In the current Conn.Close behaviour can leak file descriptors if the server has gone away before the client calls .Close

// Close implements the io.Closer interface.
func (ws *Conn) Close() error {
    err := ws.frameHandler.WriteClose(ws.defaultCloseStatus)
    if err != nil {
        return err
    }
    return ws.rwc.Close()
}

In the case that the remote side has closed the connection, WriteClose will return an error which will be returned to the caller without calling ws.rwc.Close. As it is customary to ignore the error from an io.Closer.Close, and even when not, the error is treated as opaque there is no indication that Close didn't actually release the file descriptor associated with ws.rwc. Even if the error was handled from the caller of Close, it would typically be io.EOF or some operating system specific version of "connection closed unexpectedly".

/cc @bradfitz