How to translate JS Promises to Rust

Question

at the moment I'm writing a pure Rust MQTT5 library (I know there are existing ones out there, but I'm more trying to learn Rust) and I stumpled upon this problem.

I'm using latest stable rust with tokio 1.0.1.

When I send out a packet over the wire, I often expect a response from the server (example below PingReq/PingAck, Ping/Pong).

Leaving out a lot if logic regarding timeouts and packet clashes I wrote a simplified version of the logic in JavaScript (since I know that fairly well).

How would this logic translate to Rust and its futures? Or to be more clear: Can I somehow recreate the resolve() callback function behavior of awaitPackage + onIncomingPacket?

class Client {
  awaitedPacketTypes = {};

  /**
   * a ping consist of a send ping and a receive pong
   */
  async ping(){
    await this.sendPacket("Ping");
    return await this.awaitPackage("Pong");
  }

  async sendPacket(packetType) { /*...*/ }
  
  /**
   * This expects a specific packet type to be received in the future
   * @param {*} packetType 
   */
  awaitPackage(packetType) {
    return new Promise((resolve, reject) => {
      this.awaitedPacketTypes[packetType] = {
        resolve,
        reject
      };
    });
  }

  /**
   * This gets called for every packet from the network side and calls the correct resolver if something waits for this packet type
   * @param {*} packet 
   */
  onIncomingPacket(packet) {
    if(this.awaitedPacketTypes[packet.type]) {
      this.awaitedPacketTypes[packet.type].resolve(packet);
      this.awaitedPacketTypes[packet.type] = undefined;
    } else {
      /*...*/
    }
  }
}

Masklinn · Accepted Answer · 2021-01-01T16:12:36.283

Or to be more clear: Can I somehow recreate the resolve() callback function behavior of awaitPackage + onIncomingPacket?

Kinda? A rust Future is only "something which can be polled for readiness", it's a much lower-level concept than a JS promise.

There are libraries which claim to provide JS-style promises, but most every async library probably provides a similar object named differently e.g. in Tokio, you'd probably want a oneshot channel, that is a channel on which a single value can be sent, resulting in something along the lines of:

struct Packet { r#type: &'static str }
struct Client {
  awaited: Mutex<HashMap<&'static str, Sender<Packet>>>
}

impl Client {
    async fn ping(&self) -> Packet {
        self.send_packet("Pong").await;
        self.await_package("Pong").await.unwrap()
    }
    async fn send_packet(&self, _: &'static str) {}
    fn await_package(&self, packet_type: &'static str) -> Receiver<Packet> {
        let (tx, rx) = channel();
        self.awaited.lock().unwrap().insert(packet_type, tx);
        rx
    }
    fn on_incoming_packet(&self, packet: Packet) {
        if let Some(tx) = self.awaited.lock().unwrap().remove(packet.r#type) {
            tx.send(packet);
        }
    }
}

Notice that while this is the proper equivalent to the OP's js code, it might be a better idea to queue incoming packets so that they don't get lost if `await_package` hasn't been called before they are received. In JS, we can usually rely on the event queue to handle that. — Bergi, Jan 01 '21 at 16:18
Good to mention it here. My logic already had this, but I removed it to simplify the question. — Snapstromegon, Jan 01 '21 at 16:39

How to translate JS Promises to Rust

1 Answers1