Set SO_REUSEADDR only non-Windows platforms (#3473)

* Fix: Per discussion in #2958, set `SO_REUSEADDR` only non-Windows platforms.

Add: Tests confirming that only a single webserver instance may
bind to a given address.

* Clean: Lint.

* Clean: another guess at making the formatter happy.

* Clean: More cargo fmt fun. (Running cargo fmt locally doesn't help.)

---------

Co-authored-by: Bryan A. Jones <bjones1@users.noreply.github.com>
Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-web/CHANGES.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Minimum supported Rust version (MSRV) is now 1.75.
+- On Windows platforms, produce an error when invoking `HttpServer::bind` on a socket that's already in use. See [issue 2958](https://github.com/actix/actix-web/issues/2958).
 
 ## 4.9.0
 

actix-web/src/server.rs

@@ -1085,7 +1085,10 @@ fn create_tcp_listener(addr: net::SocketAddr, backlog: u32) -> io::Result<net::T
     use socket2::{Domain, Protocol, Socket, Type};
     let domain = Domain::for_address(addr);
     let socket = Socket::new(domain, Type::STREAM, Some(Protocol::TCP))?;
-    socket.set_reuse_address(true)?;
+    #[cfg(not(windows))]
+    {
+        socket.set_reuse_address(true)?;
+    }
     socket.bind(&addr.into())?;
     // clamp backlog to max u32 that fits in i32 range
     let backlog = cmp::min(backlog, i32::MAX as u32) as i32;

actix-web/tests/test_httpserver.rs

@@ -1,13 +1,10 @@
 #[cfg(feature = "openssl")]
 extern crate tls_openssl as openssl;
 
-#[cfg(any(unix, feature = "openssl"))]
-use {
-    actix_web::{web, App, HttpResponse, HttpServer},
-    std::{sync::mpsc, thread, time::Duration},
-};
+use std::{sync::mpsc, thread, time::Duration};
+
+use actix_web::{web, App, HttpResponse, HttpServer};
 
-#[cfg(unix)]
 #[actix_rt::test]
 async fn test_start() {
     let addr = actix_test::unused_addr();
@@ -53,6 +50,27 @@ async fn test_start() {
     let response = client.get(host.clone()).send().await.unwrap();
     assert!(response.status().is_success());
 
+    // Attempt to start a second server using the same address.
+    let result = HttpServer::new(|| {
+        App::new().service(
+            web::resource("/").route(web::to(|| async { HttpResponse::Ok().body("test") })),
+        )
+    })
+    .workers(1)
+    .backlog(1)
+    .max_connections(10)
+    .max_connection_rate(10)
+    .keep_alive(Duration::from_secs(10))
+    .client_request_timeout(Duration::from_secs(5))
+    .client_disconnect_timeout(Duration::ZERO)
+    .server_hostname("localhost")
+    .system_exit()
+    .disable_signals()
+    .bind(format!("{}", addr));
+
+    // This should fail: the address is in use.
+    assert!(result.is_err());
+
     srv.stop(false).await;
 }