Timed out waiting for device /subsystem/net/devices/wlan0

[Juerg Haefliger]

I've added the following netplan yaml to a Raspberry Pi classic (groovy and focal) image:

network:
  wifis:
    wlan0:
      dhcp4: true
      optional: true
      access-points:
        "<ssid>":
          password: "<psk>"

This works fine on a Pi that has a wifi adapter but causes a 1.5 minute boot delay on a Pi 2B. With 'optional: true' that shouldn't happen, or should it?

[ OK ] Finished Wait until snapd is fully seeded.
snapd.seeded.service
[ TIME ] Timed out waiting for device /subsystem/net/devices/wlan0.
[DEPEND] Dependency failed for WPA supplicant for netplan wlan0.
[ OK ] Reached target Network.
[ OK ] Reached target Network is Online.

[Brian Murray (brian-murray)]

The Groovy Gorilla has reached end of life, so this bug will not be fixed for that release

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Dries Oeyen (driesoeyen)]

This behavior is not rPi-specific and affects Netplan in general. This happens when configuring an optional wifis entry for a Wi-Fi adapter that may not be present at boot, such as a Wi-Fi dongle.

- The following user reported it on Ubuntu Desktop 18.04 and 20.04: https://bugs.launchpad.net/netplan/+bug/1951651 (marked as duplicate)
- I encountered it on Ubuntu Core 20: https://forum.snapcraft.io/t/ubuntu-core-20-boot-delayed-by-missing-wi-fi-usb-dongle-despite-optional-true-netplan-config/27403

I got a promising reply on the Snapcraft forum thread linked above (direct link to the reply: https://forum.snapcraft.io/t/ubuntu-core-20-boot-delayed-by-missing-wi-fi-usb-dongle-despite-optional-true-netplan-config/27403/5). The relevant section is:

> The optional: true is a valid flag, but it only handles the waiting for the network to come up and be configured. The timeout we’re observing here seems to come from the netplan-wpa-wlx0123456789ab.service unit waiting for the sys-subsystem-net-devices-wlx0123456789ab.device hardware to become available, though, but that does not exist if the dongle is unplugged.
>
> According to https://github.com/systemd/systemd/issues/4413 1 we might need to change the Requires=sys-subsystem-net-devices-wlx0123456789ab.device dependency to something like Requisite=sys-subsystem-net-devices-wlx0123456789ab.device (I’m not sure about the side-effects, tho).

There's a reproduction scenario in the original post on the Snapcraft forum as well.

[Lukas Märdian (slyon)]

Thanks for this investigation! This issue has been discussed in multiple places, like here where a udev rule is recommended: https://github.com/systemd/systemd/issues/2891

I wonder if we could downgrade the "Requires=sys-subsystem-net-devices-XXX.device" to a "Wants=sys-subsystem-net-devices-XXX.device" and add an additional triggering condition on the sysfs path ("ConditionPathExists=|/sys/subsystem/net/devices/XXX"). AFAIU this way the wpa_supplicant unit should fail immediately, if the devices (& its sysfs path) is not there but start as soon as the dongle is plugged in.

https://systemd.network/systemd.unit.html#Conditions%20and%20Asserts

I guess this needs some experimentation to fully understand the side-effects.