Why oneshot channel is blocked?

[panguangming2024]

I'm wondering why the following code doesn't work, it doesn't go to the last println!().
However when I move "drop(tx)" to front of "manager.await.unwrap()", it works well.

use tokio::sync::{mpsc, oneshot};

#[derive(Debug)]
enum Command {
    Get {
        key: String,
        resp: oneshot::Sender<String>,
    },
}
#[tokio::main]
async fn main() -> () {
    let (tx, mut rx) = mpsc::channel(2);
    let manager = tokio::spawn(async move {
        while let Some(cmd) = rx.recv().await {
            match cmd {
                Command::Get { key, resp } => {
                    let _ = resp.send(format!("Get cmd resp to {}",key));
                }
            }
        }
    });

    for cnt in 0..3 {
        let tx3 = tx.clone();
        let t3 = tokio::spawn(async move {
            let (resp_tx, resp_rx) = oneshot::channel();
            let cmd = Command::Get {
                key: format!("Get cmd [{}] ",cnt),
                resp: resp_tx,
            };
            if tx3.send(cmd).await.is_err() {
                eprintln!("connection task shutdown at loop {}",cnt);
                return;
            }
            let res = resp_rx.await;
            println!("task 1 GOT (Get) = {:?} at loop {}", res,cnt);
        });
        t3.await.unwrap();
    }

   
    manager.await.unwrap();
    drop(tx);
   println!("main func is over.");
}

[Darksonn]

This was answered on discord