[Bug 2071363] Re: netplan apply triggers udev add events for every network interfdace

[Alberto Contreras]

cloud-init logs on aws

** Attachment added: "0-oracular-240626192140.tar.xz"
   https://bugs.launchpad.net/ubuntu/+source/netplan.io/+bug/2071363/+attachment/5792834/+files/0-oracular-240626192140.tar.xz

** Changed in: cloud-init (Ubuntu Oracular)
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/2071363

Title:
  netplan apply triggers udev add events for every network interfdace

Status in cloud-init package in Ubuntu:
  Invalid
Status in netplan.io package in Ubuntu:
  New
Status in cloud-init source package in Oracular:
  Invalid
Status in netplan.io source package in Oracular:
  New

Bug description:
  netplan.io version 1.0-3ubuntu1, on Ubuntu 24.10, triggers udev add
  events for every already present network interface:

  ```
  $ lxc launch ubuntu-daily:o oo

  # open `udevadm monitor` on another window within the container

  o: # netplan apply

  
  o: # udevadm monitor 
  monitor will print the received events for:
  UDEV - the event which udev sends out after rule processing
  KERNEL - the kernel uevent

  KERNEL[23616.687850] add      /devices/virtual/net/eth0 (net)
  KERNEL[23616.687890] add      /devices/virtual/net/lo (net)
  UDEV  [23616.691806] add      /devices/virtual/net/lo (net)
  UDEV  [23616.697810] add      /devices/virtual/net/eth0 (net)
  ```

  This is wrong by its own, but, with cloud-init on aws images is even
  worse because:

  cloud-init installs the following udev rules to configure hotplugged
  nics:

  ```
  # cat /etc/udev/rules.d/90-cloud-init-hook-hotplug.rules 
  # Installed by cloud-init due to network hotplug userdata
  ACTION!="add|remove", GOTO="cloudinit_end"

  ENV{ID_NET_DRIVER}=="vif|ena|ixgbevf", GOTO="cloudinit_hook"
  GOTO="cloudinit_end"

  LABEL="cloudinit_hook"
  SUBSYSTEM=="net", RUN+="/usr/lib/cloud-init/hook-hotplug"
  LABEL="cloudinit_end"

  ```

  which gets triggered by the udev add events, and hook-hotplug
  instructs cloud-init to configure the new nic, and during that
  reconfiguration `netplan apply` is called by cloud-init, which starts
  an infinite loop.

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