[tz] Vanguard vs. "main" vs. rearguard

[Paul Eggert]

On 6/7/21 9:21 AM, Neil Fuller wrote:

> To fix this for future releases *safely*, I think downstream code like
> Android would need metadata at runtime (preferably mastered by TZDB) that
> indicates whether a zone ID is one of the ones that has flipped its
> behavior.

Surely it'd have to be more complicated than that, as a single Zone can 
have negative DST some years, and positive DST other years. See 
Africa/Windhoek, for example.

But perhaps I'm not understanding the problem correctly, as I don't see 
why tzdb would need to maintain a separate set of data for this issue. 
Android libraries could have a flag (or multiple versions, or however 
Android folks like to do software changes) controlling whether an 
application can tolerate negative DST. If the flag says "negative DST is 
OK", the libraries can pass through data unscathed. If the flag says "I 
don't want to see negative DST", then whenever the underlying data says 
DST is negative, the Android library can swap standard and daylight time 
and give the swapped info to the app. As I understand it, Android 
libraries are already doing the latter; if so, all that needs to be 
added is a flag so that they can do the former.

Anyway, I appreciate the info about why rearguard support will be needed 
for quite some time.