CircleCI recipe

Here's some slightly redacted code for what worked for me, hopefully all the interesting parts. came out of this issue

   environment:
      PARALLEL_TEST_PROCESSORS: "12" 
...
      - run:
          name: setup parallel test databases
          command: |
            bundle exec rake parallel:create
            bundle exec rake parallel:load_schema
      - restore_cache:
          keys:
            - parallel-runtime-v1-
      - attach_workspace:
          at: workspace
...
      - persist_to_workspace:
          root: tmp
          paths:
            - parallel_runtime_rspec_*.log
...
  aggregate-parallel-test-logs:
    docker:
      - image: cimg/base:current
    steps:
      - attach_workspace:
          at: workspace
      - run:
          name: Combine runtime logs
          command: cat workspace/parallel_runtime_rspec_*.log > workspace/parallel_runtime_rspec.log
      - save_cache:
          key: parallel-runtime-v1-{{ epoch }}
          paths:
            - workspace/parallel_runtime_rspec.log
      - store_artifacts:
          path: workspace/parallel_runtime_rspec.log
          destination: parallel_runtime_rspec.log

if [[ -f workspace/parallel_runtime_rspec.log ]]; then
  cp workspace/parallel_runtime_rspec.log tmp/parallel_runtime_rspec.log
else
  echo "No runtime log found, using filesize splitting"
fi

TEST_FILES=$(circleci tests glob "spec/controllers/**/*_spec.rb" ......

cat > .rspec_parallel <<EOF
--format RspecJunitFormatter
--out /tmp/test-results/rspec_${CIRCLE_NODE_INDEX:?}_<%= ENV["TEST_ENV_NUMBER"] %>.xml
--format ParallelTests::RSpec::RuntimeLogger
--out tmp/parallel_runtime_rspec.log
EOF


RSPEC_COMMAND="bundle exec parallel_rspec \
  -n ${PARALLEL_TEST_PROCESSORS:?}\
  --runtime-log tmp/parallel_runtime_rspec.log"

echo "${TEST_FILES}" | circleci tests run --verbose --split-by=timings --command="xargs -I{} bash -c '${RSPEC_COMMAND} {}'"

rspec_status=$?

cp tmp/parallel_runtime_rspec.log tmp/parallel_runtime_rspec_"${CIRCLE_NODE_INDEX}".log
cp log/test*.log /tmp/test-results/

exit "${rspec_status}"

# database.yml
url: <%= ... %><%= ENV['TEST_ENV_NUMBER'] %>