fix: When getting the key in the slot where migration failed, codis-p…

[chejinge]

[…roxy exited abnormally](#3205)

Summary by CodeRabbit

• Bug Fixes
  • Enhanced error handling for backend connections to properly detect and log warnings when slot migration is disabled, ensuring correct connection state management.

[coderabbitai]

📝 Walkthrough

Walkthrough

The changes refactor error response byte slice declarations and introduce a new error sentinel errRespSlotMigrateClosed with corresponding error-handling logic in BackendConn.loopReader to detect and handle slot migration disabled conditions by transitioning connection state.

Changes

Cohort / File(s)	Summary
Backend Error Handling codis/pkg/proxy/backend.go	Reformatted package-level error response declarations, added new errRespSlotMigrateClosed sentinel for "ERR please open slotmigrate and reload slot", and extended error dispatch in loopReader to detect this error and transition connection state from stateConnected to stateDataStale with warning log.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 A new sentinel hops into view,
Slot migration errors, now we're through!
When stateConnected needs a change,
We hop to staleData, rearrange,
CompareAndSwap ensures the way,
Error handling saves the day! 🎯

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes a bug fix related to slot migration failures in codis-proxy, which aligns with the code changes that add error handling for slot migration state transitions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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