I'm using the async_trait
crate to define an async
method on a trait. I intend to transform the Future
it returns into a Stream
, wrap the Stream
in an Arc
and send the Arc
over to multiple threads. Minimal reproducible code is as follows:
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()`
});
}
However it compiled with the following error:
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`
It seems that async_trait
desugars the async methods' return types as Pin<Box<dyn Future<...> + Send>>
, without specifying Sync
. However I thought it would also be fairly common to require Future
s to be Sync
, in addition to Send
.
My questions are:
- How should I specify the async method's return type to be
Sync
, and
- Why does
async_trait
not specify Sync
for the return types automatically?
question from:
https://stackoverflow.com/questions/65916163/how-to-restrict-async-trait-functions-return-value-to-implement-sync 与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…