How to propagate background job failures?

[milang]

Yarn's portable shell supports running tasks in parallel with the task & syntax. This works great when tasks succeed:

// package.json
{
  "name": "yarn-test",
  "packageManager": "yarn@4.6.0",
  "scripts": {
    "thing-serial": "run thing:1 && run thing:2",
    "thing-parallel": "run thing:1 & run thing:2 &",
    "thing:1": "echo 'thing one'",
    "thing:2": "echo 'thing two'"
  }
}

---

However, when the background jobs fail, the failures are ignored and yarn run succeeds anyway:

// package.json
{
  "name": "yarn-test",
  "packageManager": "yarn@4.6.0",
  "scripts": {
    "thing-serial": "run thing:1 && run thing:2",
    "thing-parallel": "run thing:1 & run thing:2 &",
    "thing:1": "echo 'thing one'; exit 1",
    "thing:2": "echo 'thing two'; exit 2"
  }
}

Notice how yarn run thing-serial correctly propagates exit 1 from thing:1. However, yarn run thing-parallel ignores both non-zero exit codes and succeeds.

Is there a way to propagate failures from background jobs in yarn? If not, what are the intended use-cases for this feature? The fact that they always succeed disqualifies them from many scenarios IMO (for example running multiple linters in parallel).

[Labeeb2339]

The current portable shell treats & as asynchronous for status purposes: it waits for background jobs before resolving, but a background job's non-zero exit is reported and the overall shell result remains 0. Yarn's current shell tests codify that behavior, and there is no wait/status-aggregation hook exposed by the portable shell.

For CI, either chain the checks (yarn lint:a && yarn lint:b) or use a runner/script that owns the child processes and exits non-zero if any failed. & alone cannot make yarn run propagate those failures.

Refs: current background-job tests and portable-shell implementation.