On Linux, the QNetworkInterface::IsUp flag semantics has changed which breaks existing application code

On Linux, the  QNetworkInterface::IsUp flag semantics has changed which breaks existing application code.

At some point Qt changed the way that QNetworkInterface::IsUp is obtained from the Linux kernel. Qt 5.12.2 obtains the QNetworkInterface::IsUp flags using a RTM_GETLINK request over a Netlink socket which returned the flags in a ifinfomsg as follows :-

struct ifinfomsg {
    ...
    unsigned int   ifi_flags;  /* Device flags  */
    ...
}

However, if the response also includes a IFLA_OPERSTATE message, then QNetworkInterface::IsUp is overridden by the IFLA_OPERSTATE of the network interface. i.e.

static QList<QNetworkInterfacePrivate *> getInterfaces(int sock, char *buf)
{
...
            case IFLA_OPERSTATE:    // operational state
                if (*payloadPtr != IF_OPER_UNKNOWN) {
                    iface->flags &= ~QNetworkInterface::IsUp;
                    if (*payloadPtr == IF_OPER_UP)
                    {
                        iface->flags |= QNetworkInterface::IsUp;
                    }
                }
                break;
 

In the situation where a network interface is ready aside from the cable being unplugged, then QNetworkInterface::IsUp is cleared whereas in previous Qt versions (e.g. 5.6), it would be set.

I suggest that the operational state should be exposed using a separate and new attribute in QNetworkInterface rather than being used to modify QNetworkInterface::IsUp.