vyos-netlinkd: T8950: track per-interface operstate to suppress UP-to-UP DHCP restarts

_handle_dhcp_events() restarts dhclient/dhcp6c on every RTM_NEWLINK
that carries operstate=UP, regardless of whether the interface was
already UP.  Normal kernel events that re-notify UP without a preceding
DOWN (e.g. post-migration gratuitous-ARP, promiscuous-mode toggles)
trigger unnecessary DHCP restarts.  Each restart also creates a feedback
loop: dhclient-script runs "ip link set dev <if> up" during PREINIT,
which emits another RTM_NEWLINK(UP), which triggers another restart.
One seed event becomes 10+ restarts in 15 seconds.

Add a module-level dict tracking the last observed operstate per
interface.  Only proceed with the DHCP restart when the transition is
DOWN-to-UP or when no previous state is recorded (first boot / service
restart).  UP-to-UP re-notifications are suppressed with a LOG_DEBUG
message.

Validated on a production VyOS 2026.05.26-1327-rolling router: a
controlled KVM live migration after the patch produced zero DHCP
restarts (all UP-to-UP events suppressed), with no impact on normal
DOWN-to-UP DHCP recovery.

0 files changed, 0 insertions, 0 deletions