fix(bug): ns->us overflow handling

[HalFrgrd]

I ran into an odd bug on macos. If my code polls for just under 5s (say 4.999_999_123 seconds), I immediately receive an EINVAL error.

This is due to the rounding logic. With the current logic, we set tv_usec to 1_000_000 when the nanosecond remainder is >= 999_999_001. This creates an invalid timeval on macOS and the OS returns EINVAL.

tv_usec=1_000_000 is considered invalid and instead we should increase tv_sec by 1 and set tv_usec=0.

Solution

Checking if tv_usec >= 1_000_000 https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/kern/kern_time.c#L670

XNU handling this issue in the same way that I propose: https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/sys/time.h#L175-L177

Linux kernel is more forgiving if you set usec to 1_000_000: https://github.com/torvalds/linux/blob/master/fs/select.c#L723-L725. So I could revert the changes here for linux code and only keep the macos fix. Please let me know what is best.

Reproducing

This is a small example that I have tested on macOS. select with a timeout of 999_999_123 fails using rustix master but is fixed when using my branch.

use rustix::event::{fd_set_insert, fd_set_num_elements, select, FdSetElement, Timespec};
use rustix::fd::AsRawFd;
use rustix::pipe::pipe;

fn main() {
    // Create a pipe so we have a valid file descriptor to pass to select()
    let (reader, _writer) = pipe().unwrap();
    let raw_fd = reader.as_raw_fd();
    let nfds = raw_fd + 1;
    let num_elems = fd_set_num_elements(1, nfds);

    // 1. 999_999_000 ns -> (999_999_000 + 999)/1000 = 999_999 μs (< 1,000,000 μs)
    let mut readfds1 = vec![FdSetElement::default(); num_elems];
    fd_set_insert(&mut readfds1, raw_fd);
    let res1 = unsafe {
        select(
            nfds,
            Some(&mut readfds1),
            None,
            None,
            Some(&Timespec {
                tv_sec: 0,
                tv_nsec: 999_999_000,
            }),
        )
    };
    println!("999_999_000 ns result: {:?}", res1);

    // 2. 999_999_123 ns -> (999_999_123 + 999)/1000 = 1_000_000 μs
    let mut readfds2 = vec![FdSetElement::default(); num_elems];
    fd_set_insert(&mut readfds2, raw_fd);
    let res2 = unsafe {
        select(
            nfds,
            Some(&mut readfds2),
            None,
            None,
            Some(&Timespec {
                tv_sec: 0,
                tv_nsec: 999_999_123,
            }),
        )
    };
    println!("999_999_123 ns result: {:?}", res2);
}

Testing

invalid_offset::invalid_offset_fadvise library test was failing on main. All other cargo test --features=all-apis tests pass. All cargo test --test event --features all-apis tests pass.