Condvar wait not supported

Developers
1

I’m trying to execute cross-canister calls from within a function that I cannot make async. I am using the futures crate to do this. futures::executor::block_on works just fine if I call my own async functions, but when I try to call ic_cdk::api::call::call I get this error:

[Canister rrkah-fqaaa-aaaaa-aaaaq-cai] Panicked at 'condvar wait not supported', library/std/src/sys/wasm/../unsupported/condvar.rs:23:9

Here’s an example of the call I am trying to make (this is within a function that cannot be made async):

let string = futures::executor::block_on(async {
    let call_result: Result<(String,), _> = ic_cdk::api::call::call(
        ic_cdk::export::Principal::from_text("ryjl3-tyaaa-aaaaa-aaaba-cai").unwrap(),
        "testString",
        ()
    ).await;

    return call_result.unwrap().0;
});

I’ve narrowed things down to…well, I really think there’s an issue in the ic-cdk futures implementation somehow. I don’t yet understand Rust futures super well, but my own async functions work just fine, so I assume there is an issue with the futures that ic-cdk functions are using.

2 Likes
2

I am getting the same error… "condvar wait not supported " on calling ic_cdk::call below; using tokio. Any pointers would be much appreciated.

#[tokio::main(flavor="current_thread")]
async fn get_raw_rand() -> i32 {
    match call(ic_cdk::export::Principal::management_canister(),
     "raw_rand", 
     ()).await as  Result< (Vec<u8>, ), _> {
         Ok(_) => return 42,
         Err(_) => return 0
     }
}

#[update]
fn get() -> candid::Nat {
    candid::Nat::from(get_raw_rand())
}
3

Yeah, I’ve just ruled out tokio for now. I think futures need a real overhaul to make them work with the Rust ecosystem at large.

4

ok. But in the meantime, does it mean that all async functions exposed (i.e. call::call) are inaccessible in Rust?

5

No, if you remove tokio doesn’t that code above work?

6

Well, it should look like this right?

async fn get_raw_rand() -> i32 {
    match call(ic_cdk::export::Principal::management_canister(),
     "raw_rand", 
     ()).await as  Result< (Vec<u8>, ), _> {
         Ok(_) => return 42,
         Err(_) => return 0
     }
}

#[update]
async fn get() -> candid::Nat {
    candid::Nat::from(get_raw_rand().await)
}
1 Like
7

Thanks ! That works.

1 Like
8

this is within a function that cannot be made async

Sorry, there is really no way to block within a canister. If you need to make a call, you have to make the surrounding function async.

works just fine if I call my own async functions

I guess that’s because your async functions never block.

9

ic_cdk::spawn has an effect similar to futures::executor::block_on, but why the Output here is (), if it can be a generic T, it is more convenient to use

2 Likes