Pulling HTTP data requires sleep -- code snippet from Programming Rust 2nd Edition

[db666]

Hi folks, I'm currently reading "Programming Rust 2nd edition" and trying to run the blocking HTTP client code snippet. But I'm unable to read data from the server without adding a "sleep" call after writing my data. I know I should be using something like reqwest but trying to figure out what I'm doing wrong here.

fn main() {
    let host = "worldtimeapi.org";
    let path = "/api/timezone/America/Argentina/Salta";
    let response = send_request(host, 80, path);
    println!("Output is `{}`", host, response.unwrap());
}

fn send_request(host: &str, port: u16, path: &str) -> io::Result<String> {
    let mut socket = TcpStream::connect((host, port))?;
    let request = format!("GET {} HTTP/1.1\r\nHost: {}\r\n\r\n", path, host);

    socket.write_all(request.as_bytes())?;
    socket.set_nodelay(true)?;
    socket.flush()?;
    
    // Why do I need sleep here? Removing this shows me the following output on STDOUT
    // Output is ``
    thread::sleep(Duration::from_secs(1));

    socket.shutdown(Shutdown::Write)?;
    let mut response = String::new();
    socket.read_to_string(&mut response)?;

    Ok(response)
}

[Hyeonu]

It's nagle's algorithm. Since the .set_nodelay(true) applied after the .write_all(...), this write is not sent instantly. With thread::sleep(1s) it will be sent anyway, but without it you're shutting down the TCP write half before the send buffer is actually send.

If you move the socket.set_nodelay(true) one line above, it works.

Side note: socket.flush() is no-op. It can only flush things managed by userland like BufWriter<T>. But the in-kernel TCP buffer is not flushed with it.

Fulll code that works:

use std::io::{self, Read, Write};
use std::net::{TcpStream, Shutdown};

fn main() {
    let host = "worldtimeapi.org";
    let path = "/api/timezone/America/Argentina/Salta";
    let response = send_request(host, 80, path);
    println!("{} - Output is `{}`", host, response.unwrap());
}

fn send_request(host: &str, port: u16, path: &str) -> io::Result<String> {
    let mut socket = TcpStream::connect((host, port))?;
    let request = format!("GET {} HTTP/1.1\r\nHost: {}\r\n\r\n", path, host);

    socket.set_nodelay(true)?;
    socket.write_all(request.as_bytes())?;

    socket.shutdown(Shutdown::Write)?;
    let mut response = String::new();
    socket.read_to_string(&mut response)?;

    Ok(response)
}

[db666]

Thanks @Hyeonu -- it's interesting to note that moving the set_nodelay call by copy-pasting your code still doesn't work for me. Would you know why? I'm using the following rustc and cargo versions on my M1 Macbook Air (in case it makes a difference)

cargo 1.63.0 (fd9c4297c 2022-07-01)
rustc 1.63.0 (4b91a6ea7 2022-08-08)

[db666]

Hi folks, any help on this please? It seems strange that a simple HTTP client snippet doesn't work without a sleep even after enabling nodelay. This is my latest snippet

STDOUT
worldtimeapi.org - Output is ``

use std::io::{Read, Write};
use std::net::{Shutdown, TcpStream};
use std::{io};

fn main() {
    let host = "worldtimeapi.org";
    let path = "/api/timezone/America/Argentina/Salta";
    let response = send_request(host, 80, path);
    println!("{} - Output is `{}`", host, response.unwrap());
}

fn send_request(host: &str, port: u16, path: &str) -> io::Result<String> {
    let mut socket = TcpStream::connect((host, port))?;
    let request = format!("GET {} HTTP/1.1\r\nHost: {}\r\n\r\n", path, host);

    socket.set_nodelay(true)?;
    socket.write_all(request.as_bytes())?;
    socket.shutdown(Shutdown::Write)?;

    let mut response = String::new();
    socket.read_to_string(&mut response)?;

    Ok(response)
}