api: return 400 instead of 500 on duplicate changefeed creation

[rajneesh2k10]

ErrChangeFeedAlreadyExists was not in the httpBadRequestError registry, so the v2 CreateChangefeed handler returned HTTP 500 when a client posted a changefeed ID that already existed. A duplicate-create is a client-side condition and should be a 4xx.

Adds the error to the existing bad-request registry, mirroring how ErrChangeFeedNotExists is already classified, and adds a TestErrorHandleMiddleware regression test.

close #5427

What problem does this PR solve?

Issue Number: close #5427

The v2 POST /api/v2/changefeeds endpoint returned HTTP 500 when the requested changefeed ID already existed, because
ErrChangeFeedAlreadyExists was not registered in the middleware's bad-request error list. Clients could not distinguish
this duplicate condition from a real server fault.

What is changed and how it works?

• pkg/api/util.go: add ErrChangeFeedAlreadyExists to httpBadRequestError, grouped next to ErrChangeFeedNotExists. The
  existing IsHTTPBadRequestError matcher already handles direct equality, RFC code, and wrapped errors, so no other plumbing
  is needed.
• api/middleware/middleware_test.go: add TestErrorHandleMiddleware pinning the 200 / 400 / 500 buckets, including the
  duplicate-changefeed → 400 case as a regression guard.

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

Changes the HTTP response code for one error case from 500 to 400. Clients that previously treated the 500 as a transient
server fault and retried will now see a 4xx and (correctly) stop retrying. No wire-format or API-shape changes.

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

No.

Release note

Fix `POST /api/v2/changefeeds` returning HTTP 500 instead of 400 when the changefeed ID already exists.

Summary by CodeRabbit

• Bug Fixes
  • Improved API error handling to return appropriate HTTP 400 status codes for invalid requests, including scenarios involving duplicate changefeeds.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: ce1f5744-8fdc-4362-ba8a-05585409dfbf

📥 Commits

Reviewing files that changed from the base of the PR and between 50cfdb0 and 450b5e7.

📒 Files selected for processing (2)

• api/middleware/middleware_test.go
• pkg/api/util.go

---

📝 Walkthrough

Walkthrough

pkg/api/util.go adds cerror.ErrChangeFeedAlreadyExists to the httpBadRequestError list so IsHTTPBadRequestError now returns true for duplicate changefeed errors. A new test TestErrorHandleMiddleware in api/middleware/middleware_test.go validates the full HTTP status code mapping of ErrorHandleMiddleware.

Changes

ErrChangeFeedAlreadyExists HTTP 400 fix and test

Layer / File(s)	Summary
Bad-request classification fix and middleware test pkg/api/util.go, api/middleware/middleware_test.go	httpBadRequestError gains cerror.ErrChangeFeedAlreadyExists, making IsHTTPBadRequestError return true for duplicate changefeed errors. TestErrorHandleMiddleware covers 400 for invalid param and duplicate changefeed, 200 for no error, and 500 for unclassified errors.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Suggested labels

size/XS

Suggested reviewers

• tenfyzhong
• asddongmen

Poem

🐰 A changefeed twice? That's quite a blunder,
No more five-hundreds shall tear asunder!
Four hundred now greets the duplicate call,
A tiny fix, but it fixes it all.
Hop hop, the rabbit checks the test — hooray! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: correcting HTTP response code from 500 to 400 for duplicate changefeed creation, which aligns with the primary objective of the PR.
Description check	✅ Passed	The description comprehensively covers the problem statement, solution details, testing approach, compatibility considerations, and includes a release note. All required sections from the template are addressed.
Linked Issues check	✅ Passed	The PR fully addresses issue #5427 by registering ErrChangeFeedAlreadyExists in the bad-request error list and adding a regression test that validates the 400 response for duplicate changefeeds.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the HTTP 500 to 400 response code issue: adding the error to the registry and testing the fix. No unrelated modifications were introduced.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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All committers have signed the CLA.

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Code Review

This pull request adds cerror.ErrChangeFeedAlreadyExists to the list of errors mapped to an HTTP 400 Bad Request status in pkg/api/util.go. It also introduces a new unit test TestErrorHandleMiddleware in api/middleware/middleware_test.go to verify the error handling middleware's behavior under different scenarios. There are no review comments, so I have no feedback to provide.

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