fix(netplan): Fix predictable interface rename issue (canonical#5339)

When predictable naming is disabled, the following command may exit with
a non-zero exit code.

    udevadm test-builtin net_setup_link

This code only ran to check for udev rename races, which cannot happen
when systemd renaming is disabled. Skip when disabled.

Fixes canonicalGH-3950

cloudinit/net/netplan.py

@@ -381,6 +381,9 @@ def _net_setup_link(self, run=False):
         if not run:
             LOG.debug("netplan net_setup_link postcmd disabled")
             return
+        elif "net.ifnames=0" in util.get_cmdline():
+            LOG.debug("Predictable interface names disabled.")
+            return
         setup_lnk = ["udevadm", "test-builtin", "net_setup_link"]
 
         # It's possible we can race a udev rename and attempt to run

tests/unittests/test_net.py

@@ -3397,10 +3397,13 @@ def test_netplan_render_calls_postcmds(
         mock_netplan_generate.assert_called_with(run=True, config_changed=True)
         mock_net_setup_link.assert_called_with(run=True)
 
+    @mock.patch("cloudinit.util.get_cmdline")
     @mock.patch("cloudinit.util.SeLinuxGuard")
     @mock.patch.object(netplan, "get_devicelist")
     @mock.patch("cloudinit.subp.subp")
-    def test_netplan_postcmds(self, mock_subp, mock_devlist, mock_sel):
+    def test_netplan_postcmds(
+        self, mock_subp, mock_devlist, mock_sel, m_get_cmdline
+    ):
         mock_sel.__enter__ = mock.Mock(return_value=False)
         mock_sel.__exit__ = mock.Mock()
         mock_devlist.side_effect = [["lo"]]