Rust actix-web thread unsafe movement

Question

I'm trying to write an HTTP endpoint using actix-web 1.0. I've reduced the function so that it's just returning the user that is passed to it, but the compiler still gives an error.

extern crate actix_web;
extern crate chrono;
extern crate futures;
extern crate listenfd;
#[macro_use]
extern crate serde_derive;
extern crate dotenv;
use actix_web::{error, web, App, Error, HttpResponse, HttpServer};
use futures::future::Future;

#[derive(Debug, Deserialize, Serialize)]
pub struct LoginUser {
    pub username: String,
    pub password: String,
}

pub fn login(
    login_user: web::Json<LoginUser>,
) -> impl Future<Item = HttpResponse, Error = error::BlockingError<Error>> {
    web::block(move || {
        let login_user = login_user.into_inner();
        let user = LoginUser {
            username: login_user.username,
            password: login_user.password,
        };
        Ok(HttpResponse::Ok().json(user))
    })
}

pub fn router(cfg: &mut web::ServiceConfig) {
    cfg.service(web::scope("/").service(web::resource("").route(web::get().to(login))));
}

fn main() -> std::io::Result<()> {
    HttpServer::new(move || App::new().configure(router))
        .bind("127.0.0.1:3000")?
        .run()
}

Here is my cargo.toml.

[package]
name = "log"
version = "0.1.0"
authors = ["me@example.com"
edition = "2018"

[dependencies]
actix-files = "~0.1"
actix-web = "~1.0"
chrono = { version = "0.4.6", features = ["serde"] }
listenfd = "0.3"
diesel = {version = "1.4.1", features = ["postgres", "uuid", "r2d2", "chrono"]}
dotenv = "0.13"
failure = "0.1"
futures = "0.1"
scrypt = "0.2.0"
serde_derive="1.0"
serde_json="1.0"
serde="1.0"

I'm getting the compilation error

|     web::block(move || {
|     ^^^^^^^^^^ `(dyn std::any::Any + 'static)` cannot be sent between threads safely

I think it has something to do with the use of login_user in the web::block, but it's hard to tell from the error. What is the preferred way to use request parameters asynchronously in Rust or actix safely?

Answer 1

Well, first, HttpResponse does not implement Send. Since web::block() runs the closure on a threadpool, that is a problem. So you need to return a value that is Send from web::block, and then create a HttpResponse from that - using and_then() for example.

Second, in your router you are using web::get().to(login). If you want to call a function that returns a Future, that needs to be web::get().to_async(login).

Third, the closure in web::block needs to return Result. Since you never return an error value, the compiler cannot infer the error type. You need to give the compiler a hint. Usually std::io::Error will do,so return Ok::<_, std::io::Error>(...value...).

Fourth, web::block returns a BlockingError<E>. You can use from_err() to map that to something that you can return.

So, with all that, the relevant part of your code would look like:

pub fn login(
    login_user: web::Json<LoginUser>,
) -> impl Future<Item = HttpResponse, Error = Error> {
    web::block(move || {
        let login_user = login_user.into_inner();
        let user = LoginUser {
            username: login_user.username,
            password: login_user.password,
        };
        Ok::<_, std::io::Error>(user)
    })
        .from_err()
        .and_then(|user| HttpResponse::Ok().json(user))
}

pub fn router(cfg: &mut web::ServiceConfig) {
    cfg.service(web::scope("/").service(web::resource("").route(web::get().to_async(login))));
}