When doing server-side streaming in tonic, the generated async function is expected to return something which implements Stream. If I construct my stream by taking a receiver and then calling .map(my_very_expensive_function) on it, will my_very_expensive_function run on a tokio worker thread? Or somewhere it’s safe to do blocking operations?

Eg.

use proto::MyService;

struct RpcService;

impl MyService for RpcService {
  type MyFnStream = Pin<Box<dyn Stream<Item=Result<Bar, tonic::Status>> + Send>>

  async fn my_fn(&self, request: tonic::Request<Foo>) -> Result<Response<Self::MyFnStream>, tonic::Status> {
    let (sender, receiver) = tokio::sync::mpsc::channel();
    register_sender(sender);
    Box::new(ReceiverStream(receiver).map(my_very_expensive_function))
  }
}

fn register_sender(sender: tokio::sync::mpsc::Sender<Foo>) {
  ...
}

fn my_very_expensive_function(foo: Foo) -> Bar {
  ... // This should not run in a tokio worker thread
}