This is the block of code from libmariadb.c:

  if (mysql->options.connect_timeout >= 0 &&

      vio_wait_or_timeout(net->vio, FALSE, mysql->options.connect_timeout * 1000) < 1)

  {

    my_set_error(mysql, CR_SERVER_LOST, SQLSTATE_UNKNOWN,

                 ER(CR_SERVER_LOST_EXTENDED),

                 "handshake: waiting for inital communication packet",

                 errno);

    goto error;

  }

The second option of vio_wait_or_timeout() is "my_bool is_read". If set to true, it waits for a read. If set to false, it waits for a write. The code above is waiting to read a packet from the server, so it should be TRUE. However, it is false, so it waits for a write instead. Since this happens immediately after it has already waited for the connection, this always returns immediately. However, on async it passes the wrong flags (i.e. WAIT_WRITE instead of WAIT_READ) to the async caller, which messes up edge-triggered event handling. It also means that it does not actually wait for the timeout that it is supposed to be waiting for.