[英]How to restrict async_trait function's return value to implement Sync?
我正在使用async_trait
板条箱在 trait 上定义async
方法。 我打算将它返回的Future
转换为Stream
,将Stream
包装在Arc
中并将Arc
发送到多个线程。 最小可重现代码如下:
use async_trait::async_trait;
use futures::stream::{unfold, Stream};
use std::sync::Arc;
use tokio::runtime::Runtime;
#[async_trait]
trait Trait: Send + Sync {
async fn do_something(&self) -> i32;
}
async fn use_trait<T: Trait>(x: &T) {
let boxed: Arc<Box<dyn Stream<Item = i32> + Send + Sync>>;
let lazy_poller = unfold(None, move |state| async move {
if let Some(value) = state {
Some((value, Some(value)))
} else {
let value = x.do_something().await;
Some((value, Some(value)))
}
});
boxed = Arc::new(Box::new(lazy_poller));
let boxed_clone = boxed.clone();
let rt = Runtime::new().unwrap();
rt.block_on(async {
let _moved = boxed_clone;
// Do something with `_moved.next()`
});
}
但是它编译时出现以下错误:
error: future cannot be shared between threads safely
--> src/main.rs:21:22
|
21 | boxed = Arc::new(Box::new(lazy_poller));
| ^^^^^^^^^^^^^^^^^^^^^ future created by async block is not `Sync`
|
= help: the trait `Sync` is not implemented for `dyn futures::Future<Output = i32> + std::marker::Send`
note: future is not `Sync` as it awaits another future which is not `Sync`
--> src/main.rs:17:25
|
17 | let value = x.do_something().await;
| ^^^^^^^^^^^^^^^^ await occurs here on type `Pin<Box<dyn futures::Future<Output = i32> + std::marker::Send>>`, which is not `Sync`
= note: required for the cast to the object type `dyn Stream<Item = i32> + Sync + std::marker::Send`
似乎async_trait
将异步方法的返回类型脱糖为Pin<Box<dyn Future<...> + Send>>
,而没有指定Sync
。 但是,我认为除了Send
之外,要求Future
s 是Sync
也是相当普遍的。
我的问题是:
Sync
,以及async_trait
不自动为返回类型指定Sync
? 正如评论中已经提到的,对于大多数用例, Future
不需要是Sync
; 当可以从不同的任务中轮询它时, FutureExt::shared
是通往 go 的方式,这正是我的用例所需要的。 我不必将其转换为Stream
。
至于我的例子,它将是:
use async_trait::async_trait;
use futures::future::FutureExt;
use tokio::runtime::Runtime;
#[async_trait]
trait Trait: Send + Sync {
async fn do_something(&self) -> i32;
}
async fn use_trait<T: Trait>(x: &T) {
let shared = x.do_something().shared();
let shared_clone = shared.clone();
let rt = Runtime::new().unwrap();
rt.block_on(async {
let _moved = shared_clone;
// Do something with `_moved.await`
});
println!("{}", shared.await)
}
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