Use system zoneinfo data on Android

[mhsmith]

Feature or enhancement

Proposal:

The zoneinfo module can load time zone information from the common IANA format: one directory per continent, containing one file per city. These files can come from one of two sources:

• The operating system in of the following paths: /usr/share/zoneinfo:/usr/lib/zoneinfo:/usr/share/lib/zoneinfo:/etc/zoneinfo.
• The tzdata package on PyPI.

None of the well-known paths exist on Android, so the best approach on Android is currently the same as on Windows – use the tzdata package from PyPI.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

[mhsmith]

I got the following message from the responsible person on the Android team:

We do provide tzdata, but, as you noticed, in a slightly different form: there is one single
file which has all TZif files concatenated + some indexing. Its format is not documented
because it is an implementation detail and we don't want anyone to rely on it. But Android
is open-source, so certain libraries / frameworks just checked how that file is read on
the platform. It didn't go well for Mono when we switched from fat to slim TZif
format: mono/mono#21682. I am not particularly happy about
their fix, but they seem to be fine with silently returning wrong data to users in their APIs.

The overall problem is that TZif files are used as-if its format is an API, but

1. we as a platform can't be sure that all other implementations are reading TZif files correctly.
   And when they don't it might be tricky to track down the root cause.
2. There is a RFC for TZif, but it's outside of our control and it can go against
   library / framework maintainers' assumptions. For example, Java's java.util.TimeZone
   assumes that DST offset is zero or positive. But in the vanguard form of Europe/Dublin
   offset is negative (and there are a bunch of time zones where DST offset changed from
   negative to zero and to positive). So we, ICU, and Java are using rearguard form
   because it definitely might break someone's code.

So thank you for not reading Android's TZif files directly! We only provide APIs: libc, Java, and ICU.

On the subject of the ICU API, these notes in PEP 615 may be relevant.