util/async: avoid blocking Append on Exec cancellation

[lcwangchao]

What

Backport #2034 to release-nextgen-202603.

This fixes the RunLoop cancellation race from #2033, where Append could block indefinitely if a waiting Exec returned on ctx.Done() after Append had already committed to notify through ready.

Why

The branch still has the pre-fix util/async/runloop.go implementation and is therefore still affected by the race.

Tests

• go test ./util/async

Cherrypick

• 392411f

Summary by CodeRabbit

• Bug Fixes
  • Improved reliability when a running task is canceled while new work is being queued. Cancellation now returns the appropriate error, queued work remains available for a subsequent run, and adding work does not remain blocked in this race condition. These changes also make concurrent execution behavior more consistent.

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📥 Commits

Reviewing files that changed from the base of the PR and between 6e3fd8a and d0826bd.

📒 Files selected for processing (2)

• util/async/runloop.go
• util/async/runloop_test.go

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