Comment 5 for bug 1982218

Apologies for the late response:

Yes, we have a Ubuntu 22.04 LTS image that we use across a set of machines, some of which have multiple Ethernet ports, but for which we only connect a single port. For this image we use the following Netplan configuration (/etc/netplan/ethernet.yaml):

network:
  ethernets:
    en:
      match:
        name: en*
      dhcp4: true
  version: 2

Which places _all_ Ethernet interfaces under systemd-networkd management, allowing us to bring up at least one interface via DHCP without needing to know the exact interface name (which depends on the exact hardware) and enables us to use any of the available Ethernet ports on the machine.

As part of this image we also have a boot-time systemd service that requires functional network connectivity in order to successfully start. The unit file for this service depends on the network-online target ("Wants=network-online.target"). For example purposes you could replace this service with a remote NFS mount specified in /etc/fstab, since boot-time NFS mounts implicitly depend on the network-online target as well.

The default behavior of systemd-networkd-wait-online is to wait until all networkd-managed interfaces are routable before the network-online target is started, but since we only ever connect a single Ethernet interface to the network, we override its behavior here in our image (/etc/systemd/system/systemd-networkd-wait-online.service.d/any.conf):

[Service]
ExecStart=
ExecStart=/usr/local/lib/systemd-networkd-wait-online --any --timeout=0

Now, the problem introduced by the UBUNTU-wait-online-exit-if-no-links-are-managed patch is that it places us in a race against our DHCP server. During a successful boot, where networkd is able to obtain an address via DHCP _before_ the wait-online service is reached, networkctl reports this status:

$ sudo networkctl
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  2 enp0s1 ether routable configured
  3 enp0s2 ether no-carrier configuring

Since the second interface (enp0s1) is in the configured state and is routable, systemd-networkd-wait-online behaves correctly by exiting immediately. However, during an unsuccessful boot, where networkd has yet to obtain an address via DHCP at the time the wait-online service is reached, networkctl reports this status:

$ sudo networkctl
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  2 enp0s1 ether degraded configuring
  3 enp0s2 ether no-carrier configuring

Here, both of the managed Ethernet interfaces are still in the configuring state. Since UBUNTU-wait-online-exit-if-no-links-are-managed doesn't check if managed interfaces are in this state, only configured, it assumes those links are unmanaged and systemd-networkd-wait-online also exits immediately. This results in failure of our network-dependent service to start (or equivalently, failure of the remote NFS volume to mount).

With the provided patch, systemd-networkd-wait-online identifies two managed interfaces in the configuring state and waits until at least one of them becomes routable, and thus waits until the network is functional before continuing to boot.

I hope this example better illustrates the issue. Thanks!