986b2514cc
When an AP goes up, the kernel may emit a netlink event indicating that the interface has gained carrier. In that event, we should check if the SSID has changed before attempting to reconfigure. Not doing so means that the link->ssid member is not updated, leading to a potential mismatch if some of the .network configurations match on SSID=. There are however scenarios where the above heuristic is not enough. Specifically, if the interface carrier state flip-flops within a short enough interval, the internal throttling of netlink events inside the kernel may suppress intermediate linkdown+linkup events (cf. Linux net/core/link_watch.c). So there is no linkup event to react on. To improve on the latter scenario, it is proposed to make newer kernels emit an NL80211_CMD_START_AP multicast event when an AP goes up. This event will not be dropped by link_watch. systemd-networkd can then react to such events as well, and optionally reconfigure the link if the SSID has changed. This will only work with newer kernels though.
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