fix: add support for exposing functions that return objects

[namuol]

I've been experimenting with this library and ran into a problem which is best illustrated with this simple test:

test("expose function that returns object", async () => {
  const vm = (await getQuickJS()).createVm();
  const arena = new Arena(vm);

  arena.expose({
    makeObject: () => {
      return {
        myFavoriteNumber: 42,
      };
    },
  });

  expect(arena.evalCode(`makeObject().myFavoriteNumber`)).toBe(42);

  arena.dispose();
  vm.dispose();
});

While attempting to call evalCode, a Lifetime not alive error is thrown. I traced the issue to an instance where VMMap::_mapSet was attempting to be called in the VM but was not alive.

I believe the _mapSet instance was no longer alive because the original VMMap used to marshal the return value was created ephemerally here:

quickjs-emscripten-sync/src/index.ts

Lines 196 to 197 in 21a7fd8

	const map = new VMMap(this.vm);
	map.merge(this._map);

Then, it is merged with the Arena::map instance again before it is disposed via .dispose(), here:

quickjs-emscripten-sync/src/index.ts

Lines 219 to 221 in 21a7fd8

	this._map.merge(map);
	map.clear();
	map.dispose();

Then, some time after the ephemeral map is disposed of, a callback passed to .consume must be executed and at this point _mapSet has already been disposed:

quickjs-emscripten-sync/src/vmmap.ts

Lines 87 to 102 in 203f8ae

	this.vm.newNumber(counter).consume((c) => {
	if (!this._mapSet.alive) {
	throw new Error("VMMap::set: `this._mapSet` is not alive!");
	}
	this._call(
	this._mapSet,
	undefined,
	handle,
	c,
	handle2 ?? this.vm.undefined
	);
	});
	return true;
	}

To get around this, I'm avoiding the creation of the ephemeral map entirely, and instead I'm pointing directly to the Arena::map instance.

This fixed the issue and didn't appear to cause any new test failures, but I'm not certain this change is actually safe and would love to know what you think.

Thank you for your help, and for sharing your hard work - your library has been a pleasure to work with. 🙇‍♂️

[rot1024]

Thanks for your great PR. I just discovered this bug and was about to find out what caused it, but your PR saved me a lot of work! I will release a new version of quickjs-emscripten-sync as soon as possible!

[rot1024]

v1.2.0 has been released!

[namuol]

Thank you!