rust
use bson::{doc, Document};
use mongodb::options::FindOptions;
use mongodb::sync::Client;
use std::sync::mpsc;
use std::thread;
use std::io::{self, BufRead};

enum TaskResult {
    Success,
    Fail,
}

fn main() {
    {
        let source =
            Client::with_uri_str("mongodb://localhost:27017/?authSource=admin")
                .unwrap();
        let (sender, receiver) = mpsc::channel();

        for _ in 0..40 {
            let source_coll = source.database("betahub").collection::<Document>("factor_exposure");
            let sender = sender.clone();
            thread::spawn(move || {
                let res = source_coll
                    .find(doc! {}, FindOptions::builder().batch_size(10000).limit(509545).build())
                    .and_then(|cursor| {
                        let mut buf = vec![];
                        Ok(for _doc in cursor { buf.push(_doc); if buf.len() >= 10000 {buf.clear()}})
                    });
                match res {
                    Err(e) => {
                        let _ = sender.send(TaskResult::Fail);
                    }
                    Ok(_) => {
                        let _ = sender.send(TaskResult::Success);
                    }
                }
            });
        }

        let mut count = 0;
        while let Ok(event) = receiver.recv() {
            match event {
                TaskResult::Fail => {
                    println!("get fail");
                    break;
                },
                TaskResult::Success => {
                    count += 1;
                    if count == 40 {
                        break;
                    }
                }
            }
        }
    }

    println!("Every thing is done, press enter to exit.");
    let _line1 = io::stdin().lock().lines().next().unwrap().unwrap();
}