rust - 为什么 Mio 的民意调查针对用户生成的事件触发了两次？

Question

以下代码生成一个由poll 返回的“用户事件”:

extern crate mio;

use mio::event::Evented;
use mio::{Events, Poll, PollOpt, Ready, Registration, Token};
use std::thread::{sleep, spawn, JoinHandle};
use std::time::{Duration, Instant};

#[derive(Debug)]
struct Output(u32, Duration);

pub struct MioThread {
    registration: Registration,
    handle: JoinHandle<Output>,
}

impl MioThread {
    pub fn new(i: u32) -> MioThread {
        let now = Instant::now();

        let (registration, set_readiness) = Registration::new2();

        let handle = spawn(move || {
            sleep(Duration::from_millis((1000 - (100 * i)) as u64));

            set_readiness.set_readiness(Ready::readable()).unwrap();

            Output(i, now.elapsed())
        });

        MioThread {
            registration: registration,
            handle: handle,
        }
    }

    // manage the thread result
    fn eval_result(self) {
        let out = self.handle.join();
        println!("do whathever you want with: {:?}", out.unwrap());
    }
}

fn main() {
    let poll = Poll::new().unwrap();

    let mut events = Events::with_capacity(16);

    let mut tasks = Vec::new();
    for i in 0..5 {
        let mio_thread = MioThread::new(i);

        mio_thread
            .registration
            .register(&poll, Token(i as usize), Ready::readable(), PollOpt::edge())
            .unwrap();

        tasks.push(Some(mio_thread));
    }

    loop {
        let num_events = poll.poll(&mut events, None).unwrap();
        println!("poll fired: {} events", num_events);
        for event in &events {
            if event.readiness().is_readable() {
                let Token(thread_id) = event.token();

                if let Some(t) = tasks.remove(thread_id) {
                    t.eval_result();
                }

            }
        }
    }
}

输出是:

poll fired: 1 events
do whathever you want with: Output(4, Duration { secs: 0, nanos: 600967623 })
poll fired: 0 events
poll fired: 1 events
do whathever you want with: Output(3, Duration { secs: 0, nanos: 701035026 })
poll fired: 0 events
poll fired: 1 events
do whathever you want with: Output(2, Duration { secs: 0, nanos: 801089370 })
poll fired: 0 events
poll fired: 1 events
do whathever you want with: Output(1, Duration { secs: 0, nanos: 900890190 })
poll fired: 0 events
poll fired: 1 events
do whathever you want with: Output(0, Duration { secs: 1, nanos: 600076 })
poll fired: 0 events

我打开了an issue on the Mio repository .

Answer 1

如评论中所述并确认here :

dropping a Registration might wake up the loop (which it indeed does, the same as a registration) without actually triggering an event

这在大多数情况下显然不是问题，只是我在阅读 docs 后没想到的行为。 :

fn poll(&self, events: &mut Events, timeout: Option) -> Result[−] Wait for readiness events

Blocks the current thread and waits for readiness events for any of the Evented handles that have been registered with this Poll instance. The function will block until either at least one readiness event has been received or timeout has elapsed. A timeout of None means that poll will block until a readiness event has been received.

当前线程确实在删除 Registration 时也被唤醒。