         return new MemoryStream(Encoding.UTF8.GetBytes(text));
@@ -502,6 +514,29 @@ public async Task SyncReadWithOffsetWorks()
         Assert.Null(await reader.ReadNextSectionAsync());
     }
 
+    [Theory]
+    // The default buffer holds the whole body, so every read sees the full boundary.
+    [InlineData(4096)]
+    // A buffer smaller than the body forces partial boundary matches at the end of the buffered data.
+    [InlineData(64)]
+    public async Task MultipartReader_ReadSectionBodyIntoSpan_ReadsEachSection(int bufferSize)
+    {
+        var stream = MakeStream(TwoPartBody);
+        var reader = new MultipartReader(Boundary, stream, bufferSize);
+
+        var section = await reader.ReadNextSectionAsync();
+        Assert.NotNull(section);
+        Assert.Equal("form-data; name=\"text\"", section.Headers["Content-Disposition"][0]);
+        Assert.Equal("text default", ReadToEndUsingSpans(section.Body, spanLength: 5));
+
+        section = await reader.ReadNextSectionAsync();
+        Assert.NotNull(section);
+        Assert.Equal("form-data; name=\"file1\"; filename=\"a.txt\"", section.Headers["Content-Disposition"][0]);
+        Assert.Equal("Content of a.txt.\r\n", ReadToEndUsingSpans(section.Body, spanLength: 5));
+
+        Assert.Null(await reader.ReadNextSectionAsync());
+    }
+
     [Fact]
     public async Task MultipartReader_BoundaryWithUnexpectedTrailingData_ThrowsIOException()
     {