Visiting blocking handler from Browser blocks whole route

[Geo-W]

• [*] I have looked for existing issues (including closed) about this
• [*] I already read https://ryhl.io/blog/async-what-is-blocking/ and got that blocking code shouldn't appear in async function, but still feel confusing about the phenomena.

Bug Report

Version

0.6.20

Platform

64-bit Windows 11 Home 21H2

Crates

axum = { version = "0.6.20", features = ["macros"]}
tokio = { version = "1.32.0", features = ["full"]}

also tested on

hyper = { version = "1.0.0-rc.4", features = ["full"] }
tokio = { version = "1", features = ["full"] }
http-body-util = "0.1.0-rc.3"
hyper-util = { git = "https://github.com/hyperium/hyper-util.git" }

Description

I tried this code:

use tokio;
use axum::{response::Html, routing::get, Router};
use std::net::SocketAddr;


#[tokio::main(flavor = "multi_thread", worker_threads = 16)]
async fn main() {

    let app = Router::new()
        .route("/blocking/", get(get_block))
        .route("/nonblock/", get(non_block));

    let addr = SocketAddr::from(([0, 0, 0, 0], 2666));
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

async fn get_block() -> Html<&'static str> {
    dbg!(std::thread::current().id());
    let mut i = 1;
	for _ in (0..=599999999){
		i+=1;
	}
    Html("<h1>blocking!</h1>")
}

async fn non_block() -> Html<&'static str> {
    dbg!(std::thread::current().id());
    Html("<h1>Non block!</h1>")
}

I expected to see this happen:

Visiting blocking handler and non-block handler at the same time, the blocking page wait for the calculation to finish, and the non-block handler return immediately.

Instead, this happened:

When visiting http://localhost:2666/blocking/ from the browser, it blocks all of the following requests no matter they are from the browser or command line curl, waiting for blocking to finish then starts handling the following. This problem doesn't exist in actix-web.
But it works well when visiting http://localhost:2666/blocking/ with curl, and all the following requests are handled properly without waiting for the blocking thread to finish.
Test with python requests & threading works, start 1 blocking request and 15 non-block request, output all non-block first then wait for blocking to finish.
Test with rewrk -c 256 -t 12 -d 12 -h http://127.0.0.1:2666/blocking/ also works pretty well.

[davidpdrsn]

Welcome to async rust 😅 that’s just how tokio works and there is nothing axum can do about that. Basically you never wanna block.

See https://ryhl.io/blog/async-what-is-blocking/#:~:text=When%20writing%20async%20Rust%2C%20the,from%20swapping%20the%20current%20task”.

[Geo-W]

Yep i got that.
But what makes me pretty curious is this issue varies for different request agents. It is pretty strange.