[Bug 2017999] Re: tzdata 2023c-0ubuntu0.22.04.1 might have some inconsistency

[Benjamin Drung]

Thanks for the review. I tested what happens if a debconf answer is
provided that will be removed by the update:

```
$ printf "tzdata tzdata/Areas select Europe\ntzdata tzdata/Zones/Europe select Uzhgorod\n" | debconf-set-selections
$ DEBIAN_FRONTEND=noninteractive apt install ./tzdata_2023c-0ubuntu0.20.04.2_all.deb
[...]
Current default time zone: 'Europe/Amsterdam'
```

So it will select the first entry of that area instead. So we need to
keep all debconf options. I'll prepare a new set of debdiffs.

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https://bugs.launchpad.net/bugs/2017999

Title:
  tzdata 2023c-0ubuntu0.22.04.1 might have some inconsistency

Status in python-tz package in Ubuntu:
  Fix Released
Status in tzdata package in Ubuntu:
  Fix Released
Status in python-tz source package in Focal:
  Invalid
Status in tzdata source package in Focal:
  New
Status in python-tz source package in Jammy:
  Invalid
Status in tzdata source package in Jammy:
  New
Status in python-tz source package in Kinetic:
  Invalid
Status in tzdata source package in Kinetic:
  New
Status in python-tz source package in Lunar:
  Fix Committed
Status in tzdata source package in Lunar:
  Incomplete

Bug description:
  [ Impact ]

  Recent updates have been using `PACKRATDATA=backzone` in order to add
  all out-of-scope lower-quality pre-1970 data back that existed in
  earlier tzadata releases (see bug #2003797). However it also leads to
  time zones like America/Ensenada, Atlantic/Jan_Mayen, Europe/Tiraspol,
  and Pacific/Enderbury to change:

  compared to 2023c-0ubuntu0.22.04.0
  - America/Ensenada->only dst changes until 2022 are shown
  - Atlantic/Jan_Mayen->now valid data is shown
  - Europe/Tiraspol->only dst changes until 2010 are shown

  [ Test Plan ]

  Add an autopkgtest test case for post-1970 symlink consistency: Check
  that the timezones for today and in half a year (= to cover DST
  changes) do not change compared to building them without
  `PACKRATDATA=backzone`.

  The mentioned timezones that differed can also be tested manually.
  Running following commands should have the same output (the output
  mentioned here is from tzdata 2023c-0ubuntu0.22.04.1):

  ```
  $ TZ=America/Ensenada date -d "2023-04-28 13:49 Z"
  Fri Apr 28 05:49:00 PST 2023
  $ TZ=Atlantic/Jan_Mayen date -d "2023-04-28 13:49 Z"
  Fri Apr 28 12:49:00 -01 2023
  $ TZ=Europe/Tiraspol date -d "2023-04-28 13:49 Z"
  Fri Apr 28 16:49:00 MSK 2023
  ```

  [ Where problems could occur ]

  Timezone data could be wrong. Systems with incorrect timezone set may
  observe unexpected time shift when updating the package.

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