Add std::thread::add_spawn_hook.

[m-ou-se]

Implementation of rust-lang/rfcs#3642

[rustbot]

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[m-ou-se]

An alternative is to use a lock free linked list:

static SPAWN_HOOK: AtomicPtr<SpawnHook> = AtomicPtr::new(ptr::null_mut());

struct SpawnHook {
    next: Option<&'static SpawnHook>,
    hook: &'static (dyn Fn(&Thread) -> io::Result<Box<dyn FnOnce() + Send>> + Sync),
}

pub fn add_spawn_hook<F, G>(hook: F)
where
    F: 'static + Sync + Fn(&Thread) -> io::Result<G>,
    G: 'static + Send + FnOnce(),
{
    let mut next = SPAWN_HOOK.load(Ordering::Relaxed);
    let mut hook = Box::leak(Box::new(SpawnHook {
        next: unsafe { next.as_ref() },
        hook: Box::leak(Box::new(move |thread: &Thread| {
            let f: Box<dyn FnOnce() + Send> = Box::new(hook(thread)?);
            Ok(f)
        })),
    }));
    while let Err(n) =
        SPAWN_HOOK.compare_exchange_weak(next, hook, Ordering::Release, Ordering::Relaxed)
    {
        next = n;
        hook.next = unsafe { next.as_ref() };
    }
}

pub(super) fn run_spawn_hooks(thread: &Thread) -> io::Result<Vec<Box<dyn FnOnce() + Send>>> {
    let mut result = Vec::new();
    let mut next = unsafe { SPAWN_HOOK.load(Ordering::Acquire).as_ref() };
    while let Some(hook) = next {
        result.push((hook.hook)(thread)?);
        next = hook.next;
    }
    Ok(result)
}

But that uses two Boxes per hook (one for the dyn Fn and one for the linked list node (SpawnHook). We could merge them into a single Box by storing the Fn by value in a SpawnHook<F>, but that results in a lot more complexity and much more subtle unsafe code, which is probably not worth it.

[bors]

☔ The latest upstream changes (presumably #127998) made this pull request unmergeable. Please resolve the merge conflicts.