examples: add linkflap (link-flap detection in the kernel) - #621
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A notifier.netdevice callback keeps a per-interface sliding window of UP/DOWN transitions and, once an interface crosses the threshold within the window, multicasts a structured flapping event (interface name and transition count) over the "linkflap" generic netlink family. subscriber.c consumes the events in userspace, decoding the attributes. It hooks the kernel's netdevice notifier chain and runs stateful logic in the handler; emitting the multicast is safe from the callback (it does not take rtnl_lock, unlike an rtnetlink write). Also the first netlink.channel example (it had only test coverage). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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notifier.netdevicecallback keeps a per-interface sliding window of UP/DOWN transitions and, once an interface crosses the threshold within the window, multicasts a structured flapping event (interface name and transition count) over thelinkflapgeneric netlink family.subscriber.cconsumes the events in userspace, decoding the attributes and printing e.g.linkflap: dummy0 flapping (5 transitions).It hooks the kernel's netdevice notifier chain, and the value is the stateful per-interface window in the handler (plain event forwarding cannot do that). The callback runs holding
rtnl_lock, so it must not issue rtnetlink itself, but emitting a multicast is safe (it does not take the lock), so this is a single runtime.Also the first
netlink.channelexample in the tree (it had only test coverage).Validated end to end in the kernel: running
watch.luaand bouncing a dummy interface past the threshold,subscriber.c(joined to the family's multicast group) receives and decodes the event. Noprintkinvolved; the point is the netlink path.Rebased onto master now that #620 (multicast rename) is merged.