How do I gracefully exit TcpListener.incoming()?

Question

From the rust std net library:

let listener = TcpListener::bind(("127.0.0.1", port)).unwrap();

info!("Opened socket on localhost port {}", port);

// accept connections and process them serially
for stream in listener.incoming() {
    break;
}

info!("closed socket");

How does one make the listener stop listening? It says in the API that when the listener is dropped, it stops. But how do we drop it if incoming() is a blocking call? Preferably without external crates like tokio/mio.

Answer 1

You'll want to put the TcpListener into non-blocking mode using the set_nonblocking() method, like so:

use std::io;
use std::net::TcpListener;

let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
listener.set_nonblocking(true).expect("Cannot set non-blocking");

for stream in listener.incoming() {
    match stream {
        Ok(s) => {
            // do something with the TcpStream
            handle_connection(s);
        }
        Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
            // Decide if we should exit
            break;
            // Decide if we should try to accept a connection again
            continue;
        }
        Err(e) => panic!("encountered IO error: {}", e),
    }
}

Instead of waiting for a connection, the incoming() call will immediately return a Result<> type. If Result is Ok(), then a connection was made and you can process it. If the Result is Err(WouldBlock), this isn't actually an error, there just wasn't a connection pending at the exact moment incoming() checked the socket.

Note that in the WouldBlock case, you may want to put a sleep() or something before continuing, otherwise your program will rapidly poll the incoming() function checking for a connection, resulting in high CPU usage.

Code example adapted from here