slave.cc:

  {

    char buf[FN_REFLEN];

    const char *ln;

    static bool name_warning_sent= 0;

    ln= rli->relay_log.generate_name(opt_relay_logname, "-relay-bin",

                                     1, buf);

    /* We send the warning only at startup, not after every RESET SLAVE */

    if (!opt_relay_logname && !opt_relaylog_index_name && !name_warning_sent)

    {

      /*

        User didn't give us info to name the relay log index file.

        Picking `hostname`-relay-bin.index like we do, causes replication to

        fail if this slave's hostname is changed later. So, we would like to

        instead require a name. But as we don't want to break many existing

        setups, we only give warning, not error.

      */

      sql_print_warning("Neither --relay-log nor --relay-log-index were used;"

                        " so replication "

                        "may break when this MySQL server acts as a "

                        "slave and has his hostname changed!! Please "

                        "use '--relay-log=%s' to avoid this problem.", ln);

      name_warning_sent= 1;

    }

    /*

      note, that if open() fails, we'll still have index file open

      but a destructor will take care of that