Worker Threads can't read stdout

[Ethan-Arrowood]

• Version: 11.2.0
• Platform: Darwin Ethans-MacBook-Pro.local 18.0.0 Darwin Kernel Version 18.0.0: Wed Aug 22 20:13:40 PDT 2018; root:xnu-4903.201.2~1/RELEASE_X86_64 x86_64
• Subsystem:

Hard to describe this bug but I'd like to document the strange behaviour I am experiencing with this experimental feature.

const {
  Worker, isMainThread, parentPort, workerData, theadId, MessageChannel
} = require('worker_threads')

const DATA = [
  7, 12, 19, 3, 18, 16, 4, 2,
  6, 9, 18, 38, 15, 8, 123, 72,
  4, 3, 11, 33, 15, 48, 13, 17,
  23, 19, 30, 45, 31, 1, 14, 50
]

const TYPE_SORT_THREAD = 'sort_thread'

if (isMainThread) {
  console.log("I'm the main thread")

  // Split data
  const x = DATA.slice(0, DATA.length/2)
  const y = DATA.slice(DATA.length/2, DATA.length)

  const sortedArrays = []

  function sortThreadCb(sortedArray) {
    console.log('Sorted array: ', sortedArray)
    sortedArrays.push(sortedArray)
  }

  for (var subArray of [x, y]) {
    let sortThread = new Worker(__filename, { workerData: {
      array: subArray,
      type: TYPE_SORT_THREAD
    }})
    sortThread.on('message', sorted => sortThreadCb(sorted))
  }


} else {
  console.log("I'm a sub thread")

  if ( workerData.type === TYPE_SORT_THREAD) {
    console.log("I'm a sort thread")
    let { array } = workerData
    array.sort((a, b) => a - b)
    // console.log('foobar') <-- REFERENCE THIS LINE
    parentPort.postMessage(array)
  } 
}

In the above code snippet, the expected output looks something like so (due to the way threading works sometimes the console is out of order but I do not think this is relevant):

I'm the main thread
I'm a sub thread
I'm a sort thread
Sorted array:  [ 2, 3, 4, 6, 7, 8, 9, 12, 15, 16, 18, 18, 19, 38, 72, 123 ]
I'm a sub thread
I'm a sort thread
Sorted array:  [ 1, 3, 4, 11, 13, 14, 15, 17, 19, 23, 30, 31, 33, 45, 48, 50 ]

However, with the console.log('foobar') line commented out I get this output (and error message instead):

Ethans-MacBook-Pro:lab5 ethanarrowood$ node --experimental-worker lab5.js
I'm the main thread
I'm a sub thread
Sorted array:  [ 2, 3, 4, 6, 7, 8, 9, 12, 15, 16, 18, 18, 19, 38, 72, 123 ]
I'm a sub thread
I'm a sort thread
Sorted array:  [ 1, 3, 4, 11, 13, 14, 15, 17, 19, 23, 30, 31, 33, 45, 48, 50 ]
internal/worker.js:346
        return this[kParentSideStdio][stream].push(chunk, encoding);
                                     ^

TypeError: Cannot read property 'stdout' of null
    at Worker.[kOnMessage] (internal/worker.js:346:38)
    at MessagePort.Worker.(anonymous function).on (internal/worker.js:279:57)
    at MessagePort.emit (events.js:182:13)
    at MessagePort.onmessage (internal/worker.js:84:8)

The error trace does not have a line from my lab5.js to reference.

The actual output when console.log('foobar') is active is:

I'm the main thread
I'm a sub thread
Sorted array:  [ 2, 3, 4, 6, 7, 8, 9, 12, 15, 16, 18, 18, 19, 38, 72, 123 ]
I'm a sub thread
Sorted array:  [ 1, 3, 4, 11, 13, 14, 15, 17, 19, 23, 30, 31, 33, 45, 48, 50 ]
I'm a sort thread
foobar
I'm a sort thread
foobar

Order does not seem perfectly correct but I don't think that is an issue AFAIK.

I'm not exactly expecting a solution to this just wanted to bring this behavior to whichever contributors are working on this part of Node JS

[Trott]

@nodejs/workers

[benjamingr]

@Ethan-Arrowood Thanks for the report! We're taking a look. I think for the very least the error message should be better.

[yaelhe]

I've figured out what's happening and have a fix coming up