localtime_r shim uses TZ variable from the host, not the interpreted program

[saethlin]

Currently the test suite may fail to pass, unless you run it with TZ=UTC ./miri test. This is because the localtime_r shim reads the TZ variable from the host on this line: https://github.com/rust-lang/miri/blob/f26bd28c4d0e96a3b855b7f544030ee593b3090a/src/shims/time.rs#L139-L140

In other words, this test code is ineffective: https://github.com/rust-lang/miri/blob/f26bd28c4d0e96a3b855b7f544030ee593b3090a/tests/pass-dep/shims/libc-misc.rs#L220-L222

I'm going to look into this.

cc @tiif @eduardosm

[tiif]

Thanks for looking into this, and sorry for causing this. I ran into this issue while testing and discussed with @oli-obk before, but I thought there is something wrong with my machine because it passed the CI and env::set_var works when I isolated it out in a test.

This is how the test looks like: In tests/pass-dep/shims/libc-misc.rs

fn test_tz_var_setting() {
    use chrono::Local;
    use std::env;

    let key = "TZ";
    env::set_var(key, "GMT");
    assert_eq!(env::var(key), Ok("GMT".to_string()));
    let offset_in_second = Local::now().offset().local_minus_utc();
    // If the timezone is GMT, tm_gmtoff will be 0
    let tm_gmtoff = offset_in_second;
    assert_eq!(tm_gmtoff, 0);
    env::remove_var(key);
    assert!(env::var(key).is_err());
}

In test_dependencies/Cargo.toml, add chrono to dependencies:

[dependencies]
# all dependencies (and their transitive ones) listed here can be used in `tests/`.
libc = "0.2"
num_cpus = "1.10.1"
tempfile = "3"
chrono = "0.4.37"

This test still pass after I merged the latest update from master (commit f26bd28c).

[RalfJung]

This is how the test looks like:

That's using chrono::Local inside the program, so when it does env::get it sees the variables of the interpreted program.

However, the shim uses chrono::Local on the host, so that will use the host's env::get instead of the one of the interpreted program.

[tiif]

That's using chrono::Local inside the program, so when it does env::get it sees the variables of the interpreted program.

I see, so env::set_var in test in fact can't change the TZ variable in the host?

[RalfJung]

No, it's impossible to change the host environment from the interpreted program.

Mutating the environment is unsafe so it would be rather bad to let the interpreted program do that. ;)

[tiif]

This makes sense, thanks!