[1.21] (valence_protocol) LoginHelloC2s fails to decode valid packets

[bcheidemann]

Valence Version

2e4bc6e

What You Did

1. Establish TCP listener on default MC port.
2. Accept connection from 1.21.4 vanilla client
3. Read data from TCP stream into buffer
4. Queue slice into PacketDecoder
5. Get next frame with try_next_packet
6. Try decode LoginHelloC2s

Playground

use std::time::Instant;
use tokio::net::{TcpListener, TcpStream};
use tracing::{error, info, warn};
use tracing_subscriber;
use valence_protocol::PacketDecoder;
use valence_protocol::packets::login::LoginHelloC2s;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    tracing_subscriber::fmt::init();

    let listener = TcpListener::bind("0.0.0.0:25565").await?;
    info!("Listening on port 25565");

    loop {
        match listener.accept().await {
            Ok((socket, addr)) => {
                info!("New connection from {}", addr);
                tokio::spawn(handle_connection(socket, addr.to_string()));
            }
            Err(e) => {
                error!("Failed to accept connection: {}", e);
            }
        }
    }
}

async fn handle_connection(socket: TcpStream, addr: String) {
    let start_time = Instant::now();
    info!("Connection established with {} at {:?}", addr, start_time);

    loop {
        socket.readable().await.unwrap_or_else(|e| {
            warn!("Error waiting for socket readability: {}", e);
        });

        let mut decoder = PacketDecoder::new();
        let mut buf = [0; 1024];
        match socket.try_read(&mut buf) {
            Ok(0) => {
                let duration = start_time.elapsed();
                info!(
                    "Connection from {} closed after {:?} ({:.3} seconds)",
                    addr,
                    duration,
                    duration.as_secs_f64()
                );
                break;
            }
            Ok(n) => {
                info!("Received {} bytes from {}", n, addr);

                decoder.queue_slice(&buf[0..n]);

                while let Ok(Some(frame)) = decoder.try_next_packet() {
                    match frame.decode::<LoginHelloC2s>() {
                        Ok(login_packet) => {
                            info!("Bingo! Received login from: {}", login_packet.username)
                        }
                        Err(err) => info!("Not LoginHelloC2s: {err}"),
                    }
                }
            }
            Err(ref e) if e.kind() == std::io::ErrorKind::WouldBlock => {
                continue;
            }
            Err(e) => {
                let duration = start_time.elapsed();
                warn!(
                    "Connection from {} error after {:?} ({:.3} seconds): {}",
                    addr,
                    duration,
                    duration.as_secs_f64(),
                    e
                );
                break;
            }
        }
    }
}

What Went Wrong

Expected Result

2025-08-28T14:08:46.987550Z  INFO mcprox: Listening on port 25565
2025-08-28T14:08:49.420192Z  INFO mcprox: New connection from 127.0.0.1:51014
2025-08-28T14:08:49.420360Z  INFO mcprox: Connection established with 127.0.0.1:51014 at Instant { tv_sec: 6612, tv_nsec: 740116261 }
2025-08-28T14:08:49.420896Z  INFO mcprox: Received 77 bytes from 127.0.0.1:51014
2025-08-28T14:08:49.420933Z  INFO mcprox: Not LoginHelloC2s: failed to decode field `username` in `LoginHelloC2s`
2025-08-28T14:08:49.421136Z  INFO mcprox: Received 27 bytes from 127.0.0.1:51014
2025-08-28T14:08:49.421155Z  INFO mcprox: Bingo! Received login from: rohedin4
2025-08-28T14:08:52.680779Z  INFO mcprox: Connection from 127.0.0.1:51014 closed after 3.260432602s (3.260 seconds)

Actual Result

2025-08-28T14:08:23.785379Z  INFO mcprox: Listening on port 25565
2025-08-28T14:08:26.259271Z  INFO mcprox: New connection from 127.0.0.1:60720
2025-08-28T14:08:26.259399Z  INFO mcprox: Connection established with 127.0.0.1:60720 at Instant { tv_sec: 6589, tv_nsec: 579175778 }
2025-08-28T14:08:26.259655Z  INFO mcprox: Received 77 bytes from 127.0.0.1:60720
2025-08-28T14:08:26.259709Z  INFO mcprox: Not LoginHelloC2s: failed to decode field `username` in `LoginHelloC2s`
2025-08-28T14:08:26.259732Z  INFO mcprox: Received 27 bytes from 127.0.0.1:60720
2025-08-28T14:08:26.259747Z  INFO mcprox: Not LoginHelloC2s: failed to decode field `profile_id` in `LoginHelloC2s`
2025-08-28T14:08:31.326701Z  INFO mcprox: Connection from 127.0.0.1:60720 closed after 5.067285718s (5.067 seconds)

Additional Information

The Minecraft Wiki indicates that the profile ID is not optional. However, the LoginHelloC2s struct is implemented with an Option<Uuid>:

#[derive(Clone, Debug, Encode, Decode, Packet)]
#[packet(state = PacketState::Login)]
pub struct LoginHelloC2s<'a> {
    pub username: Bounded<&'a str, 16>,
    pub profile_id: Option<Uuid>,
}

Replacing Option<Uuid> with Uuid results in successfully decoding packets from the 1.20.4 client.

-    pub profile_id: Option<Uuid>,
+    pub profile_id: Uuid,

The issue seems to be in the Decode implementation for Option<T>:

impl<'a, T: Decode<'a>> Decode<'a> for Option<T> {
    fn decode(r: &mut &'a [u8]) -> anyhow::Result<Self> {
        Ok(if bool::decode(r)? {
            Some(T::decode(r)?)
        } else {
            None
        })
    }
}

This essentially expects either [username]1[profile_id] or [username]0. However, the actual packet looks like this: [username][profile_id].

I'm not sure if there is a version of MC which doesn't send the profile_id, so I'm not sure how this should be fixed.

[maxomatic458]

@bcheidemann
The latest 1.21.X work is here: https://github.com/maxomatic458/valence/tree/1.21-packets

Where this is already fixed

@dyc3 since you have write perms
Could we get an official 1.21 Branch in the valence repo (where those changes could be merged to).

[dyc3]

Already exists: https://github.com/valence-rs/valence/tree/next-1.21

[bcheidemann]

Hey @maxomatic458 @dyc3, thanks for info. I can see that LoginHelloC2s had been replaced by HelloC2s in 1.21-packets. This works as expected. However, this fix doesn't seem to have been pulled into next-1.21 yet. Edit: Didn't read the previous comments properly