Sitelet https://github.com/nodejs/node/issues/34785
Skip to content

fork child process does not receive message event when using ES modules #34785

Closed
Closed
fork child process does not receive message event when using ES modules#34785
Labels
child_processIssues and PRs related to the child_process subsystem.esmIssues and PRs related to the ECMAScript Modules implementation.wontfixIssues that will not be fixed.

Description

@dheeraj-br
dheeraj-br
opened on Aug 15, 2020
  • Version: v14.8.0
  • Platform: windows 10 pro 64 bit
  • Subsystem: ES6 module

steps to reproduce the bug

im trying to have two way messaging between child process and the parent process, the parent process works as expected it sends and receives messages from child process, but the child process is only able to send message and not receive them

ive enabled ES6 modules from package.json using this key-value "type": "module"

app.js

import { fork } from 'child_process';

const child = fork('./script.js');

child.on('message', msg => console.log('msg from child :', msg));

child.send({ msg: 'parent' }, () => console.log('parent sent msg'));

script.js


process.on('message', msg => console.log('msg from parent :', msg));

process.send({ msg: 'child' }, () => console.log('child sent msg'));

expected behavior

parent sent msg
msg from child : { msg: 'child' }
msg from parent : { msg: 'parent' }
child sent msg

output recieved

parent sent msg
msg from child : { msg: 'child' }
child sent msg

Additional information

this issue is not present while using commonJs modules (ie: require('child_process') )

same issue closed without bug fix, nodejs/help#1383
although the workaround would be to call the child.send in the next tick like shown below, this is not homologous to the commonJs behavior

setTimeout(() => {
    child.send({ msg: 'parent' }, () => console.log('parent sent msg'));
}, 0);

Activity

  1. dheeraj-br
    changed the title [-]fork child process does not receive message event when using ES6 modules[/-] [+]fork child process does not receive message event when using ES modules[/+] on Aug 15, 2020

  2. bnoordhuis commented on Aug 15, 2020

    @bnoordhuis
    bnoordhuis
    on Aug 15, 2020
    Member

    I think that's explained by the asynchronous nature of ES module loading.

    Your script has a race condition, it's just less likely to trigger with (synchronous) CJS. The solution is to have the child notify the parent that it's ready for operation and only then send the message.

    I'm going to close this as a wontfix but I can move the issue to nodejs/help if you have follow-up questions.

  3. bnoordhuis
    closed this as completedon Aug 15, 2020
  4. bnoordhuis
    added
    child_processIssues and PRs related to the child_process subsystem.
    esmIssues and PRs related to the ECMAScript Modules implementation.
    wontfixIssues that will not be fixed.
    on Aug 15, 2020

  5. dheeraj-br commented on Aug 16, 2020

    @dheeraj-br
    dheeraj-br
    on Aug 16, 2020
    Author

    @bnoordhuis i believe there is no built-in event to notify the parent, cluster has 'online' event to notify when a fork process has started. a feature where maybe the child process can implicitly emit an event once its started, if that's plausible i can follow-up, if not we can end this here.

  6. bnoordhuis commented on Aug 16, 2020

    @bnoordhuis
    bnoordhuis
    on Aug 16, 2020
    Member

    Not built-in, no, but there couldn't be - node doesn't know when your script is ready. Call process.send('ready') in the child and make the parent wait for that event before it starts sending messages of its own.

  7. joaotavora commented on Mar 20, 2021

    @joaotavora
    joaotavora
    on Mar 20, 2021

    Call process.send('ready') in the child and make the parent wait for that event before it starts sending messages of its own.

    Right. This works. But I think at least one should update the documentation (https://nodejs.org/api/child_process.html#child_process_subprocess_send_message_sendhandle_options_callback) to call attention to this very elusive gotcha.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Metadata

Metadata

Assignees

No one assigned

    Labels

    child_processIssues and PRs related to the child_process subsystem.esmIssues and PRs related to the ECMAScript Modules implementation.wontfixIssues that will not be fixed.

    Type

    No type

    Projects

    No projects

      Milestone

      No milestone

      Relationships

      None yet

      Development

      No branches or pull requests

      Issue actions