do-release-upgrade reinstalls netplan.io, breaking systemd-networkd configuration

[Dan Streetman]

[impact]

Users who have chosen to remove the 'netplan.io' package and manually configure systemd-networkd, when upgrading, will have the 'netplan.io' package reinstalled, which will create a new dynamic networkd configuration after reboot, which breaks system networking.

[test case]

install an Ubuntu release, e.g. Bionic.

Make sure some 'desktop' metapackage is installed, e.g.:
$ sudo apt install xubuntu-desktop

Remove netplan:
$ sudo apt purge netplan.io

note that the above command removes not only 'netplan.io' but also 'ubuntu-minimal'

then, configure systemd-networkd manually, in some way that would conflict with netplan's default of just using dhcp on the main interface, e.g.:

$ grep . /etc/systemd/network/*
/etc/systemd/network/10-br0.netdev:[NetDev]
/etc/systemd/network/10-br0.netdev:Name=br0
/etc/systemd/network/10-br0.netdev:Kind=bridge
/etc/systemd/network/20-ens3.network:[Match]
/etc/systemd/network/20-ens3.network:Name=ens3
/etc/systemd/network/20-ens3.network:[Network]
/etc/systemd/network/20-ens3.network:Bridge=br0
/etc/systemd/network/50-br0.network:[Match]
/etc/systemd/network/50-br0.network:Name=br0
/etc/systemd/network/50-br0.network:[Network]
/etc/systemd/network/50-br0.network:Address=192.168.122.200/24
/etc/systemd/network/50-br0.network:Gateway=192.168.122.1
/etc/systemd/network/50-br0.network:DNS=192.168.122.1

(That is a trivial example of networkd config that would conflict with netplan's default, real examples can be much more complex, even as far as causing systemd-networkd to fail to start)

reboot the system to discard old netplan networkd configuration, and pick up new manual networkd configuration. If needed, edit /etc/update-manager/release-upgrades to change Prompt to 'normal'. Then upgrade to a new release:

$ sudo do-release-upgrade

following the prompts to complete the upgrade, including the final prompt to reboot after the upgrade.

Boot into the upgraded system and check if netplan is installed and operating:

$ dpkg -l|grep netplan
ii netplan.io 0.97-0ubuntu1~19.04.1 amd64 YAML network configuration abstraction for various backends
$ ls -l /run/systemd/network/
total 8
-rw-r--r-- 1 root root 69 Sep 24 15:36 10-netplan-ens3.link
-rw-r--r-- 1 root root 129 Sep 24 15:36 10-netplan-ens3.network

[regression potential]

TBD

[other info]

This appears to require one of the 'desktop' packages installed, e.g. 'ubuntu-desktop, 'xubuntu-desktop', etc.; I reproduced it with 'xubuntu-desktop' installed, but could not reproduce it with a stock cloud image that has no 'desktop' package installed.

[Dan Streetman (ddstreet)]

I opened this against netplan, but it might be an issue that should be addressed in ubuntu-release-upgrader, not netplan, since netplan is just doing its job after being installed.

[Brian Murray (brian-murray)]

It is actually ubuntu-minimal which depends on netplan.io and ubuntu-release-upgrader will always reinstall ubuntu-minimal as it is a part of the BaseMetaPkgs configuration. If people are removing ubuntu-minimal, which happens by removing netplan.io, then they should upgrade by editing /etc/apt/sources.list and running 'sudo apt-get dist-upgrade' rather than using ubuntu-release-upgrader.

[Dan Streetman (ddstreet)]

> It is actually ubuntu-minimal which depends on netplan.io and ubuntu-release-upgrader will always reinstall ubuntu-minimal as it is a part of the BaseMetaPkgs configuration

this is what i initially thought, but when i tested it, the upgrade did *not* reinstall ubuntu-minimal (or netplan.io).

> If people are removing ubuntu-minimal, which happens by removing netplan.io, then they should upgrade by editing /etc/apt/sources.list and running 'sudo apt-get dist-upgrade' rather than using ubuntu-release-upgrader.

er...really? And how are people supposed to know that? If do-release-upgrade should *really* not be used if ubuntu-minimal isn't installed, then it *definitely* should check that and abort before it starts with a warning...

[Felipe Reyes (freyes)]

do-release-upgrade in this aspect have 2 modes: server and desktop mode, the desktop mode is used whenever any of the packages defined in the MetaPkgs[0] section is installed[2]

So when server mode is detected is when the presence of packages defined in BaseMetaPkgs is NOT enforced[1]

[0] https://bazaar.launchpad.net/~ubuntu-core-dev/ubuntu-release-upgrader/trunk/view/head:/data/DistUpgrade.cfg.bionic#L13
[1] https://bazaar.launchpad.net/~ubuntu-core-dev/ubuntu-release-upgrader/trunk/view/head:/DistUpgrade/DistUpgradeCache.py#L624
[2] https://bazaar.launchpad.net/~ubuntu-core-dev/ubuntu-release-upgrader/trunk/view/head:/DistUpgrade/DistUpgradeCache.py#L359

[Steve Langasek (vorlon)]

Users who remove netplan.io are removing a core component of the definition of a base Ubuntu system (that's what being a dependency of ubuntu-minimal means). This should be considered unsupported.

Any affected user should have dealt with not wanting netplan to manage their network by removing the configuration files from /etc/netplan, not by removing the package.