Is it possible to use LocalSet from within a spawned task?

[Alovchin91]

Version

tokio-macros v0.2.3
tokio v0.2.9

Platform

MacOS 10.15.2:

Darwin 19.2.0
Darwin Kernel Version 19.2.0: Sat Nov 9 03:47:04 PST 2019; root:xnu-6153.61.1~20/RELEASE_X86_64 x86_64

Description

I can't find a way to use LocalSet inside a task spawned by tokio::spawn inside a #[tokio::main] main fn.

I tried this code:

#[tokio::main]
async fn main() {
    tokio::spawn(async move {
        let local = tokio::task::LocalSet::new();
        local.run_until(async {}).await;
    }).await.unwrap()
}

In this case, the compiler complains that:

rror[E0277]: `std::rc::Rc<tokio::task::local::Scheduler>` cannot be sent between threads safely
   --> src/main.rs:3:5
    |
3   |     tokio::spawn(async move {
    |     ^^^^^^^^^^^^ `std::rc::Rc<tokio::task::local::Scheduler>` cannot be sent between threads safely
    | 
   ::: /Users/me/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.9/src/task/spawn.rs:123:21
    |
123 |         T: Future + Send + 'static,
    |                     ---- required by this bound in `tokio::task::spawn::spawn`
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<tokio::task::local::Scheduler>`
    = note: required because it appears within the type `tokio::task::local::LocalSet`
    = note: required because it appears within the type `{tokio::task::local::LocalSet, tokio::task::local::LocalSet, tokio::task::local::LocalSet, ()}`
    = note: required because it appears within the type `[static generator@src/main.rs:3:29: 6:6 {tokio::task::local::LocalSet, tokio::task::local::LocalSet, tokio::task::local::LocalSet, ()}]`
    = note: required because it appears within the type `std::future::GenFuture<[static generator@src/main.rs:3:29: 6:6 {tokio::task::local::LocalSet, tokio::task::local::LocalSet, tokio::task::local::LocalSet, ()}]>`
    = note: required because it appears within the type `impl std::future::Future`

From my understanding, it tries to capture a Scheduler from the outer context (the main function). (And Rc is obviously !Send.)

The only other way to run LocalSet that I know of is to call LocalSet::block_on, but it requires a Runtime which I don't think is possible to get from inside a context of #[tokio::main], and also creating a new Runtime is not possible because I'm already in a context of a runtime.

Also, if I try to remove await and just pass the Future returned by LocalSet::run_until to tokio::spawn like this:

tokio::spawn({
    let local = tokio::task::LocalSet::new();
    local.run_until(async {})
}).await.unwrap()

then the compiler panics: rust-lang/rust#68109

What I'm trying to achieve is to run Actix inside Tokio runtime in a separate thread so I could send it a shutdown signal at some point.

Do you think there is a way to run LocalSet inside Tokio-spawned task? Or am I getting the whole idea of LocalSet wrong? 😬

Thanks!

[carllerche]

You cannot use regular tokio::spawn as spawned futures migrate across threads thus require Send. You can do something like:

#[tokio::main]
async fn main() {
    tokio::spawn_blocking(move {
        let rt = tokio::runtime::Handle::current();
        rt.block_on(async {
            let local = tokio::task::LocalSet::new();
            local.run_until(async {}).await;
        });
    }).await.unwrap()
}

[Alovchin91]

Thank you for the explanation, @carllerche!

Unfortunately, latest version of tokio::runtime::Handle doesn't have the block_on method: https://docs.rs/tokio/0.2.9/tokio/runtime/struct.Handle.html

How can I do the same with the current API? Thanks!

[carllerche]

Oh sorry... I thought we were going to add it. Somebody still needs to write that PR :)

You can do this instead:

let rt = runtime::Handle::current();
rt.enter(|| {
    futures::executor::block_on(...)
})

[Alovchin91]

Thank you! Does exactly what I need 🙂 Hopefully there'll be a corresponding PR for block_on 🙂