[tz] [PROPOSED] Avoid backward links in zone.tab

[Michael H Deckers]

    On 2021-05-10 08:38, Paul Eggert wrote:


>
> The idea is to move Zones into 'backward' if they're identical to some 
> other Zone after 1970.


    If these rules really hold for backzone (are
    they listed anywhere?), then
       America/Curacao
    also should go to backzone, and one of
       Africa/Johannesburg and Africa/Maputo
    and three of
       Pacific/Majuro, Pacific/Tarawa,
       Pacific/Funafuti, Pacific/Mata-Utu
    should go to backzone.

    By the way, do these rules imply that
       Pacific/Yap
    with the data as of 2005k belongs to backzone?

    A remark on backward compatibility:
    The proposed change is not conservative in the sense that
    information that could automatically be extracted from
    previous versions is no longer available in this way.
    To wit:

    It is no longer possible to automatically find geographical
    coordinates for the tzdb timezones described in backzone.

    In previous versions, the name of a tzdb timezone in
    backzone could also be found in zone.tab together with
    its country code and the coordinates for its location
    (except for backzone/Asia/Hanoi).

    After the change, there is no systematic way to find
    the right line in zone.tab with the coordinates, even
    if one supposes that one can automatically deduce the
    ISO country code for the name of a tzdb timezone in
    backzone (which is quite tricky and not so simple as
    in previous versions). Only the comment entry in a
    line of zone.tab (such as "Vietnam (south)") gives
    some indication on the name of the timezone (eg,
    Asia/Ho_Chi_Minh) whose geographic coordinates
    are contained in that line.

    I am aware that zone.tab is deprecated, so that
    one could say that the previously possible method was
    not guaranteed to work across versions -- but then the
    locations for backzone timezones could be omitted or put
    somewhere where they can be found automatically.

    Michael Deckers.