Can I safely use tokio's task_local on a per-request basis?

[edgarogh]

Summary

Hello, I'm wondering if axum handles requests within tokio tasks or not? If yes, is it safe to rely on this property to have a task_local value set by a middleware? (and only accessible for the current request)

If that's not the case, I know how to implement my own custom middleware future that sets and unsets a thread-local value respectively before and after polling. I simply prefer to use existing framework features, if any.

I am 100% aware that using thread_local/task_local stuff is very unidiomatic and could a big code smell, but in my specific case, I'm pretty sure that my current situation is way worse in terms of simplicity and maintainability.

axum version

0.6.18

[jplatte]

Yes, you can use a task local to pass information about the request to inner middlewares, handlers and IntoResponse impls. Though for all but the last one, you can also use request extensions.

If is however also possible for inner middleware to do parts of its work (including calling the inner service) on different tasks though, making task locals unavailable. This is rare and something I would expect the middleware's docs to point out, though.

[edgarogh]

That's perfect, thank you! Extensions are unfortunately out of the question here, that's why I want to use global state (despite its disadvantages).

Is it mentioned in the docs somewhere, that task_local! is safe to use in handlers/middlewares/IntoResponse? I think it can be worth documenting, not without mentioning that other means of passing information around are still much more preferable. I'd happily submit a PR (I'm not sure on which doc(s) item, though).

[jplatte]

It's not currently mentioned in the docs. A PR would be appreciated, I think the "Passing state from middleware to handlers" section would be a good place. And I think we should have an example like

axum::middleware::from_fn(|..., next| async move {
    task_local! { ... };
    // Make MY_TASK_LOCAL available to inner middleware,
    // handlers, `FromRequest` and `IntoResponse` impls
    MY_TASK_LOCAL.scope(async move {
        next.run().await
    })
})