Partition RSpec files across runners in a stable way to support deterministic sharding.

.circleci_config.yml

jobs:
  build:
    docker: [image: cimg/ruby:3.4.3]
    parallelism: 4
    steps:
      - checkout
      - run: script/test spec:ci[$((${CIRCLE_NODE_INDEX}+1))/${CIRCLE_NODE_TOTAL}]

.github_workflows_test.yml

jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        group: [1, 2, 3, 4, 5]
    steps:
      - uses: actions/checkout
      - uses: ruby/setup-ruby
      - run: script/test spec:ci[${{ matrix.group }}/${{ strategy.job-total }}]

rspec.rake

if defined?(RSpec)
  SPECS = FileList[ENV.fetch "SPEC", RSpec::Core::RakeTask::DEFAULT_PATTERN].uniq

  namespace :spec do
    desc "Run default ci specs - no flaky specs [job='N/total' (default: 1/1)]"
    RSpec::Core::RakeTask.new(:ci, [:job]) do |t, args|
      args.with_defaults(job: "1/1")
      group, total = args[:job].split("/").map(&:to_i)

      # partition such that the grouping is stable regardless of file count
      # hash the filename taking the first 64 bits as an int, modulo the number of groups
      # offset by 1 b/c job number is 1-index (so that '5/5' makes sense)
      ENV["SPEC"] = SPECS.select { |s| Digest::SHA2.digest(s).unpack1("Q") % total == group - 1 }.join(" ")

      t.rspec_opts = %w[
      --tag ~flaky
      --force-color
      --format doc
      --profile 2
      --warnings
    ]
    end

    desc "Run only flaky specs (need made deterministic or deleted)"
    RSpec::Core::RakeTask.new(:flaky) do |t|
      t.rspec_opts = %w[
        --tag flaky
        --force-color
        --format doc
      ]
    end
  end
end

Invocation of the rake task looks like:

rake spec:ci[1/4]