network: fix detection of netplan on recent Ubuntu releases

Subiquity has a mechanism to detect the presence of netplan. It does so
by checking the existence of the file /lib/netplan/generate. This
mechanism is used in the network screen to validate the YAML
configuration.

However, since netplan 0.107 (present in mantic and noble), the file
/lib/netplan/generate is no longer present. It used to be provided as an
alias for /usr/libexec/netplan/generate ; starting in jammy.

This made Subiquity unable to detect that netplan is running ; and
therefore skip the YAML validation against netplan.

Since we support focal in Subiquity, let's change the detection code so
that we look for both locations. When we stop supporting Focal in the
future, we can drop the reference to /lib/netplan/generate.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquitycore/controllers/network.py

@@ -18,6 +18,7 @@ import asyncio
 import contextlib
 import logging
 import os
+import pathlib
 import subprocess
 from typing import Optional
 
@@ -315,7 +316,14 @@ class BaseNetworkController(BaseController):
             if self.opts.dry_run:
                 delay = 1 / self.app.scale_factor
                 await arun_command(["sleep", str(delay)])
-                if os.path.exists("/lib/netplan/generate"):
+
+                # /usr/libexec/netplan/generate exists starting with 22.04.
+                # When we stop supporting 20.04, we should drop the
+                # reference to /lib/netplan/generate.
+                netplan_generator_legacy = pathlib.Path("/lib/netplan/generate")
+                netplan_generator = pathlib.Path("/usr/libexec/netplan/generate")
+
+                if netplan_generator.exists() or netplan_generator_legacy.exists():
                     # If netplan appears to be installed, run generate to
                     # at least test that what we wrote is acceptable to
                     # netplan but clear the SNAP environment variable to