JSPI/Asyncify - how to call from JavaScript

[isaackwan]

Let's say I have this cpp file:

#include <emscripten/bind.h>
#include <emscripten/val.h>
#include <emscripten.h>

// em++ --bind -O3 repro.cpp -o repro.html -s ASYNCIFY=2

using namespace emscripten;

EM_ASYNC_JS(int, run, (), {
  return Promise.resolve(42);
});

int cppRun() {
  return run();
};


EMSCRIPTEN_BINDINGS() {
  emscripten::function("run", &run);
  emscripten::function("cppRun", &cppRun);
}

EMSCRIPTEN_KEEPALIVE int main() {
  printf("start\n");
  printf("Out %d\n", cppRun());
  printf("end\n");
  return 0;
}

During startup (ie calling main()) it should print this, all good:

start
Out 42
end

Now however if I call it using Module.cppRun() it doesn't work.

 Uncaught RuntimeError: invalid suspender object for suspend
    at repro.wasm:0x49b
    at repro.wasm:0x3d4e
    at repro.wasm:0x37c4
    at Object.cppRun (eval at newFunc (repro.js:1:15672), <anonymous>:7:19)
    at <anonymous>:1:8

What's weird is that if I compile with Asyncify=1 it works.

Any idea why? Is this supported at all? I would expect cppRun() to return a JS promise.

[brendandahl]

Since the binding cpprun can suspend you need to mark it as async:

emscripten::function("cppRun", &cppRun, emscripten::async());