Rust CDK - How to catch panic/trap when calling another canister which traps?

Developers
1

Hey,

I just want to know if it is possible to catch a panic/trap produced by calling another canister which traps (in Motoko it should be possible with a try-catch).
I tried out catch_unwind but my caller canister still traps.

Here is the panic message:
[Canister r7inp-6aaaa-aaaaa-aaabq-cai] Panicked at 'called Result::unwrap() on an Err value: (
CanisterError, "IC0503: Canister ryjl3-tyaaa-aaaaa-aaaba-cai trapped explicitly: assertion failed

Here is the code:

use futures::FutureExt;

#[import(
        candid_path = ".dfx/local/canisters/counter/counter.did",
        canister_id = "ryjl3-tyaaa-aaaaa-aaaba-cai"
    )]
    pub struct CounterCanister;

let result: std::result::Result<
            (ic_cdk::export::candid::Nat,),
            std::boxed::Box<dyn std::any::Any + std::marker::Send>,
        > = CounterCanister::increment().catch_unwind().await;

I think the generated code (CounterCanister) calls the following method in ic_cdk:

pub async fn call<T: ArgumentEncoder, R: for<'a> ArgumentDecoder<'a>>(
    id: Principal,
    method: &str,
    args: T,
) -> CallResult<R> {
    let args_raw = encode_args(args).expect("Failed to encode arguments.");
    let bytes = call_raw(id, method, args_raw, 0).await?;
    decode_args(&bytes).map_err(|err| trap(&format!("{:?}", err)))
}

So on error trap function is called. Is it why I can’t catch the panic with catch_unwind()? How to catch the panic then?

Thanks

2 Likes
2

It should be possible to catch the error if another canister is calling into the trapping canister. The trapping canister should return an error type to the outer canister.

So just pattern match the error rather than catch unwind.

3

I asked a similar question before and it does not appear its possible to catch this error:

Here is my code:

1 Like
4

I don’t know how to pattern match it because the generated code (with import macro from ic-cdk-macros) doesn’t produce any error result when calling. If I call the other canister like CounterCanister::increment().await, the compiler says that the result is a tupel (ic_cdk::export::candid::Nat,), so there is nothing I can pattern match

5

It looks like this is the code that produce the generated code (ic-cdk-macros-0.3.2\src\import.rs):

fn actor_function_body(
        &self,
        name: &str,
        arguments: &[(String, String)],
        _returns: &str,
        _is_query: bool,
    ) -> Result<String, candid::error::Error> {
      ...

        let call = "ic_cdk::call";

        // We check the validity of the canister_id early so it fails if the
        // ID isn't in the right text format.
        let principal: ic_cdk::export::Principal =
            ic_cdk::export::Principal::from_text(canister_id).unwrap();

        Ok(format!(
            r#"
            {{
                {call}(
                  ic_cdk::export::Principal::from_text("{principal}").unwrap() as ic_cdk::export::Principal,
                  "{name}",
                  {arguments}
                 )
                 .await
                 .unwrap()
            }}
        "#,
            call = call,
            principal = &principal.to_text(),
            name = name.escape_debug(),
            arguments = arguments,
        ))
    }

It directly calls await.unwrap()…

1 Like
6

I can catch the error with

let principal =
            ic_cdk::export::Principal::from_text("ryjl3-tyaaa-aaaaa-aaaba-cai").unwrap();
let result = ic_cdk::api::call::call(principal, "increment()", ()).await
            as CallResult<(ic_cdk::export::candid::Nat,)>;

if result.is_ok() {
           ...
        } else {
           ...
        }

Unfortunately it just works with call() function but not with the import macro where the candid-file can be utilized

2 Likes