While looking at a recent failure of test_fileno() in test_urllibnet, I discovered that socket.close() ignores the return value of the close() system call. It looks like it has always been this way: <https://docs.python.org/3/library/socket.html#socket.socket.fileno>.

On the other hand, both FileIO.close() and os.close() raise an exception if the underlying close() call fails. So I propose to make socket.close() also raise an exception for any failure indicated by the underlying close() call. The benefit is that a programming error causing EBADF would be more easily noticed.