Call trace occurred when running `netplan apply` after an yaml is removed

[Vic Liu]

Call trace occurred when running `netplan apply` after an yaml was removed.

System: Intel NUC7CJYH
Image: Ubuntu Core 20 beta daily build (20200514.2)

Steps to Reproduce:
1. $ sudo rm /etc/netplan/n-wpa.yaml
2. $ sudo netplan apply, then the following call trace occurred:

Warning: The unit file, source configuration file or drop-ins of netplan-wpa-wlo2.service changed on disk. Run 'systemctl daemon-reload' to reload units.
Failed to start netplan-wpa-wlo2.service: Unit netplan-wpa-wlo2.service not found.
Traceback (most recent call last):
  File "/usr/sbin/netplan", line 23, in <module>
    netplan.main()
  File "/usr/share/netplan/netplan/cli/core.py", line 50, in main
    self.run_command()
  File "/usr/share/netplan/netplan/cli/utils.py", line 179, in run_command
    self.func()
  File "/usr/share/netplan/netplan/cli/commands/apply.py", line 46, in run
    self.run_command()
  File "/usr/share/netplan/netplan/cli/utils.py", line 179, in run_command
    self.func()
  File "/usr/share/netplan/netplan/cli/commands/apply.py", line 173, in command_apply
    utils.systemctl_networkd('start', sync=sync, extra_services=netplan_wpa)
  File "/usr/share/netplan/netplan/cli/utils.py", line 86, in systemctl_networkd
    subprocess.check_call(command)
  File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['systemctl', 'start', '--no-block', 'systemd-networkd.service', 'netplan-wpa-wlo2.service']' returned non-zero exit status 5.

[Lukas Märdian (slyon)]

What is the netplan version this is happening with?

Might it be a duplicate of https://bugs.launchpad.net/ubuntu/+source/netplan.io/+bug/1874494 ?
If so, the call trace should have been fixed with netplan.io_0.99-0ubuntu3~20.04.1

[Dimitri John Ledkov (xnox)]

Fix for this issue is available in the edge channel only at the moment.

Please verify that you do not observe the crash, before we promote to beta.

(calling daemon-reload is still needed however, until next netplan.io SRU, but it's minor in UX compared with out right crash, see other bug report for details)

[Claudio Matsuoka (cmatsuoka)]

Fix seems good, I couldn't reproduce this bug using core20 663/665 from edge.

[Claudio Matsuoka (cmatsuoka)]

Systems tested: qemu, rpi4 (1GB)