Avoiding closure - encapsulating thread variables in a struct - multithreading

Question

I am writing a simple websocket server named BoltServer based on Rust websocket crate(Code is incomplete, I just started). I am using the example program as the base. However the example programs are not modular (having very long methods). So I am trying to break them up into structs and methods. I want to spawn two threads for each client. One sends messages and the other one receives messages. So here, I want to capture all variables used by the thread in a struct and then call the run method in the impl.
extern crate websocket;
extern crate time;
extern crate rustc_serialize;
pub mod ws {
use std::thread;
use std::sync::{Arc, Mutex};
use std::sync::mpsc;
use std::net::ToSocketAddrs;
use websocket;
use websocket::{Server, Message, Sender, Receiver};
use websocket::server::Connection;
use websocket::stream::WebSocketStream;
use std::str::from_utf8;
struct BoltUser {
user_id: u32,
my_tx: mpsc::Sender<String>,
}
struct Broadcaster {
my_rx: mpsc::Receiver<String>,
}
impl Broadcaster {
fn new(receiver: mpsc::Receiver<String>) -> Broadcaster {
Broadcaster { my_rx: receiver }
}
fn run(self) {
while let Ok(msg) = self.my_rx.recv() {
println!("Broadcaster got message: {}", msg);
}
}
}
struct SocketReader {}
impl SocketReader {
fn run(self) {}
}
struct SocketWriter {
my_rx: mpsc::Receiver<String>,
sender: Sender,
}
impl SocketWriter {
fn run(self) {
while let Ok(message) = self.my_rx.recv() {
}
}
}
pub struct BoltServer {
address: String,
connected_users: Arc<Mutex<Vec<BoltUser>>>,
}
impl BoltServer {
pub fn new(address: &str) -> BoltServer {
BoltServer {
address: address.to_string(),
connected_users: Arc::new(Mutex::new(vec![])),
}
}
fn handshake(&mut self,
connection: Connection<WebSocketStream, WebSocketStream>)
-> (SocketWriter, SocketReader) {
let request = connection.read_request().unwrap();
// println!("thread-> Accepting request...");
let response = request.accept();
let (mut sender, mut receiver) = response.send().unwrap().split();
let (user_tx, user_rx) = mpsc::channel::<String>();//Create a channel for writer
let socket_writer = SocketWriter {
my_rx: user_rx,
sender: sender,
};
let socket_reader = SocketReader {};
(socket_writer, socket_reader)
}
pub fn start(&mut self) {
println!("Starting");
let (broadcaster_tx, broadcaster_rx) = mpsc::channel::<String>();
let broadcaster = Broadcaster::new(broadcaster_rx);
let handle = thread::Builder::new()
.name("Broadcaster".to_string())
.spawn(move || broadcaster.run());
let server = Server::bind(&*self.address).unwrap();
let mut user_id: u32 = 0;
// Block and process connection request from a new client
for connection in server {
user_id = user_id + 1;//Create a new user id
let (socket_writer, socket_reader) = self.handshake(connection);
thread::Builder::new()
.name("Socket writer".to_string())
.spawn(move || socket_writer.run());
thread::Builder::new()
.name("Socket reader".to_string())
.spawn(move || socket_reader.run());
}
handle.unwrap().join();
println!("Finished");
}
}
}
The following code gives an idea of what I want to achieve.
// Block and process connection request from a new client
for connection in server {
user_id = user_id + 1;//Create a new user id
let (socket_writer, socket_reader) = self.handshake(connection);
thread::Builder::new().name("Socket writer".to_string()).spawn(move || {
socket_writer.run()
});
thread::Builder::new().name("Socket reader".to_string()).spawn(move || {
socket_reader.run()
});
}
Here I am stuck in the handshake method. I am not able to initialize the SocketWriter struct with the sender that I am getting by calling the split method in the library. I am getting the following compilation error:
error[E0038]: the trait `websocket::Sender` cannot be made into an object
--> src/lib.rs:46:9
|
46 | sender:Sender,
| ^^^^^^^^^^^^^ the trait `websocket::Sender` cannot be made into an object
|
= note: method `send_dataframe` has generic type parameters
= note: method `send_message` has generic type parameters

Answer 1

The error is telling you the immediate problem:
46 | sender:Sender,
| ^^^^^^^^^^^^^ the trait `websocket::Sender` cannot be made into an object
First of all, a variable/field can't have a plain trait type (but &Trait can be possible), but also the websocket::Sender trait is not object safe; it has generic methods which can't work dynamically (ie vtable methods have to have a fixed type).
Instead, you have to have a concrete type (you could also make it a generic struct).
It's not obvious what the right type is, so I like to get the compiler to tell me. So first try the simplest possible:
sender: (),
The compiler replies with some information:
| ^^^^^^ expected (), found struct `websocket::client::Sender`
Ok, let's plug that in:
sender: websocket::client::Sender,
That gives:
46 | sender: websocket::client::Sender,
| ^^^^^^^^^^^^^^^^^^^^^^^^^ expected 1 type arguments, found 0
Ok, that's a generic type. Next try:
sender: websocket::client::Sender<()>,
Finally it gives us the real type:
74 | sender:sender,
| ^^^^^^ expected (), found enum `websocket::WebSocketStream`
So finally we can finish SocketWriter:
struct SocketWriter {
my_rx: mpsc::Receiver<String>,
sender: websocket::client::Sender<websocket::WebSocketStream>,
}
There's a following compile error since the connection you get is a Result<> so you need to check for errors (it compiles if I change to self.handshake(connection.unwrap()), but that's obviously not the best practice.