PollFd implementing Copy is a footgun

[JonathonReinhart]

I think this code speaks for itself:

fn example(file: &std::fs::File) -> io::Result<Option<()>> {
    use nix::poll::{poll, PollFd, PollFlags, PollTimeout};
    use std::os::fd::AsFd;

    let poll_fd = PollFd::new(file.as_fd(), PollFlags::POLLIN);

    let mut poll_fds = [poll_fd];                          // <<<< Copy happens here

    let poll_res = poll(&mut poll_fds, PollTimeout::NONE)?;
    if poll_res == 0 {
        // Timeout
        return Ok(None);
    }
    assert_eq!(poll_res, 1);

    // This fails because poll_fd is not the object that was actually updated
    //assert_eq!(poll_fd.any(), Some(true));

    // You have to access the item in the array
    assert_eq!(poll_fds[0].any(), Some(true));

    Ok(Some(()))
}

The fact that PollFd implements Copy makes it easy to make this mistake.

Normally, Rust doesn't let you make mistakes like this. If PollFd didn't implement Copy, rust would complain if you tried to write this code. You'd have to call .clone() and then there would be an explicit clue that you're not operating on the same object.

error[E0382]: borrow of moved value: `poll_fd`
  --> src/main.rs:23:16
   |
11 |     let poll_fd = PollFd::new(file.as_fd(), PollFlags::POLLIN);
   |         ------- move occurs because `poll_fd` has type `PollFd<'_>`, which does not implement the `Copy` trait
12 |
13 |     let mut poll_fds = [poll_fd];                          // <<<< Copy happens here
   |                         ------- value moved here
...
23 |     assert_eq!(poll_fd.any(), Some(true));
   |                ^^^^^^^ value borrowed here after move
   |
help: consider cloning the value if the performance cost is acceptable
   |
13 |     let mut poll_fds = [poll_fd.clone()];                          // <<<< Copy happens here
   |                                ++++++++

[asomers]

Good point.