emscripten-core/emscripten

Feature request: MAIN_THREAD_EM_ASYNC_JS

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#16,694 opened on Apr 11, 2022

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good first bughelp wanted

Description

EM_ASYNC_JS is really useful for writing async JS code with await, however it runs on the thread it was called on. It would be nice if there were a version that proxied the call to the main thread.

The alternative is to use MAIN_THREAD_ASYNC_EM_ASM (extra boilerplate with Asyncify.handleAsync and doesn't define a C function like EM_ASYNC_JS does):

MAIN_THREAD_ASYNC_EM_ASM({
    return Asyncify.handleAsync(async () => {
        // ....
    });
});

and then setting -s 'ASYNCIFY_IMPORTS=["emscripten_receive_on_main_thread_js"]'

related https://github.com/emscripten-core/emscripten/issues/8991#issuecomment-846440750

Contributor guide

Tech stack
javascriptccppwasm
Domain
toolingdeveloper experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Emscripten build systemAsyncify APIWebAssembly threadingJavaScript async/await
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Understand the current EM ASYNC JS implementation, likely in src/emscripten settings.js or src/asyncify.js. Examine how MAIN THREAD ASYNC EM ASM uses Asyncify.handleAsync. Review the linked issue #8991 and comment for prior discussion. Propose a new macro that combines the EM ASYNC JS syntax with main thread proxying, possibly by leveraging the existing `emscripten receive on main thread js` import.
Feature request: MAIN_THREAD_EM_ASYNC_JS · emscripten-core/emscripten#16694 | Good First Issue