[Bug 1650688] Re: timedatectl set-timezone fails on UC16

[Ratchanan Srirattanamet]

** Also affects: systemd (Ubuntu)
   Importance: Undecided
       Status: New

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Title:
  timedatectl set-timezone fails on UC16

Status in Snappy:
  Triaged
Status in systemd package in Ubuntu:
  New

Bug description:
  Impish feature freeze bug fix (sort of) exception request
  =========================================================

  I (Ratchanan) would like ubuntu-release to verify if the proposed
  merge for Impish qualify as a bug fix per [1].

  Strictly speaking, as there's no core snap from an interim release,
  this issue doesn't directly effect Impish. However, to comply with the
  SRU requirement, and to let the patches carry to the 22.04 release, I
  would like this to be included in Impish. One of the patch has a small
  possibility to create a problem on classic systems, so testing in a
  development release is a good idea.

  Note that one can simulate core snap's /etc/writable redirection by
  running this image creation hook [2] on the system.

  [1] https://wiki.ubuntu.com/FreezeExceptionProcess#FeatureFreeze_for_bugfix-only_updates
  [2] https://git.launchpad.net/livecd-rootfs/tree/live-build/ubuntu-core/hooks/08-etc-writable.chroot?h=ubuntu/focal

  ---------------------

  SRU
  ===

  [Impact]

   * The bug prevents timedated from recognizing and correctly set the
  system's timezone when running Ubuntu Core 16, 18 and 20.

   * This causes by timedated fails to take Ubuntu Core's /etc/writable
  redirection into account.

   * The recognizing part is fixed by making the code take writable
  redirection into account.

   * The set part is fixed by making the code link to the absolute path
  instead of a relative one.

   * Currently core snaps worked around the set part by providing a
  wrapper script which re-create /etc/writable/localtime afterward.
  However this does not cover DBus users.

  [Test Plan]

   * On classics systems: ensure the proposed systemd package is installed.
     On Ubuntu Core systems: build a new core snap including proposed package, and install it. Replaces timedatectl with timedatectl.real to test skipping the wrapper.

   * On freshly boot system: query the timezone using `timedatectl`. The
  timezone should corresponds to `readlink -f /etc/localtime` and does
  not show `n/a`.

   * Set a new timezone: `sudo timedatectl set-timezone Asia/Bangkok`.
  `readlink -f /etc/localtime` should points to an existing file.

   * Run `sudo systemctl restart systemd-timedated.service`. Then, query
  the timezone again: `timedatectl`. It should show the previously set
  timezone and not `n/a`.

   * Run `sudo systemctl status systemd-timedated.service`. This should
  show no sign of timedated crashing.

  [Where problems could occur]

   * It's possible that the redirection handling code will be sub-par
  and causes crash. However, it's not likely because the similar pieces
  of code is in the previous patch since Ubuntu 16.04.

   * If it does: the patched `get_timezone()` function is used in 2
  places: the networkd's DHCP server [3] and the timedated itself.

     - Networkd is used primarily on servers where NetworkManage is
  absent. It's possible that this patch causes the user to loss access
  to the server due to networkd crash when setting up network
  interfaces, and requires physical access to fix. However, the code
  path is executed when DHCP is enabled only. I think it's not common
  for users to have networkd's DHCP server enabled: the feature seems to
  gear towards desktop users wanting to share internet connection, and
  in those cases they're more likely to use NetworkManager.

     - The timedated itself is likely used by the programs that involves
  in time-related functions. If a crash occur, in the worst case users
  won't be able to set time or timezone via timedated. However, users
  should still be able to e.g. set time using `date` or set timezone
  using /etc/localtime (assuming online guides still consider systems
  without systemd). Timedated is DBus-activated, and thus a crash should
  be self-healing.

   * The set part would also affects the clasic systems. However, I believe nothing else actually rely on /etc/localtime being a relative path, otherwise the /etc/writable redirection would causes even more problem, and would have been reported.
   
  [3] Yes, I'm surprised that there's a DHCP server inside systemd codebase.

  [Other Info]
   
   * This is also useful for UBports's Ubuntu Touch. We continue using system-image system where the rootfs is read-only, and thus is affected by this bug similarly to Ubuntu Core. I've tested the Focal version of the package on our (currently in development) Focal Ubuntu Touch image, and the fix works.
   
  ------------
   
  [Original bug description]

  On a system running UC16, the file /etc/localtime is a link that
  points to /etc/writable/localtime.

  On a freshly installed system, /etc/writable/localtime is a fully-
  qualified link that points at /usr/share/zoneinfo/UTC.

  If timedatectl is used to set the timezone to something else,
  timedated updates the localtime symbolic link with a relative path to
  the zoneinfo directory, which results in an invalid link.

  $ sudo timedatectl set-timezone America/Detroit

  $ sudo timedatectl
        Local time: Fri 2016-12-16 18:18:49 EST
    Universal time: Fri 2016-12-16 23:18:49 UTC
          RTC time: Fri 2016-12-16 23:18:49
         Time zone: America/Detroit (EST, -0500)
   Network time on: yes
  NTP synchronized: yes
   RTC in local TZ: no

  $ ls -l /etc/writable/localtime
  /etc/writable/localtime --> ../usr/share/zoneinfo/America/Detroit

  admin at localhost:/etc/writable$ date
  Fri Dec 16 23:20:07 UTC 2016

  I'm running the latest core snap from the candidate channel which
  contains snapd 2.18.1.  Hardware details and/or more debug information
  can be supplied on request.

  See 'man 3 timezone' and 'man timedatectl' for more details.

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