test_write_seek_read (void)
{
   ...
   mongoc_gridfs_file_writev (file, &iov, 1, 0);

// notice we didn't save yet
   mongoc_gridfs_file_seek (file, 0, SEEK_SET);
   r = mongoc_gridfs_file_readv (file, &riov, 1, -1, 0);
// r is correct, but riov points to empty buffer(it points to page->read_buf instead page->buf)
}