feat(webapp,redis): handle UNBLOCKED during ElastiCache role change

[ericallam]

Summary

When ElastiCache demotes a primary to replica — during a Multi-AZ failover or a vertical node-type change — the demoting primary issues an UNBLOCKED reply to any in-flight blocking commands (BLPOP, BRPOP, BLMOVE, XREADGROUP ... BLOCK, etc.) to clear them before the role flips. ioredis surfaces these as ReplyError to caller code.

The shared defaultReconnectOnError added in #3548 only matches READONLY and LOADING. This extends it to UNBLOCKED so the disconnect-reconnect-retry cycle handles BLPOP-shaped errors the same way the existing two cases handle non-blocking-command errors.

Fix

export function defaultReconnectOnError(err: Error): boolean | 1 | 2 {
  const msg = err.message ?? "";
  if (
    msg.startsWith("READONLY") ||
    msg.startsWith("LOADING") ||
    msg.startsWith("UNBLOCKED")
  ) {
    return 2;
  }
  return false;
}

Returning 2 tells ioredis to disconnect, reconnect, and re-issue the command. For a BLPOP that means a fresh BLPOP against the new primary instead of the UNBLOCKED error escaping to the caller.

Test plan

• CI green
• Trigger a Multi-AZ failover or a vertical scale event on an ElastiCache replication group whose clients are running blocking commands and confirm no UNBLOCKED errors surface to caller code during the cutover.

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Latest commit: 7e49f08

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[coderabbitai]

Walkthrough

This PR extends the ioredis reconnect-on-error hook to recognize UNBLOCKED responses alongside existing READONLY and LOADING prefixes. The defaultReconnectOnError function is updated to return 2 when any of these three error prefixes are detected, enabling blocking Redis commands like BLPOP to transparently recover when ElastiCache forces unblocking during node role transitions. A changelog entry documents this enhancement.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: extending error handling to include UNBLOCKED errors during ElastiCache role changes, matching the core objective of the PR.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description provides comprehensive context, including the problem (UNBLOCKED errors during ElastiCache failover), the fix implementation, and testing approach.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/tri-9217-featwebappredis-handle-unblocked-during-elasticache-scale-up

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.