[tz] Preparing to fork tzdb

[Paul Eggert]

On 9/21/21 1:49 PM, Tom Lane wrote:

> while the data may be there,
> it's no longer possible to extract it in a way that matches
> prior results.

It is possible to extract it, if you use the patches I mentioned earlier 
today <https://mm.icann.org/pipermail/tz/2021-September/030456.html>. 
Admittedly this is effectively something of a fork and I don't favor 
this approach, but it is possible.

> Consumers who value data stability are out in
> the cold.

If you value data stability, then the current tzdb repository is better 
than going to a one-Zone-per-country rule, because the latter would 
cause more data churn than the former does, due to all the Links that 
would turned into Zones that would differ before 1970.

> I just re-ran my experiment comparing the zoneinfo tree generated
> by 2021a to what I get from current tzdb git.  I get the same set
> of zone files, except for a couple of expected recent changes such
> as Pacific/Kanton.  However:
> 
> * without backzone, there are 65 zone files/links that change
> contents compared to 2021a

Yes. This mostly affects only timestamps before 1970.

> * with backzone, there are 95 zone files/links that change content.

That also sounds about right. That's the extra data churn I mentioned above.

In other words, if the goal is data stability, you're better off with 
the current development repository, than with a fork that goes to a 
one-Zone-per-country rule.

> It looks like 25 of the changes are shared between the two trees
> and so probably represent actual recent updates, rather than effects
> of the May rearrangements.

I am not sure how you're counting, but only 11 files represent updates 
other than the May rearrangements. Here's the list:

America/Barbados
America/Guyana
Atlantic/Azores
Atlantic/Madeira
Europe/Lisbon
Pacific/Apia
Pacific/Enderbury
Pacific/Niue
Pacific/Rarotonga
Pacific/Tongatapu
Portugal

I got this list by using the abovementioned patches.

> I think there is going to be a fork

Any fork won't save you from the before-1970 changes you're concerned 
about, unless you give up on equity goals.