--invoke does not call invoked function

[robinvanemden]

To run our C++ based framework, we need to invoke an exported function pred() in a WASI standalone Webassembly binary compiled using Emscripten. The main() and pred() functions are defined as follows:

extern "C" {
    int pred() __attribute__((used));
    int pred() {
        printf("Running pred()\n");
        double *prediction = cpwbart();
        double result = prediction[0] + mu;
        return (0);
    }
}

int main() {
    printf("Running main()\n");
    double *prediction = cpwbart();
    double result = prediction[0] + mu;
    return (0);
}

Running WAVM works fine:

> wavm run --nocache --function=pred bart.wasm 

Running pred()

But running the same using Wasmer results in it calling the default main() function:

> wasmer run --disable-cache bart.wasm --invoke pred

Running main()

Just to make sure, we also tested:

> wasmer run --disable-cache --invoke pred bart.wasm 

Running main()

But that also resulted in calling main.

See also the possibly related issue here: wasmerio/wasmer-go#91

[MarkMcCaskey]

It looks like we only run invoke on no-ABI modules right now. It's easy to fix, but it will do the wrong thing with WASI a lot of the time. WASI currently only supports being run as an executable which means we have to execute _start which calls main. Without that, most file system operations won't work.

I'll make it work with WASI too and add a warning saying as much!

[robinvanemden]

Thanks for making this work! So, when intending to invoke other functions in WASI modules, I presume is generally wise to first invoke _start() yourself. While keeping in mind _start() will itself call main(), of course.

[MarkMcCaskey]

Yeah, that's correct: currently to fully use WASI as a library you have to call main. If you can trust main to return then that's the easiest solution. I believe it should be possible to use metering here too, but I don't think we have a standardized, ready to use solution for this use case. It would be nice to have these types of things provided by use directly.

I think this will be added to WASI somewhat soon I'd imagine -- explicit setting up of the guest-side filesystem data structures without invoking main I mean.

edit: to clarify what I mean about metering is that you have to set up metering yourself; we don't have a single function or bool you can pass somewhere and have it just work yet for this specific use case which is unfortunate.

[robinvanemden]

Thanks for the solution and for the background information - both very useful. The PR does indeed fix calling functions in WASI ABI modules:

> wasmer run --disable-cache wasimodule.wasm --invoke pred

WARNING: Invoking a function with WASI is not officially supported in the WASI standard yet.  Use this feature at your own risk, things may not work!
Running pred()
pred returned [I32(0)]