Fix: ESLint 9+ path: handle child script errors in whitespace-async.js

[dataCenter430]

Summary

When CLIEngine is not available (ESLint 9+), whitespace.js loads config by running whitespace-async.js with execSync and parsing its stdout as JSON. The async script had no .catch() on its promise, so rejections became unhandled: the child could exit non-zero and print to stderr while the parent received empty or partial stdout and failed with a generic JSON.parse error, hiding the real failure.

Approach

• Do not add try/catch in whitespace.js; errors should still crash.
• Fix the child script so the ESLint 9/10 path fails in a clear, diagnosable way.

Changes

• packages/eslint-config-airbnb-base/whitespace-async.js: Add .catch() so that on rejection we log the error to stderr and process.exit(1).
• packages/eslint-config-airbnb/whitespace-async.js: Same change.

On success, only JSON is written to stdout, so the parent’s execSync + JSON.parse continues to work. On failure, the real error goes to stderr and the process exits 1, so execSync throws and the thrown error includes the child’s stderr, making the root cause visible without masking crashes.

Fixes #3238

[dataCenter430]

Hey, @ljharb
I'm getting the CI testing fail, even though there is no conflict.
could you pls review the PR?
Thank you

[ljharb]

All this does is exit when there’s an error - i thought the issue was an eslint 9+ bug that could be fixed?

[dataCenter430]

hey, @ljharb
thank you for your review.

All this does is exit when there’s an error - i thought the issue was an eslint 9+ bug that could be fixed?

I understood you. you really want fix the real ESLint 9+ bug so the config loads correctly; not just better behavior when it errors, right?
you’re right that the goal is to fix the underlying ESLint 9+ bug.
and pls don't close the PR. actually this is my first PR to contribute to Javascript and it's so important for me.
okay, I'll address this for sure

[dataCenter430]

Hey, @ljharb
I fixed the underlying ESLint 9+ issue by making the async whitespace path run in legacy (eslintrc) mode and using the legacy ESLint class when available.
The parent now spawns whitespace-async.js with ESLINT_USE_FLAT_CONFIG=false, and the child uses loadESLint({ useFlatConfig: false }) (falling back to ESLint/calculateConfigForFile where needed) so our existing baseConfig is valid.
The result is that the ESLint 9+ path now successfully loads the whitespace config and still crashes on real errors instead of just exiting.

could you pls review the PR again? 🙏
Best regards.