fix: add dhcp-server-detector fallback using dhclient

fixes sonic_vs with older (202505) image

Signed-off-by: Benjamin Ritter <benjamin.ritter@x-cellent.com>

Author: l0wl3vel

deploy_partition.yaml

@@ -218,7 +218,23 @@
       ansible.builtin.shell: |
         systemctl restart isc-dhcp-server
         sleep 2
-        dhcp-server-detector Vlan4000 --exit-on-first-offer --duration 10
+
+        # dhcp-server-detector is a pyroute2 >= 0.9.3 console script, and SONiC 202505 pins
+        # pyroute2 0.7.12, so fall back to dhclient there. dhclient has no timeout flag and
+        # ignores `timeout` from a config file -- it always waits ~60s -- hence timeout(1).
+        # -sf /bin/true keeps it from configuring the SVI, and on success it forks a renewing
+        # daemon with an empty pid file, so the leftover is killed by its exact command line.
+        # `pkill -x -f` matches the whole cmdline, so it cannot match this script's own shell
+        # the way a plain `pkill -f` would.
+        if command -v dhcp-server-detector >/dev/null; then
+          dhcp-server-detector Vlan4000 --exit-on-first-offer --duration 10
+        else
+          probe="dhclient -4 -1 -sf /bin/true -lf /run/dhcp-probe.leases -pf /run/dhcp-probe.pid Vlan4000"
+          timeout 15 $probe
+          rc=$?
+          pkill -x -f "$probe" || true
+          exit $rc
+        fi
       args:
         executable: /bin/bash
       when: dhcp_subnets is defined