[RFC] ifupdown2: missing bond-primary device prevents the bond from being created

[Shay Sevet <shay@sevet.net>]

Hi,

I hit this on a 3-node PVE 9.2 cluster and want to report it together with
a proposed fix.

Problem
-------
If an active-backup bond's `bond-primary` names a device that does not
exist at boot (for example a USB NIC that died, was unplugged, or was
replaced and came back under a new MAC-based name), ifupdown2 never creates
the bond at all. Every interface stacked on it fails too (vlan-aware
bridge, VLANs, addresses), so the node loses all networking, although the
other bond slave is present and has link. Same result on `ifreload -a`.

error: ifname <nic> not present in cache
error: vmbrX: bridge port bond0 does not exist

This defeats the redundancy the bond was configured for: losing the
preferred device takes the whole bond down instead of failing over.

Reproduced on ifupdown2 3.3.0-1+pmx12 and 3.3.0-1+pmx13 (Debian 13 VM and
on PVE 9.2.26 hosts). The related missing-slave case is already handled in
pmx13 (Proxmox bug 8117 / upstream PR #355); the missing primary is not.

Cause
-----
`bond-primary` is resolved with `self.cache.get_ifindex()` in
`ifupdown2/addons/bond.py` (`get_ifla_bond_attr_from_user_config()`). For
an unknown name it raises `NetlinkCacheIfnameNotFoundError`; only
`KeyError` is caught there, so the exception aborts the bond setup before
the link is created.

Proposed fix
------------
Treat a missing bond-primary device like an invalid value: log a warning,
set the interface status to warning, and create the bond without a primary.
The configuration is left untouched; a later ifreload sets the primary once
the device exists.

Upstream PR (with a unit test that fails without the change and passes with
it):
https://www.google.com/url?q=https://github.com/CumulusNetworks/ifupdown2/pull/358&source=gmail&ust=1794167497172000&sa=E

I have been running a rebuilt 3.3.0-1+pmx13 with this change on all three
nodes. A rebased quilt patch for the Proxmox packaging is available if
useful.

Workaround until then: drop `bond-primary` and set it after the bond is up,
only if the device exists:
post-up echo <nic> > /sys/class/net/bond0/bonding/primary 2>/dev/null ||
true

I found the bug on my own setup; the fix was developed with AI assistance
(Claude) and reviewed and tested by me.

Thanks,
Shay Sevet