fix: hanging server (#806)

Since the update to actix-rt@2.3 (f413996a), the web server hasn't been
started, when we executed `cargo run -p rust-code-analysis-web`.

It froze once `actix_rt::System::new().run()?;` was called,
effectively preventing the execution of the code below.

By looking at the actix-rt examples and changing the code to be more
like the examples, I was able to fix this.
The webserver now behaves as expected.

rust-code-analysis-web/src/main.rs

@@ -9,7 +9,8 @@ use clap::{crate_version, App, Arg};
 
 use web::server;
 
-fn main() {
+#[actix_web::main]
+async fn main() {
     let matches = App::new("rust-code-analysis-web")
         .version(crate_version!())
         .author(&*env!("CARGO_PKG_AUTHORS").replace(':', "\n"))
@@ -51,7 +52,7 @@ fn main() {
         eprintln!("Invalid port number");
         return;
     };
-    if let Err(e) = server::run(host.to_string(), port, num_jobs) {
+    if let Err(e) = server::run(host.to_string(), port, num_jobs).await {
         eprintln!("Cannot run the server at {}:{}: {}", host, port, e);
     }
 }

rust-code-analysis-web/src/web/server.rs

@@ -1,4 +1,3 @@
-use actix_rt::Runtime;
 use actix_web::{
     dev::Body,
     guard, http,
@@ -220,12 +219,9 @@ fn ping() -> HttpResponse {
     HttpResponse::Ok().body(Body::Empty)
 }
 
-pub fn run(host: String, port: u16, n_threads: usize) -> std::io::Result<()> {
-    actix_rt::System::new().run()?;
-    let rt = Runtime::new()?;
+pub async fn run(host: String, port: u16, n_threads: usize) -> std::io::Result<()> {
     let max_size = 1024 * 1024 * 4;
 
-    rt.block_on(async move {
     HttpServer::new(move || {
         App::new()
             .service(
@@ -284,7 +280,6 @@ pub fn run(host: String, port: u16, n_threads: usize) -> std::io::Result<()> {
     .bind((host.as_str(), port))?
     .run()
     .await
-    })
 }
 
 // curl --header "Content-Type: application/json" --request POST --data '{"id": "1234", "file_name": "prova.cpp", "code": "int x = 1;", "comment": true, "span": true}' http://127.0.0.1:8081/ast