Should socket.sendfile() always return number of bytes sent because file.tell() may be changed by something else that uses the same file descriptor?

What happens if the file grows?

Instead of returning `(was_os_sendfile_used, os_sendfile_error)`, you could specify `no_fallback=False` that could be set to `True` to assert that the fallback is not used (with `no_fallback=True` the `os_sendfile_error` is raised instead of using socket.send() as a fallback). 

If possible; always include number of bytes sent in any error that is raised.