docs(example): add comments to the examples/hello.rs example (#3150)

Author: technetos

examples/hello.rs

@@ -10,6 +10,8 @@ use hyper::service::service_fn;
 use hyper::{Request, Response};
 use tokio::net::TcpListener;
 
+// An async function that consumes a request, does nothing with it and returns a
+// response.
 async fn hello(_: Request<hyper::body::Incoming>) -> Result<Response<Full<Bytes>>, Infallible> {
     Ok(Response::new(Full::new(Bytes::from("Hello World!"))))
 }
@@ -18,14 +20,29 @@ async fn hello(_: Request<hyper::body::Incoming>) -> Result<Response<Full<Bytes>
 pub async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     pretty_env_logger::init();
 
+    // This address is localhost
     let addr: SocketAddr = ([127, 0, 0, 1], 3000).into();
 
+    // Bind to the port and listen for incoming TCP connections
     let listener = TcpListener::bind(addr).await?;
     println!("Listening on http://{}", addr);
     loop {
+        // When an incoming TCP connection is received grab a TCP stream for
+        // client<->server communication.
+        //
+        // Note, this is a .await point, this loop will loop forever but is not a busy loop. The
+        // .await point allows the Tokio runtime to pull the task off of the thread until the task
+        // has work to do. In this case, a connection arrives on the port we are listening on and
+        // the task is woken up, at which point the task is then put back on a thread, and is
+        // driven forward by the runtime, eventually yielding a TCP stream.
         let (stream, _) = listener.accept().await?;
 
+        // Spin up a new task in Tokio so we can continue to listen for new TCP connection on the
+        // current task without waiting for the processing of the HTTP1 connection we just received
+        // to finish
         tokio::task::spawn(async move {
+            // Handle the connection from the client using HTTP1 and pass any
+            // HTTP requests received on that connection to the `hello` function
             if let Err(err) = http1::Builder::new()
                 .serve_connection(stream, service_fn(hello))
                 .await