`Epoll::add` works differently comparing to `epoll::epoll_ctl`

[slhmy]

Hi, I'm trying to upgrade nix from 0.26 to 0.27, the one thing which bothered me is the new Epoll.

I used to have a function add_epoll_fd like:

fn add_epoll_fd(epoll_fd: RawFd, fd: RawFd) -> Result<(), JudgeCoreError> {
    let mut event = EpollEvent::new(EpollFlags::EPOLLIN, fd as u64);
    log::debug!("Adding fd={} to epoll", fd);
    Ok(epoll_ctl(
        epoll_fd,
        EpollOp::EpollCtlAdd,
        fd,
        Some(&mut event),
    )?)
}

// Then in 0.27 I did a bit change to make it still working
let epoll = Epoll::new(EpollCreateFlags::EPOLL_CLOEXEC)?;
add_epoll_fd(epoll.0 as RawFd, user_exit_read)?;

But when I changed epoll::epoll_ctl to Epoll::add, my epoll seems to be blocked:

fn add_epoll_fd(epoll: &Epoll, fd: RawFd) -> Result<(), JudgeCoreError> {
    let event = EpollEvent::new(EpollFlags::EPOLLIN, fd as u64);
    log::debug!("Adding fd={} to epoll", fd);
    Ok(epoll.add(&eventfd(fd as u32, EfdFlags::empty())?, event)?)
}

let epoll = Epoll::new(EpollCreateFlags::EPOLL_CLOEXEC)?;
add_epoll_fd(&epoll, user_exit_read)?;

[slhmy]

Closing this issue for I found eventfd is a single one, I made a mistake!

For cases similar with me use unsafe { BorrowedFd::borrow_raw(fd) } to convert RawFd to AsFd impl.