Extract timezone info from tzdata file on Android

[RumovZ]

Replacement for #975. As per @djc's suggestion, the tzdata parser now resides in its own crate.

Closes #922.

[RumovZ]

I'm almost afraid to ask after that embarassing typo, but does anyone have an idea why that test segfaults?

[dae]

Might be worth closing and then reopening this PR to rerun the tests to see if it was a flake?

[dae]

The test passes for me when I run it in another repo. I suspect the rust-cache action is the culprit; try replacing the line in .github/workflows/test.yml with

- uses: Swatinem/rust-cache@v2
  with:
    prefix-key: "v1-rust"

[RumovZ]

Perfect, thanks for the help. @dae!

[esheppa]

This looks good to me, only thing I'd like to have a further discussion on is the ordering of the options in from_posix_tz. Thanks for creating the android-tzdata library @RumovZ

[esheppa]

should this moved up to above line 46? Also what do we do in the case that the tz_string is "localtime" on android?

[RumovZ]

I have yet to find exhaustive documentation on this, but from what I can tell, tzdata doesn't contain a localtime entry, and there is /etc/localtime like on other Unix systems. Moving the tzdata block up sounds good to me.

[dae]

I've checked an Android device here, and it does not have an /etc/localtime - perhaps they did away with it in later Android releases, or it varies by manufacturer? Android appears to set TZ to the timezone configured in the settings by default, so presumably users would not hit this unless they explicitly overwrote the default value.

[jtmoon79]

In PR code for src/offset/local/tz_info/timezone.rs (above) the fn from_posix_tz code attempts do things the "Android Way" but if that doesn't succeed it falls back to the current implementation. Specifically, if android_tzdata::find_tz_data(tz_string) fails it will fall through and use the current general implementation.

Should fn fallback_timezone code allow a similar fallback? e.g.

fn fallback_timezone() -> Option<TimeZone> {
    let tz_name = iana_time_zone::get_timezone().ok()?;
    #[cfg(target_os = "android")]
    if let Ok(bytes) = android_tzdata::find_tz_data(&tz_name).ok() {
        return TimeZone::from_tz_data(&bytes).ok();
    }
    let bytes = fs::read(format!("{}/{}", TZDB_LOCATION, tz_name)).ok()?;
    TimeZone::from_tz_data(&bytes).ok()
}

[RumovZ]

In from_posix_tz(), we try to parse the tz string as a transition rule as a fallback, which should or shouldn't work regardless of the OS. On the other hand, trying to find a tz file on Android won't ever work, as far as I know, so I don't think it makes sense as a fallback.

[jtmoon79]

Okay. 👍

[djc]

Thanks!