time/sys/refresh_tz/
unix.rs

1/// Whether the operating system has a thread-safe environment. This allows bypassing the check for
2/// if the process is multi-threaded.
3// This is the same value as `cfg!(target_os = "x")`.
4// Use byte-strings to work around current limitations of const eval.
5const OS_HAS_THREAD_SAFE_ENVIRONMENT: bool = match std::env::consts::OS.as_bytes() {
6    // https://github.com/illumos/illumos-gate/blob/0fb96ba1f1ce26ff8b286f8f928769a6afcb00a6/usr/src/lib/libc/port/gen/getenv.c
7    b"illumos"
8    // https://github.com/NetBSD/src/blob/f45028636a44111bc4af44d460924958a4460844/lib/libc/stdlib/getenv.c
9    // https://github.com/NetBSD/src/blob/f45028636a44111bc4af44d460924958a4460844/lib/libc/stdlib/setenv.c
10    | b"netbsd"
11    // https://github.com/apple-oss-distributions/Libc/blob/63976b830a836a22649b806fe62e8614fe3e5555/stdlib/FreeBSD/getenv.c#L118
12    // https://github.com/apple-oss-distributions/Libc/blob/63976b830a836a22649b806fe62e8614fe3e5555/stdlib/FreeBSD/setenv.c#L446
13    // https://blog.rust-lang.org/2023/09/25/Increasing-Apple-Version-Requirements/
14    | b"macos"
15    => true,
16    _ => false,
17};
18
19/// Update time zone information from the system.
20///
21/// For safety documentation, see [`time::util::refresh_tz`].
22#[inline]
23pub(super) unsafe fn refresh_tz_unchecked() {
24    extern "C" {
25        #[cfg_attr(target_os = "netbsd", link_name = "__tzset50")]
26        fn tzset();
27    }
28
29    // Safety: The caller must uphold the safety requirements.
30    unsafe { tzset() };
31}
32
33/// Attempt to update time zone information from the system. Returns `None` if the call is not known
34/// to be sound.
35#[inline]
36pub(super) fn refresh_tz() -> Option<()> {
37    // Refresh $TZ if and only if the call is known to be sound.
38    //
39    // Soundness can be guaranteed either by knowledge of the operating system or knowledge that the
40    // process is single-threaded. If the process is single-threaded, then the environment cannot
41    // be mutated by a different thread in the process while execution of this function is taking
42    // place, which can cause a segmentation fault by dereferencing a dangling pointer.
43    //
44    // If the `num_threads` crate is incapable of determining the number of running threads, then
45    // we conservatively return `None` to avoid a soundness bug.
46
47    if OS_HAS_THREAD_SAFE_ENVIRONMENT || num_threads::is_single_threaded() == Some(true) {
48        // Safety: The caller must uphold the safety requirements.
49        unsafe { refresh_tz_unchecked() };
50        Some(())
51    } else {
52        None
53    }
54}
time/sys/refresh_tz/unix.rs

time/sys/refresh_tz/
unix.rs
