Inconsistent behavior of nextTick and queueMicrotask

[mcollina]

Consider this snippet:

const { EventEmitter } = require('node:events')

function run (name) {
  return new Promise((resolve) => {
    const e = new EventEmitter()
    process.nextTick(() => {
      e.emit('error', new Error())
    });
    resolve(e)
  }).then(((e) => {
    e.on('error', err => {
      console.error(`### ${name} ###`, err)
    })
  }))
}

queueMicrotask(run.bind(null, 'queueMicrotask'))
setImmediate(run.bind(null, 'setImmediate'))

This results in

### queueMicrotask ### Error
    at /Users/matteo/c.cjs:7:23
    at process.processTicksAndRejections (node:internal/process/task_queues:77:11)
node:events:492
      throw er; // Unhandled 'error' event
      ^

Error
    at /Users/matteo/c.cjs:7:23
    at process.processTicksAndRejections (node:internal/process/task_queues:77:11)
Emitted 'error' event at:
    at /Users/matteo/c.cjs:7:9
    at process.processTicksAndRejections (node:internal/process/task_queues:77:11)

Node.js v20.10.0

Why is this an issue? In most part of Node.js core,
we use process.nextTick(() => ee.emit('error')) to let users install event handlers.
However, if an EventEmitter is returned by an async function, there is a significant possibility that the error could not be caught.

(This is essentially a problem for Node.js streams).

[mcollina]

@jasnell what do you think?

[ronag]

Just to expand a bit. There is an assumption that process.nextTick tasks always run before micro-tasks. However, as demonstrated above. This is a false assumption. The reality is more inconsistent and depends whether or not the caller is inside our outside a micro tick.

[ronag]

I'm adding this to the TSC agenda for visibility and help with triaging the severity of the issue.

[ronag]

Would probably be a good idea to update https://nodejs.org/en/guides/event-loop-timers-and-nexttick to also include promises and microticks.

[joyeecheung]

It's not entirely clear to me what is the expected output in the OP, did you mean to say that you expect a ### setImmediate ### Error log in addition to the queueMicrotask one? Or did you mean to expect to see just the setImmediate one?

[ronag]

It's not entirely clear to me what is the expected output in the OP, did you mean to say that you expect a ### setImmediate ### Error log in addition to the queueMicrotask one? Or did you mean to expect to see just the setImmediate one?

The expectation is that:

queueMicrotask(run.bind(null, 'queueMicrotask'))

and

setImmediate(run.bind(null, 'setImmediate'))

Should have the same output, which they don't.

[joyeecheung]

You mean neither of them should log anything or they should both log or either is fine as long as they are the same? I am guessing from the OP (in the context of error handling) that you want both to log?

[ronag]

I think you could argue that either behavior is correct (or at least not incorrect).

The problem here is that they are not consistent.

[NiharPhansalkar]

Hello @ronag, I would like to continue working on this issue, however, as mentioned on your PR, I guess a simple search and replace won't work?
I am actually new to contributing to NodeJS, so I am a little confused as to what we can implement here. If you could give me some pointers, it would be of great help.

[ronag]

@NiharPhansalkar I don't think this is a good starter issue. Not even us in the technical steering are sure what to do about this issue.

[mcollina]

@benjamingr here is the patch that fixes the behavior... but it breaks a lot of tests:

diff --git a/lib/internal/process/task_queues.js b/lib/internal/process/task_queues.js
index bcb5eef841..04bdb07122 100644
--- a/lib/internal/process/task_queues.js
+++ b/lib/internal/process/task_queues.js
@@ -67,6 +67,7 @@ function runNextTicks() {
 function processTicksAndRejections() {
   let tock;
   do {
+    runMicrotasks();
     while ((tock = queue.shift()) !== null) {
       const asyncId = tock[async_id_symbol];
       emitBefore(asyncId, tock[trigger_async_id_symbol], tock);
@@ -92,7 +93,6 @@ function processTicksAndRejections() {

       emitAfter(asyncId);
     }
-    runMicrotasks();
   } while (!queue.isEmpty() || processPromiseRejections());
   setHasTickScheduled(false);
   setHasRejectionToWarn(false);