TypeError: start is not a function when using emscripten_sleep() and asyncify

[ohnx]

Version(s) affected: 2.0.3, 2.0.4

I'm getting this error when trying to use emscripten_sleep():
TypeError: start is not a function. (In 'start()', 'start' is undefined)

The line is coming from handleSleep; it looks like var start = Asyncify.getDataRewindFunc(Asyncify.currData); is returning nothing... Someone who was helping me debug mentioned this:

stepping through the asyncify js in a situation where it fails, it seems that when emscripten_sleep is called, that calls handleSleep, which then calls allocateData, which IIUC should store the resume target by calling setDataRewindFunc, and the latter gets that target by taking the top (bottom?) of the exportCallStack
however, due to reasons I don't understand yet, the exportCallStack is empty at that point

zip file of sample code to reproduce issue

[gwenya]

I bisected it to 43fcfd2.

IIUC the embind setup creates some indirect invocation code to call exports, and it skips that if the signature does not contain 'j'. The signature is 'vi' for the function you export, so it directly returns the wasm table entry instead of creating the invocation code. But that invocation code is needed for async because it pushes the export that was called onto a stack which is later used by the asyncify code to find out what it needs to call.

[sbc100]

@kleisauke