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]

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[liuzix]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5988080

[liuzix]

/run-integration-tests
/run-unit-tests