Sync-Api doesn't work with multiple workers

[rchiodo]

Suppose you have something like so:

Main Thread - Extension code
Worker 1 - Extension sub piece, which starts Worker 2
Worker 2 - Needs to use the sync api

Worker 2 cannot communicate with the Main Thread because its MessageChannel is only with Worker 1.

For CPython I have this exact situation when trying to run the debugger:

• Main thread - Extension code
• Worker 1 - Load of python.js
• Worker 2 - Reader thread for debugger
• Worker 3 - Writer thread for debugger

Worker 2 and 3 cannot send messages to the Main Thread.

• Note: Emscripten (by default proxies) all FS (or socket) API calls to its main thread (worker 1 in this case) but that causes deadlock issues because the Atomics.wait in the sync-api block the main worker thread, therefore no other fs API calls can be handled while waiting. Took me a while to figure this out as most of it was working until Worker 3 tried to acquire a lock.

[rchiodo]

I believe there's a solution to this, use a BroadcastChannel instead. (supported by node and web browsers).

Essentially every worker uses the BroadcastChannel instead. When sending messages, a target could be set, something like so:

{
    "dest": "main",
    "src": "worker-<threadid>"
}

Main thread would listen on the same broadcast channel and handle messages with "dest" : "main".
Worker threads would listen on the same broadcast channel and handle messages with "dest": "worker-<threadid>" where the thread id matches their own.

I believe that would let messages to go to and from an arbitrary worker to the main thread.