events: timing issue awaiting multiple events emitted in same nextTick

[jasnell]

Just leaving this here as an issue worth investigating. Not sure there's anything we can do about it other than documenting it...

Multiple events emitted on the same process.nextTick() may be missed when using await once(). For example, when running the following, both the 'bar' and 'foo' events are emitted but the async function foo() never completes because await once(myEE, 'foo') occurs after the foo event is actually emitted.

'use strict';

const { EventEmitter, once } = require('events');

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

setTimeout(() => {}, 1000);

The only way to catch both events is to use Promise.all() or Promise.allSettled(), or to not use await with once(myEE, 'bar')

'use strict';

const { EventEmitter, once } = require('events');

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  await once(myEE, 'foo');
  console.log('foo');
}

async function foo2() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo2().then(() => console.log('done'));

setTimeout(() => {}, 1000);

Why does this happen? It is because the process.nextTick is processed before the microtask queue. Both events are emitted synchronously before the microtask queue is processed. The await once(myEE, 'bar') does not continue until the microtask queue is executed, after the 'foo' event is emitted, so await once(myEE, 'foo') ends up waiting forever, having missed the actual event.

[mcollina]

I agree that this looks like a problem to investigate. I don’t think we have a solution, or a solution is possible without a perf cost.

[addaleax]

I don’t think there’s anything we can do about this besides maybe documenting this – I would say it’s expected behaviour that can still be surprising sometimes.

[jasnell]

Yeah, there's really nothing that can be done without a major refactor or a major perf hit. I think documenting is the only way to go

[hassaanp]

Hi,

I was wondering where in the docs should this go. If someone can point me to the appropriate section, I'd be happy to log this behavior.

[jasnell]

@hassaanp ... my apologies for not seeing your comment earlier. This should likely best be added as a subsection to the documentation for the once function in the doc/api/events.md file.

Note that the same issue arises when using the on() function in a for await loop. The only solution there is that any event handlers for other events you may be interested in must be attached before entering the for await loop or they will end up being lost.