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.

[sunfishcode]

Thanks!

[sunfishcode]

This is now released in rustix 1.1.5.

[ogoffart]

Looks like this broke the build on android.

This was catched in our CI:

error[E0433]: cannot find `timespec` in `crate`
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.5/src/backend/libc/net/sockopt.rs:237:29
    |
237 |             let ts = crate::timespec::Timespec {
    |                             ^^^^^^^^ could not find `timespec` in the crate root
    |
note: found an item that was configured out
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.5/src/lib.rs:410:5
    |
389 |   #[cfg(any(
    |  __________-
390 | |     feature = "fs",
391 | |     feature = "event",
392 | |     feature = "process",
...   |
409 | | ))]
    | |_- the item is gated here
410 |   mod timespec;
    |       ^^^^^^^^

error[E0433]: cannot find `timespec` in `crate`
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.5/src/backend/libc/net/sockopt.rs:241:39
    |
241 |                     .unwrap_or(crate::timespec::Secs::MAX),
    |                                       ^^^^^^^^ could not find `timespec` in the crate root
    |
note: found an item that was configured out
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.5/src/lib.rs:410:5
    |
389 |   #[cfg(any(
    |  __________-
390 | |     feature = "fs",
391 | |     feature = "event",
392 | |     feature = "process",
...   |
409 | | ))]
    | |_- the item is gated here
410 |   mod timespec;
    |       ^^^^^^^^

error[E0433]: cannot find `timespec` in `crate`
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.5/src/backend/libc/net/sockopt.rs:244:66
    |
244 |             let (sec, usec) = ts.to_sec_usec().unwrap_or((crate::timespec::Secs::MAX, 0));
    |                                                                  ^^^^^^^^ could not find `timespec` in the crate root
    |
note: found an item that was configured out
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.5/src/lib.rs:410:5
    |
389 |   #[cfg(any(
    |  __________-
390 | |     feature = "fs",
391 | |     feature = "event",
392 | |     feature = "process",
...   |
409 | | ))]
    | |_- the item is gated here
410 |   mod timespec;
    |       ^^^^^^^^