Fix: Prevent concurrent deployments by excluding main branch from check workflow

[umihico]

Summary

Fixed deployment failures caused by concurrent workflow executions racing to deploy to the same CloudFormation stack.

Root Cause Analysis

The Problem

When PRs were merged to main, both auto-update and check workflows triggered simultaneously, causing race conditions:

2025-11-13T01:24:08Z - auto-update (push to main) - FAILED ❌
2025-11-13T01:24:08Z - check (push to main) - SUCCEEDED ✅
2025-11-13T01:59:03Z - auto-update (schedule) - SUCCEEDED ✅

Why It Failed

1. Concurrent Triggers: Both workflows triggered at exactly 2025-11-13T01:24:08Z on push to main
2. Race Condition: Both tried to update the same CloudFormation stack docker-selenium-lambda-prod
3. Deployment Conflict: One deployment failed with:

   CREATE_FAILED: DemoLambdaFunction (AWS::Lambda::Function)
   Resource handler returned message: "Lambda does not have permission to access the ECR image"
   

4. Stack Rollback: CloudFormation rolled back to UPDATE_ROLLBACK_COMPLETE state

Why The Error Was Misleading

The error message suggested an ECR permission issue, but the real problem was the concurrent deployment race condition. The "permission denied" error was a side effect of CloudFormation being in an inconsistent state during the conflict.

Solution

Modified check.yml to exclude main branch:

on:
  push:
    branches-ignore:
      - main
  workflow_dispatch:

Rationale:

• auto-update workflow already handles main branch deployments
• check workflow is now for feature branch validation only
• Added workflow_dispatch for manual testing when needed

Workflow Responsibilities

• auto-update.yml: Handles main branch deployments, scheduled updates, version updates
• check.yml: Validates feature branches before merge
• demo-test.yml: Tests README instructions on schedule

Test Plan

• Analyzed workflow execution logs
• Confirmed concurrent execution timing
• Verified subsequent scheduled run succeeded
• Merge PR and verify only auto-update runs
• Push to feature branch and verify only check runs

🤖 Generated with Claude Code