Race conditions between setTimeout() & setImmediate() #48163

Test

test-timers-immediate-queue.js

Platform

Linux x64

Console output

02:49:14 not ok 2722 parallel/test-timers-immediate-queue
02:49:14   ---
02:49:14   duration_ms: 140.40000
02:49:14   severity: fail
02:49:14   exitcode: 1
02:49:14   stack: |-
02:49:14     hit 20
02:49:14     node:assert:125
02:49:14       throw new AssertionError(obj);
02:49:14       ^
02:49:14     
02:49:14     AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:
02:49:14     
02:49:14     20 !== 10
02:49:14     
02:49:14         at process.<anonymous> (/home/iojs/build/workspace/node-test-commit-linux/test/parallel/test-timers-immediate-queue.js:56:10)
02:49:14         at process.emit (node:events:523:35) {
02:49:14       generatedMessage: true,
02:49:14       code: 'ERR_ASSERTION',
02:49:14       actual: 20,
02:49:14       expected: 10,
02:49:14       operator: 'strictEqual'
02:49:14     }
02:49:14     
02:49:14     Node.js v21.0.0-pre

Build links

• https://ci.nodejs.org/job/node-test-commit-linux/nodes=fedora-last-latest-x64/52202

Additional information

This seems to be some kinda potential race condition between setTimeout() and setImmediate() ?

For the following code, it seems TIMEOUT or IMMEDIATE either could be printed first, since over a few runs the order will flip.

setTimeout(() => {
    console.log('TIMEOUT')
}, 1);
setImmediate(() => {
    console.log('IMMEDIATE');
})

However on my machine, the following code will always result in TIMEOUT being printed first - the while loop strategy is what the test uses

setTimeout(() => {
    console.log('TIMEOUT')
}, 1);
const now = Date.now();
while (Date.now() - now < 2);
setImmediate(() => {
    console.log('IMMEDIATE');
})

I was wondering: is it possible due to some conditions on the host system, for IMMEDIATE to print before TIMEOUT in the second case?

I do not know much about the Event Loop internals, but from some rough reading, setTimeout() will put calbacks in the timers queue, setImmediate() in the I/O queue, and the timers are always checked first (https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick)

But if in the first case (setTimeout without while waiting loop) it is non-predictable, I wonder if there are other factors at play, which make the test flaky.