tai64: wrong results -- 10s is not right offset and doesn't account for past or future leap seconds in SystemTime conversion

[programmerjake]

The offset between UTC time and TAI time isn't currently 10s, it's 37s and may change to something other than 37s in the future.

The current code assumes the offset is always 10s.

[programmerjake]

comparison on my system (after installing ntp so linux has the correct offset):
note how the result from linux and Tai64N::now() differ -- linux: 1655618465 vs. tai64: 1655618438 -- SystemTime: 1655618428
code:

use tai64::Tai64N;
use std::time::SystemTime;
use libc::{clock_gettime, CLOCK_TAI, timespec};

fn now_tai() -> std::io::Result<timespec> {
    let mut retval = timespec { tv_sec: 0, tv_nsec: 0 };
    let r = unsafe { clock_gettime(CLOCK_TAI, &mut retval) };
    if r != 0 {
        Err(std::io::Error::last_os_error())
    } else {
        Ok(retval)
    }
}

fn main() {
    let t = dbg!(Tai64N::now());
    let t = (t.0.0 as i64).wrapping_sub(1 << 62);
    dbg!(t);
    dbg!(SystemTime::now());
    let tai = now_tai().unwrap();
    dbg!(tai.tv_sec);
    dbg!(tai.tv_nsec);
}

output:

[src/main.rs:16] Tai64N::now() = Tai64N(
    Tai64(
        4611686020083006342,
    ),
    259280636,
)
[src/main.rs:18] t = 1655618438
[src/main.rs:19] SystemTime::now() = SystemTime {
    tv_sec: 1655618428,
    tv_nsec: 259298279,
}
[src/main.rs:21] tai.tv_sec = 1655618465
[src/main.rs:22] tai.tv_nsec = 259305372

[programmerjake]

See also:
https://lists.zx2c4.com/pipermail/wireguard/2022-June/007657.html

[tarcieri]

The implementation is indeed naive/broken. I thought we had an open tracking issue for this, but apparently not.

I'm not sure the best way to handle this, especially given the existing published crates.

[programmerjake]

how about downloading the official leap-second table and converting it to rust in a build.rs? you could have that enabled by an enabled-by-default feature.

apparently an official source is https://www.ietf.org/timezones/data/leap-seconds.list

[tarcieri]

I'd probably suggest against making network connections from a build script, especially in any security-related package (indeed there are renewed efforts underway to run build scripts under WASM).

However, I think it would be okay to use AOT conversion of the table, leveraging the build script as a sort of "time bomb" which makes the crate fail to compile after the table expires.

Or we could add some sort of automation to yank the crate releases containing expired tables.

[programmerjake]

for wireguard compatibility, you could add a disabled-by-default feature assume-incorrect-10s-offset and release a new major version, or maybe instead just add a now_with_assumed_offset function (and similar for other UTC-based interfaces).

[programmerjake]

I'd probably suggest against making network connections from a build script, especially in any security-related package (indeed there are renewed efforts underway to run build scripts under WASM).

ok, at least on linux you can usually get an up-to-date table from the tzdata package which is usually installed by default:
/usr/share/zoneinfo/leap-seconds.list

[programmerjake]

apparently recent versions of windows store the leap second table in the registry:
https://github.com/microsoft/STL/blob/7f04137880e1c3df5125c1baf808f16f399dee2e/stl/inc/chrono#L2040
https://github.com/microsoft/STL/blob/b9505b9818c8bd53148922730fc2a426e8f38620/stl/src/tzdb.cpp#L556

[tarcieri]

Perhaps it could try to query OS facilities to find the table, and if it can't, fall back on an internal table.