[Bug 1842902] Re: FFe: create zfs dataset for each user automatically

[Didier Roche]

Thanks a lot Łukasz! The MIR is available at
https://bugs.launchpad.net/ubuntu/+source/zsys/+bug/1839271, the MIR
part has been acked, it's pending a security review for now.

Anyway, let's proceed with this so that we can test it early in the
wild, thanks again!

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Title:
  FFe: create zfs dataset for each user automatically

Status in shadow package in Ubuntu:
  Triaged
Status in zsys package in Ubuntu:
  Triaged

Bug description:
  Part of the zsys spec is creating/associating one user dataset for
  each HOME user.

  As zsys is an official experimentation for 19.10, we would like to
  include this feature in a safe way, and reachable for any tool
  creating users (adduser, gnome-control-center, ubiquity…). Those are
  using useradd under the scene.

  For this, the proposed implementation:
  - patch useradd trying to execute "zsys useradd create USER HOMEDIR". If zsys isn't present or zsys returns a status code != 0 (which will be the case if the running system isn't a zsys one: pure zfs or non zfs like / on ext4), it will fallback to mkdir. Then the code does the usual chmod()
  - patch usermod, trying as well to execute "zsys useradd rename-home OLDHOME NEWHOME". Same failing reason (not a zsys system, not installed, OLDHOME isn't a zsys handled datasets) and fallback to rename(). Then the code does the usual chmod().

  Tested with and without zsys installed, the code does what we expect.

  I'm attaching the shadow (useradd/usermod) patches, as you can see it's very minimal.
  A new ZSYS release will be needed (https://github.com/ubuntu/zsys). As you can see, there are quite some commits since the last release, but it's all baked (as usual) by a huge suite of tests (in ZFS and machine layers) with corner cases tested and such. I'm confident on that change.

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