--watch mode disables IPC communication between a spawning process and the actual child process

[znewsham]

Version

v18.17.1

Platform

Linux zacknewsham-xps 6.2.0-33-generic #33~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Sep 7 10:33:52 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

No response

What steps will reproduce the bug?

This isn't quite a bug, nor is it quite a feature request, but somewhere in between when using watch mode.

in parent.js

import { spawn } from 'child_process';
spawn(
  'node',
  '--watch',
  'child.js'
);

child.sendMessage({ type: "whatever" }, console.log);

in child.js

process.on("message", console.log);

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

I expect to be able to communicate with a spawned child process, even when using --watch.

What do you see instead?

Because the --watch mode uses IPC itself (to listen to messages from the child process) it doesn't pass through messages from the child to the parent, or the parent to the child.

Additional information

As I said, this isn't quite a bug - but not quite a feature request either. I believe the fix is relatively simple, if the process has IPC enabled, then pass through messages (that aren't watch related) from the child -> parent and all messages from parent -> child

[znewsham]

Ok, it turns out the fix is extremely simple, I'm happy to open a PR for this, I'm just not sure how to go about testing it. I think it's probably fine to require the parent process filter out unrelated (e.g., messages with watch: prefixed keys) IPC messages.

in 'lib/internal/files_watcher.js', add:

  #setupIPC(child) {
    process.on("message", message => child.send(message));
    child.on("message", message => process.send(message));
  }

then modify constructor to have:

  #wantsPassthroughIPC = false;

  constructor({ throttle = 500, mode = 'filter' } = kEmptyObject) {
    super();
    this.#wantsPassthroughIPC = !!process.send;

then modify watchChildProcessModules to have:

  watchChildProcessModules(child, key = null) {
    if (this.#wantsPassthroughIPC) {
      this.#setupIPC(child);
    }

[znewsham]

@MoLow not sure what your thoughts are on this

[MoLow]

Sound like a great solution,
for testing I would just add an extra case to https://github.com/nodejs/node/blob/main/test/sequential/test-watch-mode.mjs
with a fixture printing messages, validating it receives messages