feat: make zero-copy GetObject possible via sendfile

From #919, This provides the *os.File handle to io.Copy() for the
case where the full file range is requested. This prevents hiding
the *os.File type for io.Copy() optimizations.

This still requires the change to valyala/fasthttp#1889 to expose
the net.Conn similarly to enable the linux sendfile optimization.

backend/posix/posix.go

@@ -2931,7 +2931,12 @@ func (p *Posix) GetObject(_ context.Context, input *s3.GetObjectInput) (*s3.GetO
 		return nil, fmt.Errorf("open object: %w", err)
 	}
 
-	rdr := io.NewSectionReader(f, startOffset, length)
+	// using an os.File allows zero-copy sendfile via io.Copy(os.File, net.Conn)
+	var body io.ReadCloser = f
+	if startOffset != 0 || length != objSize {
+		rdr := io.NewSectionReader(f, startOffset, length)
+		body = &backend.FileSectionReadCloser{R: rdr, F: f}
+	}
 
 	return &s3.GetObjectOutput{
 		AcceptRanges:    &acceptRange,
@@ -2945,7 +2950,7 @@ func (p *Posix) GetObject(_ context.Context, input *s3.GetObjectInput) (*s3.GetO
 		ContentRange:    &contentRange,
 		StorageClass:    types.StorageClassStandard,
 		VersionId:       &versionId,
-		Body:            &backend.FileSectionReadCloser{R: rdr, F: f},
+		Body:            body,
 	}, nil
 }