docs(example): add an example for server using auto conn (#110)

Cargo.toml

@@ -76,3 +76,6 @@ __internal_happy_eyeballs_tests = []
 name = "client"
 required-features = ["client-legacy", "http1", "tokio"]
 
+[[example]]
+name = "server"
+required-features = ["server", "http1", "tokio"]

examples/server.rs

@@ -0,0 +1,75 @@
+//! This example runs a server that responds to any request with "Hello, world!"
+
+use std::{convert::Infallible, error::Error};
+
+use bytes::Bytes;
+use http::{header::CONTENT_TYPE, Request, Response};
+use http_body_util::{combinators::BoxBody, BodyExt, Full};
+use hyper::{body::Incoming, service::service_fn};
+use hyper_util::{
+    rt::{TokioExecutor, TokioIo},
+    server::conn::auto::Builder,
+};
+use tokio::{net::TcpListener, task::JoinSet};
+
+/// Function from an incoming request to an outgoing response
+///
+/// This function gets turned into a [`hyper::service::Service`] later via
+/// [`service_fn`]. Instead of doing this, you could also write a type that
+/// implements [`hyper::service::Service`] directly and pass that in place of
+/// writing a function like this and calling [`service_fn`].
+///
+/// This function could use [`Full`] as the body type directly since that's
+/// the only type that can be returned in this case, but this uses [`BoxBody`]
+/// anyway for demonstration purposes, since this is what's usually used when
+/// writing a more complex webserver library.
+async fn handle_request(
+    _request: Request<Incoming>,
+) -> Result<Response<BoxBody<Bytes, Infallible>>, Infallible> {
+    let response = Response::builder()
+        .header(CONTENT_TYPE, "text/plain")
+        .body(Full::new(Bytes::from("Hello, world!\n")).boxed())
+        .expect("values provided to the builder should be valid");
+
+    Ok(response)
+}
+
+#[tokio::main(flavor = "current_thread")]
+async fn main() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
+    let listen_addr = "127.0.0.1:8000";
+    let tcp_listener = TcpListener::bind(listen_addr).await?;
+    println!("listening on http://{listen_addr}");
+
+    let mut join_set = JoinSet::new();
+    loop {
+        let (stream, addr) = match tcp_listener.accept().await {
+            Ok(x) => x,
+            Err(e) => {
+                eprintln!("failed to accept connection: {e}");
+                continue;
+            }
+        };
+
+        let serve_connection = async move {
+            println!("handling a request from {addr}");
+
+            let result = Builder::new(TokioExecutor::new())
+                .serve_connection(TokioIo::new(stream), service_fn(handle_request))
+                .await;
+
+            if let Err(e) = result {
+                eprintln!("error serving {addr}: {e}");
+            }
+
+            println!("handled a request from {addr}");
+        };
+
+        join_set.spawn(serve_connection);
+    }
+
+    // If you add a method for breaking the above loop (i.e. graceful shutdown),
+    // then you may also want to wait for all existing connections to finish
+    // being served before terminating the program, which can be done like this:
+    //
+    // while let Some(_) = join_set.join_next().await {}
+}