Fail cleanly when systemd-networkd is not installed

[jonathanspw]

Description

This patch makes the failure case of netplan status cleaner when systemd-networkd is not present. This is at least relevant to Fedora/EPEL use-cases.

Before:

[root@alma-9-srpm-test 1.el9]# netplan status
Failed to start systemd-networkd.service: Unit systemd-networkd.service not found.
Traceback (most recent call last):
  File "/usr/sbin/netplan", line 23, in <module>
    netplan.main()
  File "/usr/share/netplan/netplan_cli/cli/core.py", line 58, in main
    self.run_command()
  File "/usr/share/netplan/netplan_cli/cli/utils.py", line 332, in run_command
    self.func()
  File "/usr/share/netplan/netplan_cli/cli/commands/status.py", line 77, in run
    self.run_command()
  File "/usr/share/netplan/netplan_cli/cli/utils.py", line 332, in run_command
    self.func()
  File "/usr/share/netplan/netplan_cli/cli/commands/status.py", line 829, in command
    system_state = SystemConfigState(self.ifname, self.all)
  File "/usr/share/netplan/netplan_cli/cli/state.py", line 437, in __init__
    utils.systemctl('start', ['systemd-networkd.service'], True)
  File "/usr/share/netplan/netplan_cli/cli/utils.py", line 118, in systemctl
    subprocess.check_call(command)
  File "/usr/lib64/python3.9/subprocess.py", line 373, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['systemctl', 'start', 'systemd-networkd.service']' returned non-zero exit status 5.

After:

[root@alma-9-srpm-test 1.el9]# netplan status
systemd-networkd is required for 'netplan status' functionality

It also fixes a bug in the systemctl_is_installed def. systemctl is-enabled only returns 1/0 so it needs to use systemctl status instead to be able to use the 4 exit code to determine if a given service is installed.

Checklist

• Runs make check successfully.
• Retains code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[Conan-Kudo]

@slyon is there a reason netplan status doesn't work with NetworkManager?

[jonathanspw]

Signed the CLA now.

[slyon]

Thanks for this improvement!
I left two smaller inline comments below.

@Conan-Kudo IIRC systemd-networkd is not needed in principle for netplan status to function. But it's an implementation detail that we rely on systemd-networkd data heavily (in netplan_cli/cli/state.py:__init__()), besides iproute2 and NetworkManager data.

In theory, it could be changed to use only iproute2 data as ground truth and amend that with systemd-networkd and/or NetworkManager data - whatever is available. And I would be open about that when somebody wants to drive such change.

[jonathanspw]

Rebased PR against main.

[slyon]

Thanks for addressing my remarks. LGTM!