apply: don't assume the NM loopback connection is called "lo"

It might have any name, so we need to get the connection name
dynamically.

netplan_cli/cli/commands/apply.py

@@ -176,9 +176,12 @@ def command_apply(self, run_generate=True, sync=False, exit_on_error=True, state
         else:
             logging.debug('no netplan generated networkd configuration exists')
 
+        loopback_connection = ''
         if restart_nm:
             logging.debug('netplan generated NM configuration changed, restarting NM')
             if utils.nm_running():
+                if 'lo' in nm_ifaces:
+                    loopback_connection = utils.nm_get_connection_for_interface('lo')
                 # restarting NM does not cause new config to be applied, need to shut down devices first
                 for device in devices:
                     if device not in nm_ifaces:
@@ -293,8 +296,10 @@ def command_apply(self, run_generate=True, sync=False, exit_on_error=True, state
             # re-set via an udev rule setting "NM_UNMANAGED=1"
             shutil.rmtree('/run/NetworkManager/devices', ignore_errors=True)
             utils.systemctl_network_manager('start', sync=sync)
-            # If 'lo' is in the nm_interfaces set we flushed it's IPs (see above) and will need to bring it
-            # back manually. For that, we need NM up and ready to accept commands
+
+            # If 'lo' is in the nm_interfaces set we flushed it's IPs (see above) and disconnected it.
+            # NM will not bring it back automatically after restarting and we need to do that manually.
+            # For that, we need NM up and ready to accept commands
             if 'lo' in nm_interfaces:
                 sync = True
 
@@ -315,13 +320,12 @@ def command_apply(self, run_generate=True, sync=False, exit_on_error=True, state
                         break
                     time.sleep(0.5)
 
-            # If "lo" is managed by NM through Netplan, apply will flush its addresses and NM
-            # will not bring it back automatically like other connections.
+            # If "lo" is managed by NM through Netplan, apply will flush its addresses and disconnect it.
+            # NM will not bring it back automatically.
             # This is a possible scenario with netplan-everywhere. If a user tries to change the 'lo'
             # connection with nmcli for example, NM will create a persistent nmconnection file and emit a YAML for it.
-            if 'lo' in nm_interfaces:
-                cmd = ['nmcli', 'con', 'up', 'lo']
-                subprocess.call(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+            if 'lo' in nm_interfaces and loopback_connection:
+                utils.nm_bring_interface_up(loopback_connection)
 
     @staticmethod
     def is_composite_member(composites, phy):

netplan_cli/cli/utils.py

@@ -78,6 +78,20 @@ def nm_interfaces(paths, devices):
     return interfaces
 
 
+def nm_get_connection_for_interface(interface: str) -> str:
+    output = nmcli_out(['-m', 'tabular', '-f', 'GENERAL.CONNECTION', 'device', 'show', interface])
+    lines = output.strip().split('\n')
+    connection = lines[1]
+    return connection if connection != '--' else ''
+
+
+def nm_bring_interface_up(connection: str) -> None:  # pragma: nocover (must be covered by NM autopkgtests)
+    try:
+        nmcli(['connection', 'up', connection])
+    except subprocess.CalledProcessError:
+        pass
+
+
 def systemctl_network_manager(action, sync=False):
     # If the network-manager snap is installed use its service
     # name rather than the one of the deb packaged NetworkManager

tests/test_utils.py

@@ -371,3 +371,15 @@ def test_ip_addr_flush(self):
         self.assertEqual(self.mock_cmd.calls(), [
             ['ip', 'addr', 'flush', 'eth42']
         ])
+
+    @patch('netplan_cli.cli.utils.nmcli_out')
+    def test_nm_get_connection_for_interface(self, nmcli):
+        nmcli.return_value = 'CONNECTION \nlo         \n'
+        out = utils.nm_get_connection_for_interface('lo')
+        self.assertEqual(out, 'lo')
+
+    @patch('netplan_cli.cli.utils.nmcli_out')
+    def test_nm_get_connection_for_interface_no_connection(self, nmcli):
+        nmcli.return_value = 'CONNECTION \n--         \n'
+        out = utils.nm_get_connection_for_interface('asd0')
+        self.assertEqual(out, '')