Add example for multiple listen addrs (#1179)

examples/listen-multiple-addrs/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "listen-multiple-addrs"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+axum = { path = "../../axum" }
+hyper = { version = "0.14", features = ["full"] }
+tokio = { version = "1", features = ["full"] }

examples/listen-multiple-addrs/src/main.rs

@@ -0,0 +1,60 @@
+//! Showcases how listening on multiple addrs is possible by
+//! implementing Accept for a custom struct.
+//!
+//! This may be useful in cases where the platform does not
+//! listen on both IPv4 and IPv6 when the IPv6 catch-all listener is used (`::`),
+//! [like older versions of Windows.](https://docs.microsoft.com/en-us/windows/win32/winsock/dual-stack-sockets)
+
+use axum::{routing::get, Router};
+use hyper::server::{accept::Accept, conn::AddrIncoming};
+use std::{
+    net::{Ipv4Addr, Ipv6Addr, SocketAddr},
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+#[tokio::main]
+async fn main() {
+    let app = Router::new().route("/", get(|| async { "Hello, World!" }));
+
+    let localhost_v4 = SocketAddr::new(Ipv4Addr::LOCALHOST.into(), 8080);
+    let incoming_v4 = AddrIncoming::bind(&localhost_v4).unwrap();
+
+    let localhost_v6 = SocketAddr::new(Ipv6Addr::LOCALHOST.into(), 8080);
+    let incoming_v6 = AddrIncoming::bind(&localhost_v6).unwrap();
+
+    let combined = CombinedIncoming {
+        a: incoming_v4,
+        b: incoming_v6,
+    };
+
+    axum::Server::builder(combined)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+struct CombinedIncoming {
+    a: AddrIncoming,
+    b: AddrIncoming,
+}
+
+impl Accept for CombinedIncoming {
+    type Conn = <AddrIncoming as Accept>::Conn;
+    type Error = <AddrIncoming as Accept>::Error;
+
+    fn poll_accept(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<Self::Conn, Self::Error>>> {
+        if let Poll::Ready(Some(value)) = Pin::new(&mut self.a).poll_accept(cx) {
+            return Poll::Ready(Some(value));
+        }
+
+        if let Poll::Ready(Some(value)) = Pin::new(&mut self.b).poll_accept(cx) {
+            return Poll::Ready(Some(value));
+        }
+
+        Poll::Pending
+    }
+}