ethernet: T7813: remove VLAN interfaces first on deletion

When an Ethernet interface is deleted from the configuration, its associated
VLAN subinterfaces must be removed first. This ordering allows VLAN interfaces
to gracefully release any active DHCP or DHCPv6 leases before the parent
interface disappears, ensuring proper cleanup and avoiding potential lease
retention issues.

Author: c-po

python/vyos/ifconfig/ethernet.py

@@ -131,19 +131,17 @@ def remove(self):
         >>> i.remove()
         """
 
+        # T7813: we do need to remove the VLAN subinterfaces first so we can
+        # properly stop the DHCP client and inform the DHCP server that we are
+        # returning the lease.
+        for vlan in Section.sub_interfaces(self.ifname):
+            Interface(vlan).remove()
+
         if self.exists(self.ifname):
             # interface is placed in A/D state when removed from config! It
             # will remain visible for the operating system.
             self.set_admin_state('down')
 
-        # Remove all VLAN subinterfaces - filter with the VLAN dot
-        for vlan in [
-            x
-            for x in Section.interfaces('ethernet')
-            if x.startswith(f'{self.ifname}.')
-        ]:
-            Interface(vlan).remove()
-
         super().remove()
 
     def set_flow_control(self, enable):

python/vyos/ifconfig/section.py

@@ -144,9 +144,17 @@ def interfaces(cls, section='', vlan=True):
         if no section is provided, then it returns all configured interfaces.
         If vlan is True, also Vlan subinterfaces will be returned
         """
-
         return cls._sort_interfaces(cls._intf_under_section(section, vlan))
 
+    @classmethod
+    def sub_interfaces(cls, interface : str) -> list:
+        """
+        return a list of subinterfaces (e.g. VLAN) derived from a given interface
+        """
+        if_type = cls.section(interface)
+        res = [x for x in cls.interfaces(if_type) if x.startswith(f'{interface}.')]
+        return res
+
     @classmethod
     def _intf_with_feature(cls, feature=''):
         """