codec: move into tokio-util (#1675)

Related to #1318, Tokio APIs that are "less stable" are moved into a new
`tokio-util` crate. This crate will mirror `tokio` and provide
additional APIs that may require a greater rate of breaking changes.

As examples require `tokio-util`, they are moved into a separate
crate (`examples`). This has the added advantage of being able to avoid
example only dependencies in the `tokio` crate.

1 files changed, 77 insertions, 0 deletions

diff --git a/examples/echo.rs b/examples/echo.rs
new file mode 100644
index 00000000..455aebde
--- /dev/null
+++ b/examples/echo.rs
@@ -0,0 +1,77 @@
+//! A "hello world" echo server with Tokio
+//!
+//! This server will create a TCP listener, accept connections in a loop, and
+//! write back everything that's read off of each TCP connection.
+//!
+//! Because the Tokio runtime uses a thread pool, each TCP connection is
+//! processed concurrently with all other TCP connections across multiple
+//! threads.
+//!
+//! To see this server in action, you can run this in one terminal:
+//!
+//!     cargo run --example echo
+//!
+//! and in another terminal you can run:
+//!
+//!     cargo run --example connect 127.0.0.1:8080
+//!
+//! Each line you type in to the `connect` terminal should be echo'd back to
+//! you! If you open up multiple terminals running the `connect` example you
+//! should be able to see them all make progress simultaneously.
+
+#![warn(rust_2018_idioms)]
+
+use tokio;
+use tokio::io::{AsyncReadExt, AsyncWriteExt};
+use tokio::net::TcpListener;
+
+use std::env;
+use std::error::Error;
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn Error>> {
+    // Allow passing an address to listen on as the first argument of this
+    // program, but otherwise we'll just set up our TCP listener on
+    // 127.0.0.1:8080 for connections.
+    let addr = env::args().nth(1).unwrap_or("127.0.0.1:8080".to_string());
+
+    // Next up we create a TCP listener which will listen for incoming
+    // connections. This TCP listener is bound to the address we determined
+    // above and must be associated with an event loop.
+    let mut listener = TcpListener::bind(&addr).await?;
+    println!("Listening on: {}", addr);
+
+    loop {
+        // Asynchronously wait for an inbound socket.
+        let (mut socket, _) = listener.accept().await?;
+
+        // And this is where much of the magic of this server happens. We
+        // crucially want all clients to make progress concurrently, rather than
+        // blocking one on completion of another. To achieve this we use the
+        // `tokio::spawn` function to execute the work in the background.
+        //
+        // Essentially here we're executing a new task to run concurrently,
+        // which will allow all of our clients to be processed concurrently.
+
+        tokio::spawn(async move {
+            let mut buf = [0; 1024];
+
+            // In a loop, read data from the socket and write the data back.
+            loop {
+                let n = socket
+                    .read(&mut buf)
+                    .await
+                    .expect("failed to read data from socket");
+
+                if n == 0 {
+                    return;
+                }
+
+                socket
+                    .write_all(&buf[0..n])
+                    .await
+                    .expect("failed to write data to socket");
+            }
+        });
+    }
+}