parser: add IANA time_zone support for naive timestamps

[arnikola]

Add optional parser setting time_zone (IANA name, e.g. from tzdata) so naive timestamps from time_format are converted with local rules and DST, instead of only fixed offset or system timezone.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Skip time_zone conversion when timestamp already has offset

The new time_zone branch always uses mktime in the configured zone, even when time_format includes %z/%Z and the parsed record already carries an explicit offset. In that case flb_parser_time_lookup() has already parsed the source zone, but this path ignores it and reinterprets the wall clock in parser->time_zone, producing shifted epochs (for example, a +0000 input gets converted as local New York time). Please either reject time_zone with %z/%Z formats in flb_parser_create() or bypass this branch when parser->time_with_tz is true.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Avoid mutating process TZ during per-record parsing

This code changes TZ and calls tzset() for every parsed record, which is process-global state. The mutex only serializes this function, but other paths still call mktime without this lock (e.g. flb_parser_tm2time_simple for time_system_timezone), so concurrent parsing can observe the temporary timezone and compute wrong epochs. This creates cross-parser timestamp corruption when one thread uses time_zone and another thread parses local/system timestamps at the same time.

Useful? React with 👍 / 👎.

[arnikola]

Example config files:
config.conf:

[SERVICE]
    Parsers_File  parsers.conf

[INPUT]
    Name          tail
    Path          tz-example.log
    Tag           tz.test
    Parser        tz_naive
    Read_from_Head true
    Exit_On_Eof   true

[OUTPUT]
    Name          stdout
    Match         tz.test
    Format        json_lines

parsers.conf

[PARSER]
    Name         tz_naive
    Format       regex
    Regex        ^(?<ts>[^ ]+) (?<msg>.+)$
    Time_Key     ts
    Time_Format  %Y-%m-%dT%H:%M:%S
    Time_Keep    On
    Time_Strict  On
    Time_Zone    Australia/Sydney

[cosmo0920]

The patch direction looks great but we need to add Signed-off line on every commit. So, could you add this line?
This can be achieved by:

To add your Signed-off-by line to every commit in this branch:

    Ensure you have a local copy of your branch by [checking out the pull request locally via command line](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally).
    In your local branch, run: git rebase HEAD~1 --signoff
    Force push your changes to overwrite the branch: git push --force-with-lease origin arnikola/parser-timezones-dst

ref: https://github.com/fluent/fluent-bit/pull/11639/checks?check_run_id=69300784903