pkg/workerpool(cdc): fix race between handling error and unregistering handler (#4605)

[ti-chi-bot]

This is an automated cherry-pick of #4605

What problem does this PR solve?

Issue Number: close #4447

What is changed and how it works?

• Fixed race between handling error and unregistering handler in pkg/workerpool. The root cause is that, if an error has already caused a handler to be automatically unregistered, Unregister() will return immediately, regardless of whether the error handler has returned. This has caused race in unified sorter.

Check List

Tests

• Unit test
• Integration test

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

Release note

Fix stability problem in workerpool, which is used by Unified Sorter.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.