Parallelise MiniTest workflow

This configures the MiniTest workflow to run
using a matrix strategy. It run separate parts
of the test suite on different nodes and hence
speeds up total run time of the CI/CD pipeline.

.github/workflows/ci.yml

@@ -44,6 +44,10 @@ jobs:
   test-ruby:
     name: Test Ruby
     uses: ./.github/workflows/minitest.yml
+
+  test-ruby-parallel:
+    name: Test Ruby (parallelised)
+    uses: ./.github/workflows/minitest-parallel.yml
 
   integration-tests-parallel:
     name: Integration tests (parallelised)

.github/workflows/minitest-parallel.yml

@@ -0,0 +1,79 @@
+name: Run Minitest Parallel
+
+on:
+  workflow_call:
+    inputs:
+      ref:
+        description: 'The branch, tag or SHA to checkout'
+        required: false
+        type: string
+      publishingApiRef:
+        description: 'The branch, tag or SHA to checkout Publishing API'
+        required: false
+        default: 'main'
+        type: string
+
+jobs:
+  run-minitest:
+    name: Run Minitest
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ci_node_total: [4]
+        ci_node_index: [0, 1, 2, 3]
+    steps:
+      - name: Setup MySQL
+        id: setup-mysql
+        uses: alphagov/govuk-infrastructure/.github/actions/setup-mysql@main
+
+      - name: Setup Redis
+        uses: alphagov/govuk-infrastructure/.github/actions/setup-redis@main
+
+      - name: Install additional system dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends ghostscript
+
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          repository: alphagov/whitehall
+          ref: ${{ inputs.ref || github.ref }}
+
+      - name: Checkout Publishing API (for Content Schemas)
+        uses: actions/checkout@v4
+        with:
+          repository: alphagov/publishing-api
+          ref: ${{ inputs.publishingApiRef }}
+          path: vendor/publishing-api
+
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+
+      - name: Setup Node
+        uses: alphagov/govuk-infrastructure/.github/actions/setup-node@main
+
+      - name: Precompile assets
+        uses: alphagov/govuk-infrastructure/.github/actions/precompile-rails-assets@main
+
+      - name: Initialize database
+        env:
+          RAILS_ENV: test
+          TEST_DATABASE_URL: ${{ steps.setup-mysql.outputs.db-url }}
+        run: bundle exec rails db:setup
+
+      - name: Make bin/minitest-ci executable
+        run: chmod +x ./bin/minitest-ci
+
+      - name: Run Minitest
+        env:
+          RAILS_ENV: test
+          GOVUK_CONTENT_SCHEMAS_PATH: vendor/publishing-api/content_schemas
+          TEST_DATABASE_URL: ${{ steps.setup-mysql.outputs.db-url }}
+          CI_NODE_TOTAL: ${{ matrix.ci_node_total }}
+          CI_NODE_INDEX: ${{ matrix.ci_node_index }}
+        run: |
+          ./bin/minitest-ci

bin/minitest-ci

@@ -0,0 +1,11 @@
+#!/usr/bin/env ruby
+
+tests = Dir["test/**/*_test.rb"].
+  sort.
+  shuffle(random: Random.new(ENV['GITHUB_SHA'].to_i(16))).
+  select.
+  with_index do |el, i|
+    i % ENV["CI_NODE_TOTAL"].to_i == ENV["CI_NODE_INDEX"].to_i
+  end
+
+exec "bundle exec rails test #{tests.join(" ")}"