[Bug 1746842] [NEW] No network devices configured if YAML error in any file

[Daniel Axtens]

Public bug reported:

- Start a VM with a network interface created by cloud-init (say ens3), and assume you have no physical console (e.g. cloud VM)
- Add another network interface (say ens7)
- Create another YAML file to configure ens7, but make a small error. This file need not reference ens3 at all! 
- Reboot machine

- Observe the following message on boot
systemd[332]: /lib/systemd/systemd-generators/netplan failed with error code 1.

- Observe that you can no longer SSH in to the machine because the error
in the new yaml file prevents all generation, so ens3 is not configured.

This is observed with the latest code on git. It renders the system
entirely unusable unless there is an alternative way to get in.

 1) An error in the configuration of device X should not stop device Y from being configured!
 2) Users should be instructed to always run netplan-generate to test for bugs. (Maybe a wrapper like visudo or crontab -e would be good?)

** Affects: nplan (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1746842

Title:
  No network devices configured if YAML error in any file

Status in nplan package in Ubuntu:
  New

Bug description:
  - Start a VM with a network interface created by cloud-init (say ens3), and assume you have no physical console (e.g. cloud VM)
  - Add another network interface (say ens7)
  - Create another YAML file to configure ens7, but make a small error. This file need not reference ens3 at all! 
  - Reboot machine

  - Observe the following message on boot
  systemd[332]: /lib/systemd/systemd-generators/netplan failed with error code 1.

  - Observe that you can no longer SSH in to the machine because the
  error in the new yaml file prevents all generation, so ens3 is not
  configured.

  This is observed with the latest code on git. It renders the system
  entirely unusable unless there is an alternative way to get in.

   1) An error in the configuration of device X should not stop device Y from being configured!
   2) Users should be instructed to always run netplan-generate to test for bugs. (Maybe a wrapper like visudo or crontab -e would be good?)

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