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如何将等待“tokio::sync::watch::Receiver”变为“Some”?

rust-tokio borrow-checker async-await rust

dspyz · 技术社区 · 4 月前

我想写一篇 async 功能 recv_some 这需要一个 watch::Receiver<Option<T>> ,等待值为 Some ,并返回以下内容 Deref 的 T 。出于我所能理解的原因,当我以显而易见的方式这样做时,借阅检查器会抱怨。我似乎也无法解决这个问题。

use std::ops::Deref;

use tokio::sync::watch;

pub struct WatchSomeRef<'a, T>(watch::Ref<'a, Option<T>>);

impl<T> Deref for WatchSomeRef<'_, T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        self.0.as_ref().unwrap()
    }
}

pub async fn recv_some<T>(
    rx: &mut watch::Receiver<Option<T>>,
) -> Result<WatchSomeRef<T>, watch::error::RecvError> {
    loop {
        {
            let guard = rx.borrow_and_update();
            if guard.is_some() {
                return Ok(WatchSomeRef(guard));
            }
        }
        rx.changed().await?;
    }
}

当我这样做时,我得到了错误:

error[E0499]: cannot borrow `*rx` as mutable more than once at a time
  --> src/lib.rs:20:25
   |
16 |     rx: &mut watch::Receiver<Option<T>>,
   |         - let's call the lifetime of this reference `'1`
...
20 |             let guard = rx.borrow_and_update();
   |                         ^^ `*rx` was mutably borrowed here in the previous iteration of the loop
21 |             if guard.is_some() {
22 |                 return Ok(WatchSomeRef(guard));
   |                        ----------------------- returning this value requires that `*rx` is borrowed for `'1`

error[E0499]: cannot borrow `*rx` as mutable more than once at a time
  --> src/lib.rs:25:9
   |
16 |     rx: &mut watch::Receiver<Option<T>>,
   |         - let's call the lifetime of this reference `'1`
...
20 |             let guard = rx.borrow_and_update();
   |                         -- first mutable borrow occurs here
21 |             if guard.is_some() {
22 |                 return Ok(WatchSomeRef(guard));
   |                        ----------------------- returning this value requires that `*rx` is borrowed for `'1`
...
25 |         rx.changed().await?;
   |         ^^ second mutable borrow occurs here

ETA:

我认为这是因为我们想要保护的寿命取决于潜在价值。如果是 一些 ,它需要比函数调用更持久,但如果 None 它需要马上扔掉,这样我们才能等待下一次更改。

不过,我仍然不知道如何解决这个问题(除了使用不安全的mem::transmute来改变生命周期)。

1 回复 | 直到 4 月前

Finomnis 4 月前

这似乎是借用检查器的误报,可能与所讨论的已知局限性密切相关 here .

如果你使用 watch::Receiver::wait_for ,它编译得很好:

use std::ops::Deref;

use tokio::sync::watch;

pub struct WatchSomeRef<'a, T>(watch::Ref<'a, Option<T>>);

impl<T> Deref for WatchSomeRef<'_, T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        self.0.as_ref().unwrap()
    }
}

pub async fn recv_some<T>(
    rx: &mut watch::Receiver<Option<T>>,
) -> Result<WatchSomeRef<T>, watch::error::RecvError> {
    let value = rx.wait_for(|val| val.is_some()).await?;
    Ok(WatchSomeRef(value))
}

或者更紧凑的版本:

pub async fn recv_some<T>(
    rx: &mut watch::Receiver<Option<T>>,
) -> Result<WatchSomeRef<T>, watch::error::RecvError> {
    rx.wait_for(Option::is_some).await.map(WatchSomeRef)
}