在libp2p-rs上开发新协议

本文以floodsub为例，讨论如何在libp2p-rs上开发新协议，详细代码请查看源码。

实现两个trait

在libp2p-rs中，swarm提供了两个trait：

• Notifiee用于接收swarm的通知，当有新的连接创建或者连接关闭时，swarm会调用connected()或者disconnected();
• ProtocolHandler用于读写协议的数据，协议协商成功后，swarm会调用handle()。

/// Notifiee is an trait for an object wishing to receive notifications from swarm
pub trait Notifiee {
    /// called when a connection opened
    fn connected(&mut self, _conn: &mut Connection) {}
    /// called when a connection closed
    fn disconnected(&mut self, _conn: &mut Connection) {}
}

/// Common trait for upgrades that can be applied on inbound substreams, outbound substreams,
/// or both.
/// Possible upgrade on a connection or substream.
#[async_trait]
pub trait ProtocolHandler: UpgradeInfo + Notifiee {
    /// After we have determined that the remote supports one of the protocols we support, this
    /// method is called to start handling the inbound. Swarm will start invoking this method
    /// in a newly spawned task.
    ///
    /// The `info` is the identifier of the protocol, as produced by `protocol_info`.
    async fn handle(&mut self, stream: Substream, info: <Self as UpgradeInfo>::Info) -> Result<(), Box<dyn Error>>;
    /// This is to provide a clone method for the trait object.
    fn box_clone(&self) -> IProtocolHandler;
}

floodsub handler实现Notifiee和ProtocolHandler

#[derive(Clone)]
pub struct Handler {
    incoming_tx: mpsc::UnboundedSender<RPC>,
    new_peer: mpsc::UnboundedSender<PeerEvent>,
}

impl Handler {
    pub(crate) fn new(incoming_tx: mpsc::UnboundedSender<RPC>, new_peer: mpsc::UnboundedSender<PeerEvent>) -> Self {
        Handler { incoming_tx, new_peer }
    }
}

impl UpgradeInfo for Handler {
    type Info = &'static [u8];

    fn protocol_info(&self) -> Vec<Self::Info> {
        vec![FLOOD_SUB_ID]
    }
}

impl Notifiee for Handler {
    fn connected(&mut self, conn: &mut Connection) {
        let peer_id = conn.remote_peer();
        let mut new_peers = self.new_peer.clone();
        task::spawn(async move {
            let _ = new_peers.send(PeerEvent::NewPeer(peer_id)).await;
        });
    }
}

#[async_trait]
impl ProtocolHandler for Handler {
    async fn handle(&mut self, mut stream: Substream, _info: <Self as UpgradeInfo>::Info) -> Result<(), Box<dyn Error>> {
        loop {
            /* recv, decode and send to msg process mainloop */
            self.incoming_tx.send(rpc).await.map_err(|_| FloodsubDecodeError::ProtocolExit)?;
        }
    }

    fn box_clone(&self) -> IProtocolHandler {
        Box::new(self.clone())
    }
}

注册到swarm

let floodsub = FloodSub::new(FloodsubConfig::new(local_peer_id));
let handler = floodsub.handler();

let mut swarm = Swarm::new(local_key.public()).with_protocol(Box::new(handler))

还需要做什么

简单的协议，比如echo，那么所有事情都在ProtocolHandler.handle()中处理即可，到这里就结束了。

稍微复杂的协议，比如floodsub，最好将swarm的通知和收到的数据，发送到消息处理主循环进行处理，实时更新状态；

impl floodsub {
    pub fn start(mut self, control: Swarm_Control) {
        self.control = Some(control);
    
        // well, self 'move' explicitly,
        let mut floodsub = self;
        task::spawn(async move {
            let _ = floodsub.process_loop().await;
        });
    }

    /// Message Process Loop.
    pub async fn process_loop(&mut self) -> Result<()> {
        loop {
            select! {
                cmd = self.peer_rx.next() => {
                    self.handle_peer_event(cmd).await;
                }
                rpc = self.incoming_rx.next() => {
                    self.handle_incoming_rpc(rpc).await?;
                }
                cmd = self.control_rx.next() => {
                    self.on_control_command(cmd).await?;
                }
                sub = self.cancel_rx.next() => {
                    self.un_subscribe(sub).await?;
                }
            }
        }
    }
}

从上面可以看到，floodsub消息处理主循环运行在一个task里面，start()时需要将self传递进去，因此后续的发布订阅等操作只能通过channel发消息，这就是control和handler包裹channel的原因。

#[derive(Clone)]
pub struct Control {
    config: FloodsubConfig,
    control_sender: mpsc::UnboundedSender<ControlCommand>,
}

impl Control {
    /// Subscribe to messages on a given topic.
    pub async fn subscribe(&mut self, topic: Topic) -> Option<Subscription> {
        let (tx, rx) = oneshot::channel();
        self.control_sender
            .send(ControlCommand::Subscribe(topic, tx))
            .await
            .expect("control send subscribe");
        rx.await.expect("Subscribe")
    }
}

当新的连接创建时，floodsub会主动创建流，协商通过后向对方发送本节点感兴趣的topic。因此这里需要swarm的control。

self.control.as_mut().unwrap().new_stream(pid, vec![FLOOD_SUB_ID]).await;

总结

在libp2p-rs上面开发简单的协议，只需要两步，对于稍微复杂的协议，需要handler和control这类包裹channel的结构，将消息发送到协议消息处理主循环，以驱动整个协议的运转，完成特定的功能。

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