Dealing with future chains, I am aware of 3 places of errors and exceptions (from Scala world, but I see the same is in Rust):
- exception during construction of the future chain
- exception in the closure during execution of the chain
- explicit return of completed unsuccessful future
I have attempted to implement recovery in one single place using panic::catch_unwind, but this is tough to break through the compiler errors. I have concluded that I am doing something wrong and there is better way of doing this.
If it was Scala, there is dedicated recover call on a Future to recover cases 2 and 3. It maps an error/exception to a successful future result. The 1 is covered by try ... catch or by util::Try.
Could you please advise what mechanisms are available for recovering 1, 2 and 3 in Rust?
Here is my code (which compiles if I remove catch_unwind line and block where the result is handled). All 3 places where I attempt to cause an error or exception are marked in comments by #1, #2 and #3 markers in comments.
extern crate futures;
extern crate hyper;
extern crate tokio_core;
extern crate serde;
extern crate serde_json;
use futures::future::Future;
use hyper::server::{Http, Request, Response, Service};
use futures::Stream;
use std::panic;
use std::result;
use std::error;
#[derive(Serialize, Deserialize, Debug)]
struct Point {
x: i32,
y: i32,
}
struct AgentService;
impl AgentService {
fn get_body(&self, body: hyper::Body) -> Box<Future<Item=String, Error=<Self as hyper::client::Service>::Error>> {
let full_body = body.concat2()
.map(|chunk| {
let v = chunk.to_vec();
String::from_utf8_lossy(&v).to_string()
});
Box::new(full_body)
// PLACE #1: panics during the construction of the future chain
// unimplemented!()
}
}
impl Service for AgentService {
type Request = Request;
type Response = Response;
type Error = hyper::Error;
type Future = Box<Future<Item=Self::Response, Error=Self::Error>>;
fn call(&self, _req: Request) -> Box<Future<Item=Self::Response, Error=Self::Error>> {
let result = panic::catch_unwind(|| {
println!("Received {:?}", _req);
let resp: Box<Future<Item=Self::Response, Error=Self::Error>> = match _req.path() {
"/avl" if _req.method() == &hyper::Method::Get => {
Box::new(self.get_body(_req.body())
.map(|r| {
println!("{:?}", r);
// PLACE #2: panics during future chain execution if no body content is sent
let deserialized: Point = serde_json::from_str(&r).unwrap();
println!("deserialized: {:?}", deserialized);
let resp = Response::new().with_status(hyper::StatusCode::Created);
resp
}))
}
_ => {
let resp = Response::new().with_status(hyper::StatusCode::NotFound);
Box::new(futures::future::ok(resp))
// PLACE #3: returns failed future result intentionally either:
// - during construction of the chain
// - or during execution of the chain
// Box::new(futures::future::err(hyper::Error::Method))
}
};
resp
});
//println!("Caught {:?}", result);
// PLACE where final recovery happens for all of the cases
match result {
Ok(f) => {
return f;
}
Err(e) => {
return Box::new(futures::future::ok(Response::new().with_status(hyper::StatusCode::InternalServerError)));
}
}
}
}
pub fn run() {
let addr = "127.0.0.1:3000".parse().unwrap();
let server = Http::new().bind(&addr, || Ok(AgentService)).unwrap();
server.run().unwrap();
}
Here are the compiler errors:
error[E0277]: the trait bound `std::cell::UnsafeCell<*mut futures::sync::mpsc::queue::Node<std::option::Option<std::result::Result<hyper::Chunk, hyper::Error>>>>: std::panic::RefUnwindSafe` is not satisfied in `futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>`
--> src\server.rs:47:22
|
47 | let result = panic::catch_unwind(|| {
| ^^^^^^^^^^^^^^^^^^^ the type std::cell::UnsafeCell<*mut futures::sync::mpsc::queue::Node<std::option::Option<std::result::Result<hyper::Chunk, hyper::Error>>>> may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
|
= help: within `futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>`, the trait `std::panic::RefUnwindSafe` is not implemented for `std::cell::UnsafeCell<*mut futures::sync::mpsc::queue::Node<std::option::Option<std::result::Result<hyper::Chunk, hyper::Error>>>>`
= note: required because it appears within the type `futures::sync::mpsc::queue::Queue<std::option::Option<std::result::Result<hyper::Chunk, hyper::Error>>>`
= note: required because it appears within the type `futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>`
= note: required because of the requirements on the impl of `std::panic::UnwindSafe` for `std::sync::Arc<futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>>`
= note: required because it appears within the type `futures::sync::mpsc::Receiver<std::result::Result<hyper::Chunk, hyper::Error>>`
= note: required because it appears within the type `tokio_proto::streaming::body::Inner<hyper::Chunk, hyper::Error>`
= note: required because it appears within the type `tokio_proto::streaming::body::Body<hyper::Chunk, hyper::Error>`
