Rewite caching in local::unix module

[pitdicker]

Existing code

At a high level, the unix module tries to find the current time zone, load the data for it and cache the result.

1. To detect changes the caching code looks if the TZ environment variable has changed or if the modification time of the /etc/localtime symlink has changed.
2. To update the time zone info, the first step is in tz_info::timezone::TimeZone::local. If the TZ variable is set it can load TZ data from the following sources:

• a POSIX TZ string
• a named time zone, loaded from the ZONE_INFO_DIRECTORIES
• an absolute path
• the /etc/localtime symlink
• an empty TZ variable is an error

3. If the TZ variable is not set it uses the /etc/localtime symlink.
4. unix::current_zone then specifies unix::fallback as a third fallback.
   This gets the time zone name using the iana_time_zone crate, and tries to load it from TZDB_LOCATION (which is currently not the same as ZONE_INFO_DIRECTORIES).
5. A final fallback is to default to UTC.

Problems with this setup are that the logic lives in different places, that the caching code does not detect changes except the most limited form, and that the logic to look up the zoneinfo directory is implemented in two different ways.

New code

Everything related to loading the time zone data is implemented as methods on a CachedTzInfo type.
Caching and updating happens with the methods:

• CachedTzInfo::tz_info()
  • refresh_cache()
    • needs_update()
      • tz_env_var_changed()
      • symlink_changed()
      • tz_name_changed()
    • read_tz_info()
      • read_from_tz_env()
      • read_from_symlink()
      • read_with_tz_name()
        • tzdb_dir()

I kept methods such as read_with_tz_name() and tz_name_changed() close together so it is easier to see they work similar.

The CachedTzInfo type remembers the last source that was succesfull, and stores all information needed to check the cache is up to date: TZ variable, time zone name, path, and zoneinfo directory.

New functionality is that we cache the directory with the time zone database, and respect the TZDIR environment variable (fixes #1265).

All potential error sources are returned with a Result</* */, ()> type.

---

This is a rewrite, I can't pretend it to be a refactor.

[codecov]

Codecov Report

Attention: Patch coverage is 44.63277% with 98 lines in your changes are missing coverage. Please review.

Project coverage is 91.41%. Comparing base (e05ba8b) to head (b485ac2).

Files	Patch %	Lines
src/offset/local/unix.rs	44.88%	97 Missing ⚠️
src/offset/local/tz_info/timezone.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1457      +/-   ##
==========================================
- Coverage   91.82%   91.41%   -0.42%     
==========================================
  Files          40       40              
  Lines       18345    18382      +37     
==========================================
- Hits        16846    16804      -42     
- Misses       1499     1578      +79     

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[djc]

This is a rewrite, I can't pretend it to be a refactor.

That makes it very tricky to review. Pretty sure there are ways to break it down into smaller pieces.

[pitdicker]

That makes it very tricky to review.

Yes, I realize that 😞.

Maybe I can split it in:

• Drop the existing caching logic, and move all time zone loading logic into CachedTzInfo.
• Add new caching code.

And maybe that can be divided in more than two commits, hard to say.

Does that sound better reviewable?

[pitdicker]

I'll see what I can do to split it up more.

[pitdicker]

Setting to draft for now. This is only tangentially related to the work of converting the API to Results.
I'll concentrate on the time_delta and parsing modules first.