No new connections after max_connections reached

[EtienneBruines]

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Expected Behavior

After handling a request, the server should be able to handle another request.

Current Behavior

With max_connections set to 1, the server will handle one single request, and then no more.

(Note that this is with workers(1), backlog(0) and keep_alive(None))

Possible Solution

Steps to Reproduce (for bugs)

1. Use the example code below
2. Make one HTTP request using curl --no-keepalive -v http://localhost:8082/ (will return a 404, which is fine)
3. Make a second HTTP request (this is where it fails)

➜  ~ curl http://localhost:8082/ -v --no-keepalive
*   Trying 127.0.0.1:8082...
* Connected to localhost (127.0.0.1) port 8082 (#0)
> GET / HTTP/1.1
> Host: localhost:8082
> User-Agent: curl/7.74.0
> Accept: */*
> 

5. Make a third HTTP request (will also fail, but slightly different look)

➜  ~ curl http://localhost:8082/ -v --no-keepalive
*   Trying 127.0.0.1:8082...
* connect to 127.0.0.1 port 8082 failed: Connection timed out
* Failed to connect to localhost port 8082: Connection timed out
* Closing connection 0
curl: (28) Failed to connect to localhost port 8082: Connection timed out

Example code

use actix_web::{get, web, App, HttpServer};
use tracing::Level;

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    let _collector = tracing_subscriber::fmt()
        .with_max_level(Level::TRACE)
        .init();

    HttpServer::new(|| App::new())
        .workers(1)
        .max_connections(1)
        .backlog(0)
        .bind("127.0.0.1:8082")?
        .shutdown_timeout(5)
        .keep_alive(None)
        .run()
        .await
}

[package]
name = "my-project"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
actix-web = "4"
tokio = { version = "1.17.0" , features = ["time"] }
tracing = "0.1.32"
tracing-subscriber = "0.3.9"

Context

Your Environment

rustc 1.61.0-nightly (68369a041 2022-02-22)

Logs

2022-03-19T18:58:20.376299Z  INFO actix_server::builder: Starting 1 workers
2022-03-19T18:58:20.376378Z  INFO actix_server::server: Actix runtime found; starting in Actix runtime
2022-03-19T18:58:20.376430Z TRACE actix_server::worker: starting server worker 0
2022-03-19T18:58:20.376675Z TRACE mio::poll: registering event source with poller: token=Token(0), interests=READABLE | WRITABLE    
2022-03-19T18:58:20.377098Z TRACE mio::poll: registering event source with poller: token=Token(0), interests=READABLE    
2022-03-19T18:58:20.377112Z TRACE actix_server::worker: Service "actix-web-service-127.0.0.1:8082" is available
2022-03-19T18:58:20.377191Z TRACE actix_server::signals: setting up OS signal listener
2022-03-19T18:58:23.849938Z TRACE mio::poll: registering event source with poller: token=Token(1), interests=READABLE | WRITABLE    
2022-03-19T18:58:23.850138Z TRACE actix_http::h1::dispatcher: start flags: (empty)    
2022-03-19T18:58:23.850175Z TRACE actix_http::h1::dispatcher: start timers:    
2022-03-19T18:58:23.850197Z TRACE actix_http::h1::dispatcher:   head timer is inactive    
2022-03-19T18:58:23.850219Z TRACE actix_http::h1::dispatcher:   shutdown timer is inactive    
2022-03-19T18:58:23.850312Z TRACE actix_http::h1::dispatcher: end timers:    
2022-03-19T18:58:23.850341Z TRACE actix_http::h1::dispatcher:   head timer is active and due to expire in 4526.723 milliseconds    
2022-03-19T18:58:23.850367Z TRACE actix_http::h1::dispatcher:   shutdown timer is inactive    
2022-03-19T18:58:23.850389Z TRACE actix_http::h1::dispatcher: end flags: STARTED    
2022-03-19T18:58:23.850502Z TRACE actix_http::h1::dispatcher: start flags: STARTED    
2022-03-19T18:58:23.850533Z TRACE actix_http::h1::dispatcher: start timers:    
2022-03-19T18:58:23.850555Z TRACE actix_http::h1::dispatcher:   head timer is active and due to expire in 4526.51 milliseconds    
2022-03-19T18:58:23.850579Z TRACE actix_http::h1::dispatcher:   shutdown timer is inactive    
2022-03-19T18:58:23.851110Z TRACE actix_http::h1::dispatcher: start flags: STARTED | SHUTDOWN    
2022-03-19T18:58:23.851151Z TRACE actix_http::h1::dispatcher: start timers:    
2022-03-19T18:58:23.851174Z TRACE actix_http::h1::dispatcher:   head timer is inactive    
2022-03-19T18:58:23.851196Z TRACE actix_http::h1::dispatcher:   shutdown timer is inactive    
2022-03-19T18:58:23.851265Z TRACE actix_http::h1::dispatcher: end flags: STARTED | SHUTDOWN    
2022-03-19T18:58:23.851304Z TRACE mio::poll: deregistering event source from poller    

[fakeshadow]

https://github.com/actix/actix-net/blob/855e3f96fe250d1cbba6acd49bdd6a4519c810b8/actix-server/src/worker.rs#L103
It should have been fetch_sub - 1