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

[Paul Eggert]

On 5/11/21 5:32 AM, Michael H Deckers wrote:

>       Rather, the corresponding geographic coordinates in zone.tab
>       are no longer connected with the timezone name, only with
>       an ISO country code.

I wouldn't put it that way. In zone.tab, a line's geographic coordinates 
are connected with both the country code and the timezone name. That is, 
the coordinates identify a location that is both (a) in the country, and 
(b) has clocks that have agreed with the timezone since 1970.

> The same holds for several proposed lines
>       in zone1070.tab such as
>             AQ,KW,SA,YE  +2438+04643   Etc/GMT-3   Arabia, Syowa
>       What are these geographical coordinates supposed to indicate?

They indicate a major location within the designated area. This is the 
same as the non-Etc lines in zone1970.tab, and it's the same role they 
play in zone.tab.

>       One of the successful design choices of the tzdb
>       database was to identify timezones by locations (rather than
>       by areas or countries), so that location names and location
>       coordinates are equivalent means of identifying timezones
>       of locations. Is this still true?

That depends on what one means by "equivalent" :-). Timezone names like 
Etc/GMT never identified single locations all that well.

An alternative strategy to what's proposed in the patch, is to keep a 
single location-based Zone in place of each Etc/* name that the patch 
uses. For example, where the patch proposes making several names 
(Asia/Dubai, Asia/Muscat, Indian/Mahe, Indian/Reunion) 
backward-compatibility aliases for Etc/GMT-4, we could instead keep 
Asia/Dubai (the most-populous location) as a Zone, and have the other 
names be backward-compatibility aliases for Asia/Dubai.

In the long run this approach would be a bit more work than what the 
patch proposes, as it'd mean we should continue to maintain out-of-scope 
(pre-1970) data for Asia/Dubai, Asia/Bangkok, etc. The main advantage of 
it would be avoiding Etc/* names as link targets and in zone*.tab files.