coordinator: retry SetChangefeedProgress on etcd CAS conflict

[wlwilliamx]

What problem does this PR solve?

Issue Number: close #4049

What is changed and how it works?

Retry EtcdBackend.SetChangefeedProgress when etcd ModRevision CAS fails due to concurrent writers updating the same key, so admin APIs (remove/pause) won’t flake with ErrMetaOpFailed. Also adds a unit test covering a CAS-conflict-then-success retry.

Check List

Tests

• Unit test
• Manual test

Questions

Will it cause performance regression or break compatibility?

No. The retry loop is bounded and only triggers on CAS conflicts; the common path is unchanged.

Do you need to update user documentation, design documentation or monitoring documentation?

N/A

Release note

Fix flaky changefeed remove/pause failures caused by etcd CAS conflicts when persisting changefeed progress.

[gemini-code-assist]

Summary of Changes

Hello @wlwilliamx, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness of changefeed administration by introducing a retry mechanism for SetChangefeedProgress operations that interact with etcd. By gracefully handling concurrent write conflicts on etcd keys, it eliminates a source of flakiness in critical user-facing APIs like changefeed removal and pausing, ensuring more reliable system behavior.

Highlights

• Retry Mechanism for Etcd CAS Conflicts: Implemented a retry loop in the SetChangefeedProgress function to gracefully handle etcd Compare-And-Swap (CAS) conflicts that occur due to concurrent writes.
• Improved Reliability of Admin APIs: This change prevents ErrMetaOpFailed errors, which previously caused flakiness in changefeed administration APIs such as remove and pause.
• Unit Test for Retry Logic: A new unit test, TestSetChangefeedProgressRetriesOnCASConflict, has been added to verify the correct behavior of the retry mechanism when a CAS conflict occurs and is subsequently resolved.

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[wlwilliamx]

/test all

[gemini-code-assist]

Code Review

This pull request introduces a retry mechanism for SetChangefeedProgress when an etcd Compare-And-Swap (CAS) operation fails due to concurrent updates. This significantly improves the robustness of admin APIs like remove and pause by preventing ErrMetaOpFailed flakiness. A dedicated unit test has been added to cover the CAS conflict and successful retry scenario, which is excellent. The retry logic correctly incorporates a maximum number of attempts and a delay with context cancellation. Overall, this is a valuable improvement for the stability of the system.

[wlwilliamx]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tenfyzhong, wk989898

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Details

Needs approval from an approver in each of these files:

• OWNERS [tenfyzhong,wk989898]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-29 04:26:26.042716923 +0000 UTC m=+1245613.656673779: ☑️ agreed by wk989898.
• 2026-01-30 10:55:53.953213423 +0000 UTC m=+1355381.567170269: ☑️ agreed by tenfyzhong.