how can I use a random number and a tokio::time::sleep within an async function in rust

Question

I have a gRPC server method for which I want to simulate some work for this I have this

async fn write(&self, request: Request<WriteRequest>)
                     -> Result<Response<WriteResponse>, Status> {
    let mut rng = rand::thread_rng();
    let random = rng.gen_range(1000..5000);
    // Sleep for the random duration
    sleep(Duration::from_millis(random)).await;
    return Ok(Response::new(WriteResponse { ... }));
}

but the code is not happy about the random value, I get the error

future cannot be sent between threads safely future created by async block is not `Send`

I know I can add + Send in the function but in this case, I cannot modify the signature of the method because it is an interface generated by the gRPC compiler

When I used hardcoded value within the Duration in works.

How can I call the random number generation? or how can I sleep for a random value using sleep? is there another way?

Answer 1

This happens because rng is held past an await point. You can get around this by enclosing it in a block, which ensures it is not stored in the async function's state machine.

let random_secs = {
    let mut rng = rand::thread_rng();
    rng.gen_range(1000..5000)
};

If you do need to store an RNG, you can store a normal PRNG (such as StdRng) that is seeded from thread_rng.

use rand::{Rng, SeedableRng, rngs::StdRng};
    
let mut rng = {
    let rng = rand::thread_rng();
    StdRng::from_rng(rng).unwrap()
};
    
loop {
    let millis = rng.gen_range(1..10);
    tokio::time::sleep(std::time::Duration::from_millis(millis)).await;
    println!("{millis}");
}