fish-shell/fish-rust/src/threads.rs

//! The rusty version of iothreads from the cpp code, to be consumed by native rust code. This isn't
//! ported directly from the cpp code so we can use rust threads instead of using pthreads.

use crate::flog::FLOG;

/// The rusty version of `iothreads::make_detached_pthread()`. We will probably need a
/// `spawn_scoped` version of the same to handle some more advanced borrow cases safely, and maybe
/// an unsafe version that doesn't do any lifetime checking akin to
/// `spawn_unchecked()`[std::thread::Builder::spawn_unchecked], which is a nightly-only feature.
///
/// Returns a boolean indicating whether or not the thread was successfully launched. Failure here
/// is not dependent on the passed callback and implies a system error (likely insufficient
/// resources).
pub fn spawn<F: FnOnce() + Send + 'static>(callback: F) -> bool {
    // The spawned thread inherits our signal mask. Temporarily block signals, spawn the thread, and
    // then restore it. But we must not block SIGBUS, SIGFPE, SIGILL, or SIGSEGV; that's undefined
    // (#7837). Conservatively don't try to mask SIGKILL or SIGSTOP either; that's ignored on Linux
    // but maybe has an effect elsewhere.
    let saved_set = unsafe {
        let mut new_set: libc::sigset_t = std::mem::zeroed();
        let new_set = &mut new_set as *mut _;
        libc::sigfillset(new_set);
        libc::sigdelset(new_set, libc::SIGILL); // bad jump
        libc::sigdelset(new_set, libc::SIGFPE); // divide-by-zero
        libc::sigdelset(new_set, libc::SIGBUS); // unaligned memory access
        libc::sigdelset(new_set, libc::SIGSEGV); // bad memory access
        libc::sigdelset(new_set, libc::SIGSTOP); // unblockable
        libc::sigdelset(new_set, libc::SIGKILL); // unblockable

        let mut saved_set: libc::sigset_t = std::mem::zeroed();
        let result = libc::pthread_sigmask(libc::SIG_BLOCK, new_set, &mut saved_set as *mut _);
        assert_eq!(result, 0, "Failed to override thread signal mask!");
        saved_set
    };

    // Spawn a thread. If this fails, it means there's already a bunch of threads; it is very
    // unlikely that they are all on the verge of exiting, so one is likely to be ready to handle
    // extant requests. So we can ignore failure with some confidence.
    // We don't have to port the PTHREAD_CREATE_DETACHED logic. Rust threads are detached
    // automatically if the returned join handle is dropped.

    let result = match std::thread::Builder::new().spawn(|| callback()) {
        Ok(handle) => {
            let id = handle.thread().id();
            FLOG!(iothread, "rust thread", id, "spawned");
            // Drop the handle to detach the thread
            drop(handle);
            true
        }
        Err(e) => {
            eprintln!("rust thread spawn failure: {e}");
            false
        }
    };

    // Restore our sigmask
    unsafe {
        let result = libc::pthread_sigmask(
            libc::SIG_SETMASK,
            &saved_set as *const _,
            std::ptr::null_mut(),
        );
        assert_eq!(result, 0, "Failed to restore thread signal mask!");
    };

    result
}

#[test]
/// Verify that spawing a thread normally via [`std::thread::spawn()`] causes the calling thread's
/// sigmask to be inherited by the newly spawned thread.
fn std_thread_inherits_sigmask() {
    // First change our own thread mask
    let (saved_set, t1_set) = unsafe {
        let mut new_set: libc::sigset_t = std::mem::zeroed();
        let new_set = &mut new_set as *mut _;
        libc::sigemptyset(new_set);
        libc::sigaddset(new_set, libc::SIGILL); // mask bad jump

        let mut saved_set: libc::sigset_t = std::mem::zeroed();
        let result = libc::pthread_sigmask(libc::SIG_BLOCK, new_set, &mut saved_set as *mut _);
        assert_eq!(result, 0, "Failed to set thread mask!");

        // Now get the current set that includes the masked SIGILL
        let mut t1_set: libc::sigset_t = std::mem::zeroed();
        let mut empty_set = std::mem::zeroed();
        let empty_set = &mut empty_set as *mut _;
        libc::sigemptyset(empty_set);
        let result = libc::pthread_sigmask(libc::SIG_UNBLOCK, empty_set, &mut t1_set as *mut _);
        assert_eq!(result, 0, "Failed to get own altered thread mask!");

        (saved_set, t1_set)
    };

    // Launch a new thread that can access existing variables
    let t2_set = std::thread::scope(|_| {
        unsafe {
            // Set a new thread sigmask and verify that the old one is what we expect it to be
            let mut new_set: libc::sigset_t = std::mem::zeroed();
            let new_set = &mut new_set as *mut _;
            libc::sigemptyset(new_set);
            let mut saved_set2: libc::sigset_t = std::mem::zeroed();
            let result = libc::pthread_sigmask(libc::SIG_BLOCK, new_set, &mut saved_set2 as *mut _);
            assert_eq!(result, 0, "Failed to get existing sigmask for new thread");
            saved_set2
        }
    });

    // Compare the sigset_t values
    unsafe {
        let t1_sigset_slice = std::slice::from_raw_parts(
            &t1_set as *const _ as *const u8,
            core::mem::size_of::<libc::sigset_t>(),
        );
        let t2_sigset_slice = std::slice::from_raw_parts(
            &t2_set as *const _ as *const u8,
            core::mem::size_of::<libc::sigset_t>(),
        );

        assert_eq!(t1_sigset_slice, t2_sigset_slice);
    };

    // Restore the thread sigset so we don't affect `cargo test`'s multithreaded test harnesses
    unsafe {
        let result = libc::pthread_sigmask(
            libc::SIG_SETMASK,
            &saved_set as *const _,
            core::ptr::null_mut(),
        );
        assert_eq!(result, 0, "Failed to restore sigmask!");
    }
}