Panic protection in Tonic (more generally for tower-based services) #885

EarvinKayonga · 2022-01-10T17:19:34Z

EarvinKayonga
Jan 10, 2022

So I am looking for a way to handle panics gracefully. This doesn't work. Do you have other suggestions ?

use tonic::{body::BoxBody,transport::Server, Request, Response, Status};

use crate::contract::provider_server::{Provider, ProviderServer};
use contract::{UserRequest, UserResponse};
use futures::FutureExt;

use hyper::Body;
use tower::{Layer, Service};
use std::{
    task::{Context, Poll},
    time::Duration,
};

pub mod contract {
    tonic::include_proto!("provider");
}

#[derive(Debug, Default)]
pub struct GRPCProvider {}

#[tonic::async_trait]
impl Provider for GRPCProvider {
    async fn userdetails(
        &self,
        request: Request<UserRequest>,
    ) -> Result<Response<UserResponse>, Status> {
        let user = request.into_inner();

        log_user(&user);

        Ok(Response::new(UserResponse { message: true }))
    }
}

fn log_user(user: &UserRequest) {
    println!(
        "user request: email = {email}, name = {name}",
        email = user.useremail,
        name = user.username
    );
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let addr = "[::1]:50051".parse()?;
    let greeter = GRPCProvider::default();

    let layer = tower::ServiceBuilder::new()
        // Apply middleware from tower
        .timeout(Duration::from_secs(30))
        // Apply our own middleware
        .layer(PanicProtectionLayer::default())
        .into_inner();

    Server::builder()
        .layer(layer)
        .add_service(ProviderServer::new(greeter))
        .serve(addr)
        .await?;

    Ok(())
}

#[derive(Debug, Clone, Default)]
struct PanicProtectionLayer;

impl<S> Layer<S> for PanicProtectionLayer {
    type Service = PanicProtection<S>;

    fn layer(&self, service: S) -> Self::Service {
        PanicProtection { inner: service }
    }
}

#[derive(Debug, Clone)]
struct PanicProtection<S> {
    inner: S,
}

impl<S> Service<hyper::Request<Body>> for PanicProtection<S>
where
    S: Service<hyper::Request<Body>,
    Response = hyper::Response<BoxBody>> + Clone + Send + 'static,
    S::Future: Send + 'static,
{
    type Response = S::Response;
    type Error = S::Error;
    type Future = futures::future::BoxFuture<'static, Result<Self::Response, Self::Error>>;

    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.inner.poll_ready(cx)
    }

    fn call(&mut self, req: hyper::Request<Body>) -> Self::Future {
        let clone = self.inner.clone();
        let mut inner = std::mem::replace(&mut self.inner, clone);

        Box::pin(async move {
            let response = async {
               inner.call(req).await?
            }.catch_unwind().await?; // doesnot work

            Ok(response)
        })
    }
}

Answered by LucioFranco

Jan 12, 2022

We tend to just have a panic handler for the entire application (usually to print the message to our logging system). Panics should not be used for regular error handling like this. In tokio the panic will catch and just cancel the task.

View full answer

LucioFranco · 2022-01-12T21:05:44Z

LucioFranco
Jan 12, 2022
Maintainer

We tend to just have a panic handler for the entire application (usually to print the message to our logging system). Panics should not be used for regular error handling like this. In tokio the panic will catch and just cancel the task.

4 replies

EarvinKayonga Jan 14, 2022
Author

Thanks for the answer @LucioFranco.

I was also looking for a way to capture, then report the panic details (à la Sentry or Datadog).
And, I had the impression the task wasn't dropped because my clients never received a response. I'll look into it.

Cheers

BatmanAoD Jul 23, 2025

It would also be good to be able to return a 500 error to clients if the request-handler panics. (Not because panics are valid for "regular error handling," but because sometimes there are bugs that manifest as panics.)

LucioFranco Jul 30, 2025
Maintainer

I wonder if this is something that should be handled in hyper/h2 rather than tonic? Since they are generally the ones that control the task usually.

BatmanAoD Jul 30, 2025

Sorry, I meant a 13 "internal error" gRPC code. So I would still expect it to be tonic-specific, actually.

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