[Bug 1968287] Re: loopback addresses disappear after running "netplan apply" multiple times

[Kazuki Suda]

Since the following document states

> but a user can "add" new addresses to the standard loopback interface,
lo, in order to have it considered a valid address on the machine as
well as for custom routing:

https://netplan.io/examples/#configuring-a-loopback-interface

I believe that the disappearance of addresses such as `127.0.0.1/8` from
the loopback address is not the expected behavior.

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Title:
  loopback addresses disappear after running "netplan apply" multiple
  times

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[Bug 1968287] [NEW] loopback addresses disappear after running "netplan apply" multiple times

[Kazuki Suda]

Public bug reported:

In `netplan.io` package (`0.104-0ubuntu2~20.04.1`) on Ubuntu 20.04, I
ran `netplan apply` command to add addresses to `lo` interface. During
the second run of `netplan apply` command, two addresses `127.0.0.1/8`
and `::1/128` disappeared from `lo` interface.

I have confirmed that `0.103-0ubuntu5~20.04.6` does not have this issue.

Below are the steps to reproduce this issue.

$ wget 
https://cloud-images.ubuntu.com/focal/20220404/focal-server-cloudimg-amd64.img
$ sudo qemu-system-x86_64 -vga std -nographic -drive 
file=focal-server-cloudimg-amd64.img,if=virtio,cache=writeback,discard=ignore,format=qcow2
 -m 4096M -smp cpus=4,sockets=4 -serial mon:stdio -boot c -machine 
type=pc,accel=kvm -fda cloud.img
...
ubuntu@ubuntu:~$ cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.4 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.4 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/";
SUPPORT_URL="https://help.ubuntu.com/";
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/";
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy";
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
ubuntu@ubuntu:~$ uname -a
Linux ubuntu 5.4.0-107-generic #121-Ubuntu SMP Thu Mar 24 16:04:27 UTC 2022 
x86_64 x86_64 x86_64 GNU/Linux
ubuntu@ubuntu:~$ dpkg -l | grep netplan.io
ii  netplan.io  0.104-0ubuntu2~20.04.1 amd64
YAML network configuration abstraction for various backends
ubuntu@ubuntu:~$ ip a show lo
1: lo:  mtu 65536 qdisc noqueue state UNKNOWN group 
default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
   valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
   valid_lft forever preferred_lft forever
ubuntu@ubuntu:~$ cat  network:
  > version: 2
  > ethernets:
  > lo:
  > addresses:
  > - 10.193.17.31/32
  > match:
  > name: l