Race between the outputs' and the engine's end-of-test finalization

[na--]

If you have a script like that:

import { Counter } from 'k6/metrics';
import http from 'k6/http';
import { sleep } from 'k6';

var teardownCounter = new Counter('teardown_counter');

export const options = {
    vus: 1,
    iterations: 1,
};

export function teardown() {
    http.get('https://test.k6.io/?stage=teardown-start');
    teardownCounter.add(10);
    sleep(8);
    http.get('https://test.k6.io/?stage=teardown-middle');
    teardownCounter.add(10);
    sleep(8);
    http.get('https://test.k6.io/?stage=teardown-end');
    teardownCounter.add(10);
}

export default function () {
    http.get('https://test.k6.io/?stage=default');
}

some times, k6 won't emit the metrics for https://test.k6.io/?stage=teardown-end or the last teardown_counter=10 count. This seems to happen because the Engine's finalization code (pushing its last buffered metrics) and the outputs' finalization code (pushing their last metrics) are triggered by cancelling the same context...

They both output as:
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/cmd/run.go#L154-L169
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/cmd/run.go#L268
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/core/engine.go#L156

Here's the output path:
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/core/engine.go#L170-L176
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/stats/cloud/collector.go#L218-L220
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/stats/cloud/collector.go#L261-L263

Here's the Engine's:
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/core/engine.go#L178-L183
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/core/engine.go#L295-L297
https://github.com/loadimpact/k6/blob/c68a4eb450aa0a806b16bef3206ba2163f05e2a7/core/engine.go#L252-L266

Basically, this Engine.processSamples() is racing with the output's finalization - the engine may push some metrics to the outputs after they have stopped listening for them...

[na--]

This is actually a big problem, since k6 run --out cloud might exit before it properly calls the TestFinished cloud API, making the test time out, since for the cloud it seems like k6 just disappears 😱 https://github.com/loadimpact/k6/blob/62665cfa87899ae210578276d7e16b4f4dd8fd7c/stats/cloud/collector.go#L636-L646