interface: T4203: prevent DHCP client restart if not necessary

In the past whenever a change happened to any interface and it was configured
as a DHCP client, VyOS always had a breif outage as DHCP released the old lease
and re-aquired a new one - bad!

This commit changes the behavior that DHCP client is only restarted if any one
of the possible options one can set for DHCP client under the "dhcp-options"
node is altered.

(cherry picked from commit 3a1a7c40a13ee9f5561823a79876d88d3f5bf053)

2 files changed, 51 insertions, 13 deletions

diff --git a/python/vyos/configdict.py b/python/vyos/configdict.py
index 486260152..b18db1fb8 100644
--- a/python/vyos/configdict.py
+++ b/python/vyos/configdict.py
@@ -387,6 +387,15 @@ def get_interface_dict(config, base, ifname=''):
     bond = is_member(config, ifname, 'bonding')
     if bond: dict.update({'is_bond_member' : bond})
 
+    # Check if any DHCP options changed which require a client restat
+    for leaf_node in ['client-id', 'default-route-distance', 'host-name',
+                 'no-default-route', 'vendor-class-id']:
+        dhcp = leaf_node_changed(config, ['dhcp-options', leaf_node])
+        if dhcp:
+            dict.update({'dhcp_options_old' : dhcp})
+            # one option is suffiecient to set 'dhcp_options_old' key
+            break
+
     # Some interfaces come with a source_interface which must also not be part
     # of any other bond or bridge interface as it is exclusivly assigned as the
     # Kernels "lower" interface to this new "virtual/upper" interface.
@@ -431,6 +440,15 @@ def get_interface_dict(config, base, ifname=''):
         bridge = is_member(config, f'{ifname}.{vif}', 'bridge')
         if bridge: dict['vif'][vif].update({'is_bridge_member' : bridge})
 
+        # Check if any DHCP options changed which require a client restat
+        for leaf_node in ['client-id', 'default-route-distance', 'host-name',
+                     'no-default-route', 'vendor-class-id']:
+            dhcp = leaf_node_changed(config, ['vif', vif, 'dhcp-options', leaf_node])
+            if dhcp:
+                dict['vif'][vif].update({'dhcp_options_old' : dhcp})
+                # one option is suffiecient to set 'dhcp_options_old' key
+                break
+
     for vif_s, vif_s_config in dict.get('vif_s', {}).items():
         default_vif_s_values = defaults(base + ['vif-s'])
         # XXX: T2665: we only wan't the vif-s defaults - do not care about vif-c
@@ -454,6 +472,15 @@ def get_interface_dict(config, base, ifname=''):
         bridge = is_member(config, f'{ifname}.{vif_s}', 'bridge')
         if bridge: dict['vif_s'][vif_s].update({'is_bridge_member' : bridge})
 
+        # Check if any DHCP options changed which require a client restat
+        for leaf_node in ['client-id', 'default-route-distance', 'host-name',
+                     'no-default-route', 'vendor-class-id']:
+            dhcp = leaf_node_changed(config, ['vif-s', vif_s, 'dhcp-options', leaf_node])
+            if dhcp:
+                dict['vif_s'][vif_s].update({'dhcp_options_old' : dhcp})
+                # one option is suffiecient to set 'dhcp_options_old' key
+                break
+
         for vif_c, vif_c_config in vif_s_config.get('vif_c', {}).items():
             default_vif_c_values = defaults(base + ['vif-s', 'vif-c'])
 
@@ -477,6 +504,16 @@ def get_interface_dict(config, base, ifname=''):
             if bridge: dict['vif_s'][vif_s]['vif_c'][vif_c].update(
                 {'is_bridge_member' : bridge})
 
+            # Check if any DHCP options changed which require a client restat
+            for leaf_node in ['client-id', 'default-route-distance', 'host-name',
+                         'no-default-route', 'vendor-class-id']:
+                dhcp = leaf_node_changed(config, ['vif-s', vif_s, 'vif-c', vif_c,
+                                                  'dhcp-options', leaf_node])
+                if dhcp:
+                    dict['vif_s'][vif_s]['vif_c'][vif_c].update({'dhcp_options_old' : dhcp})
+                    # one option is suffiecient to set 'dhcp_options_old' key
+                    break
+
     # Check vif, vif-s/vif-c VLAN interfaces for removal
     dict = get_removed_vlans(config, dict)
     return dict
@@ -501,7 +538,6 @@ def get_vlan_ids(interface):
 
     return vlan_ids
 
-
 def get_accel_dict(config, base, chap_secrets):
     """
     Common utility function to retrieve and mangle the Accel-PPP configuration
diff --git a/python/vyos/ifconfig/interface.py b/python/vyos/ifconfig/interface.py
index 036ca1413..430940c57 100644
--- a/python/vyos/ifconfig/interface.py
+++ b/python/vyos/ifconfig/interface.py
@@ -1135,12 +1135,11 @@ class Interface(Control):
         options_file = f'{config_base}_{ifname}.options'
         pid_file = f'{config_base}_{ifname}.pid'
         lease_file = f'{config_base}_{ifname}.leases'
-
-        # Stop client with old config files to get the right IF_METRIC.
         systemd_service = f'dhclient@{ifname}.service'
-        if is_systemd_service_active(systemd_service):
-            self._cmd(f'systemctl stop {systemd_service}')
 
+        # 'up' check is mandatory b/c even if the interface is A/D, as soon as
+        # the DHCP client is started the interface will be placed in u/u state.
+        # This is not what we intended to do when disabling an interface.
         if enable and 'disable' not in self._config:
             if dict_search('dhcp_options.host_name', self._config) == None:
                 # read configured system hostname.
@@ -1151,16 +1150,19 @@ class Interface(Control):
                     tmp = {'dhcp_options' : { 'host_name' : hostname}}
                     self._config = dict_merge(tmp, self._config)
 
-            render(options_file, 'dhcp-client/daemon-options.tmpl',
-                   self._config)
-            render(config_file, 'dhcp-client/ipv4.tmpl',
-                   self._config)
+            render(options_file, 'dhcp-client/daemon-options.tmpl', self._config)
+            render(config_file, 'dhcp-client/ipv4.tmpl', self._config)
 
-            # 'up' check is mandatory b/c even if the interface is A/D, as soon as
-            # the DHCP client is started the interface will be placed in u/u state.
-            # This is not what we intended to do when disabling an interface.
-            return self._cmd(f'systemctl restart {systemd_service}')
+            # When the DHCP client is restarted a brief outage will occur, as
+            # the old lease is released a new one is acquired (T4203). We will
+            # only restart DHCP client if it's option changed, or if it's not
+            # running, but it should be running (e.g. on system startup)
+            if 'dhcp_options_old' in self._config or not is_systemd_service_active(systemd_service):
+                return self._cmd(f'systemctl restart {systemd_service}')
+            return None
         else:
+            if is_systemd_service_active(systemd_service):
+                self._cmd(f'systemctl stop {systemd_service}')
             # cleanup old config files
             for file in [config_file, options_file, pid_file, lease_file]:
                 if os.path.isfile(file):