= note: required because it appears within the type `hyper::Body`
= note: required because it appears within the type `std::option::Option<hyper::Body>`
= note: required because it appears within the type `hyper::Request`
= note: required because it appears within the type `[closure@src\server.rs:47:42: 71:10 _req:hyper::Request, self:&&server::AgentService]`
= note: required by `std::panic::catch_unwind`
error[E0277]: the trait bound `std::cell::UnsafeCell<*mut futures::sync::mpsc::queue::Node<std::sync::Arc<std::sync::Mutex<futures::sync::mpsc::SenderTask>>>>: std::panic::RefUnwindSafe` is not satisfied in `futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>`
--> src\server.rs:47:22
|
47 | let result = panic::catch_unwind(|| {
| ^^^^^^^^^^^^^^^^^^^ the type std::cell::UnsafeCell<*mut futures::sync::mpsc::queue::Node<std::sync::Arc<std::sync::Mutex<futures::sync::mpsc::SenderTask>>>> may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
|
= help: within `futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>`, the trait `std::panic::RefUnwindSafe` is not implemented for `std::cell::UnsafeCell<*mut futures::sync::mpsc::queue::Node<std::sync::Arc<std::sync::Mutex<futures::sync::mpsc::SenderTask>>>>`
= note: required because it appears within the type `futures::sync::mpsc::queue::Queue<std::sync::Arc<std::sync::Mutex<futures::sync::mpsc::SenderTask>>>`
= note: required because it appears within the type `futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>`
= note: required because of the requirements on the impl of `std::panic::UnwindSafe` for `std::sync::Arc<futures::sync::mpsc::Inner<std::result::Result<hyper::Chunk, hyper::Error>>>`
= note: required because it appears within the type `futures::sync::mpsc::Receiver<std::result::Result<hyper::Chunk, hyper::Error>>`
= note: required because it appears within the type `tokio_proto::streaming::body::Inner<hyper::Chunk, hyper::Error>`
= note: required because it appears within the type `tokio_proto::streaming::body::Body<hyper::Chunk, hyper::Error>`
= note: required because it appears within the type `hyper::Body`
= note: required because it appears within the type `std::option::Option<hyper::Body>`
= note: required because it appears within the type `hyper::Request`
= note: required because it appears within the type `[closure@src\server.rs:47:42: 71:10 _req:hyper::Request, self:&&server::AgentService]`
= note: required by `std::panic::catch_unwind`
error[E0277]: the trait bound `hyper::header::Header + std::marker::Send + std::marker::Sync + 'static: std::panic::UnwindSafe` is not satisfied
--> src\server.rs:47:22
|
47 | let result = panic::catch_unwind(|| {
| ^^^^^^^^^^^^^^^^^^^ the type hyper::header::Header + std::marker::Send + std::marker::Sync + 'static may not be safely transferred across an unwind boundary
|
= help: the trait `std::panic::UnwindSafe` is not implemented for `hyper::header::Header + std::marker::Send + std::marker::Sync + 'static`
= note: required because of the requirements on the impl of `std::panic::UnwindSafe` for `std::ptr::Unique<hyper::header::Header + std::marker::Send + std::marker::Sync + 'static>`
= note: required because it appears within the type `std::boxed::Box<hyper::header::Header + std::marker::Send + std::marker::Sync + 'static>`
= note: required because it appears within the type `hyper::header::internals::cell::PtrMap<std::boxed::Box<hyper::header::Header + std::marker::Send + std::marker::Sync + 'static>>`
= note: required because it appears within the type `std::cell::UnsafeCell<hyper::header::internals::cell::PtrMap<std::boxed::Box<hyper::header::Header + std::marker::Send + std::marker::Sync + 'static>>>`
= note: required because it appears within the type `hyper::header::internals::cell::PtrMapCell<hyper::header::Header + std::marker::Send + std::marker::Sync + 'static>`
= note: required because it appears within the type `hyper::header::internals::item::Item`
= note: required because it appears within the type `(hyper::header::HeaderName, hyper::header::internals::item::Item)`
= note: required because of the requirements on the impl of `std::panic::UnwindSafe` for `std::ptr::Unique<(hyper::header::HeaderName, hyper::header::internals::item::Item)>`
= note: required because it appears within the type `alloc::raw_vec::RawVec<(hyper::header::HeaderName, hyper::header::internals::item::Item)>`
= note: required because it appears within the type `std::vec::Vec<(hyper::header::HeaderName, hyper::header::internals::item::Item)>`
= note: required because it appears within the type `hyper::header::internals::vec_map::VecMap<hyper::header::HeaderName, hyper::header::internals::item::Item>`
= note: required because it appears within the type `hyper::Headers`
= note: required because it appears within the type `hyper::Request`
= note: required because it appears within the type `[closure@src\server.rs:47:42: 71:10 _req:hyper::Request, self:&&server::AgentService]`
= note: required by `std::panic::catch_unwind`