fix(@angular/build): avoid race condition in sass importer

[hborchardt]

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: #27167

Builds under load sometimes fail with "Can't find stylesheet to import" errors.

I was able to reproduce this error in my CI pipeline consistently, by running 4 npx ng build builds in parallel on an 8 core VPS. It also occurs sometimes during regular builds/ng test, especially when other concurrent jobs put load on the system.

By adding lots and lots of logging, I found the root cause, which is a race condition when the sass worker manages to call findFileUrl again in between Atomics.store(..., 0, 1) and Atomics.notify(..., 0) invocations - I left more details as a comment.

What is the new behavior?

The race no longer causes failing builds. After being notified, the worker checks again that the signal is not set to 0, and waits for another notify if necessary, which will be the correct one.

Does this PR introduce a breaking change?

• Yes
• No

Other information

I verified that the second Atomics.wait(..., 0, 0) actually does something meaningful, by logging the return value. Under load, I observed two instances where the return value was "ok", indicating that the signal was 0 at the time of the second wait, and the build would therefore have aborted without the change.

[jkrems]

The changes were merged into the following branches: main, 18.2.x

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