discussion about how to fix child_process 'spawn' not ready to send messages on ESM

[ErickWendel]

BTW: I'd like to help fixing this bug 🤩

Is your feature request related to a problem? Please describe.

There's a current issue on spawn's child processes using ES modules mentioned in those issues (#37782, #39140, #39140, #34785, and help/issues/1383 ).

When you fork a file and immediately use the .send event. The child process doesn't receive messages because it's not ready yet.

So we need to make a workaround. The child emits an event, the parent waits for .on('message', we check the message and then we start sending messages to the child.

Describe the solution you'd like
My plan initially was to add an event like child.on("ready to specify when we can start sending messages to the child process.

I spoke to @addaleax and she suggested a behavior that is closer to what MessagePorts expose, which is queueing up messages until a message listener is installed.

I know that it would increase implementation complexity a bit, but it would be very helpful for avoiding pitfalls like the ones linked here

What do you think? Any ideas of what could we do?

[Trott]

Seems like a fine idea to me but would want to hear what @nodejs/modules and/or @nodejs/child_process folks might have to say about it.

[bmeck]

I'd prefer if we could queue things because this problem isn't unique to ESM and I've seen it in the long past using purely CJS and callbacks.

[ErickWendel]

I'd prefer if we could queue things because this problem isn't unique to ESM and I've seen it in the long past using purely CJS and callbacks.

I think I didn't get it. What do you mean by queue things?

[gabrielsimas]

I'd prefer if we could queue things because this problem isn't unique to ESM and I've seen it in the long past using purely CJS and callbacks.

I don't get it too... do you can explain please?

[bmeck]

@gabrielsimas MessagePort will queue messages (events) and wait for a listener before draining the messages. The problem in this issue is that event listeners which are used in child_process are not queueing messages and fire messages that the child process has loaded prior to the child process attaching any listeners to handle events.

[weritontmachado]

I spoke to @addaleax and she suggested a behavior that is closer to what MessagePorts expose, which is queueing up messages until a message listener is installed.

This was my first thought

[ErickWendel]

@gabrielsimas MessagePort will queue messages (events) and wait for a listener before draining the messages. The problem in this issue is that event listeners which are used in child_process are not queueing messages and fire messages that the child process has loaded prior to the child process attaching any listeners to handle events.

niiice! It's the same strategy @addaleax suggested on the post, right?

[ktfth]

You have a reproduction of the behavior @ErickWendel?

[ktfth]

Or the behavior is the same on the related content you shared?

[ErickWendel]

Or the behavior is the same on the related content you shared?

Yes, Actually they already had reproduced the behavior at all of the issues I mentioned in the post. See this one #39140

[mcollina]

I'd prefer if we could queue things because this problem isn't unique to ESM and I've seen it in the long past using purely CJS and callbacks.

This is the way to go

[Peeja]

For future searchers: This has been resolved and released in v17.4.0. (🎉)

[Methuselah96]

Also released in 16.14.0 if you're on LTS.