Fix SimpleCov coverage generation in CI parallel runs

Configure SimpleCov to generate unique coverage files for each parallel
CI runner and merge them for reporting.

Changes:
- Detect CI parallel execution via CI_NODE_INDEX environment variable
- Generate unique command names (RSpec-0, RSpec-1, etc.) per runner
- Preserve resultset files immediately after test completion
- Merge all resultsets in separate coverage job before reporting

Local parallel execution still works using TEST_ENV_NUMBER detection.

Author: mroderick

.github/workflows/ruby.yml

@@ -49,17 +49,22 @@ jobs:
     - name: Run tests in parallel
       env:
         DATABASE_URL: postgres://postgres:postgres@localhost:5432
-        TEST_ENV_NUMBER: ${{ matrix.ci_node_index }}
+        CI_NODE_INDEX: ${{ matrix.ci_node_index }}
       run: |
         bundle exec parallel_rspec spec/ \
           -n ${{ matrix.ci_node_total }} \
           --only-group ${{ matrix.ci_node_index }}
 
+    - name: Preserve coverage results
+      run: |
+        # Copy resultset immediately after tests complete to prevent deletion
+        cp coverage/.resultset.json coverage/resultset-${{ matrix.ci_node_index }}.json
+
     - name: Upload coverage artifacts
       uses: actions/upload-artifact@v4
       with:
         name: coverage-${{ matrix.ci_node_index }}
-        path: coverage/.resultset.json
+        path: coverage/resultset-${{ matrix.ci_node_index }}.json
         retention-days: 1
 
   coverage:
@@ -88,7 +93,7 @@ jobs:
           require "simplecov"
 
           resultsets = {}
-          Dir["coverage-results/coverage-*/.resultset.json"].each do |file|
+          Dir["coverage-results/coverage-*/resultset-*.json"].each do |file|
             data = JSON.parse(File.read(file))
             resultsets.merge!(data)
           end

spec/spec_helper.rb

@@ -30,8 +30,18 @@ def self.branch_coverage?
   add_filter 'spec/'
 
   # Support parallel test execution
-  if ENV['TEST_ENV_NUMBER']
-    command_name "RSpec-#{ENV['TEST_ENV_NUMBER']}"
+  # In CI: Use CI_NODE_INDEX (0, 1, 2, 3) set by GitHub Actions matrix
+  # Locally: Use TEST_ENV_NUMBER ('', '2', '3', '4') set by parallel_tests
+  if ENV['CI_NODE_INDEX']
+    command_name "RSpec-#{ENV['CI_NODE_INDEX']}"
+    use_merging true
+    merge_timeout 3600
+  elsif ENV.key?('TEST_ENV_NUMBER')
+    # TEST_ENV_NUMBER is '' for first process, '2', '3', etc. for others
+    suffix = ENV['TEST_ENV_NUMBER'].empty? ? '1' : ENV['TEST_ENV_NUMBER']
+    command_name "RSpec-#{suffix}"
+    use_merging true
+    merge_timeout 3600
   end
 end