simplecov with multiple Ruby executions

configure_simplecov.rb

require 'securerandom'
require 'simplecov'

# Ensure a unique ID for each ruby process execution
SimpleCov.command_name("#{SecureRandom.uuid}_#{$0} #{ARGV}")
# Just save result, do not output anything here
SimpleCov.at_exit { SimpleCov.result }
# Ensure correct root always
SimpleCov.root File.dirname(__FILE__)
SimpleCov.start

Rakefile

# Ensure that all ruby process instances use Simplecov
task :simplecov_setup do
  ENV['RUBYLIB'] = "#{Dir.pwd}:#{ENV['RUBYLIB']}"
  ENV['RUBYOPT'] = '-r configure_simplecov'
  sh 'rm -rf coverage/'
end

# Generate a final report, with all merged results
task :simplecov_report do
  require 'simplecov'
  SimpleCov.command_name('rake')
  SimpleCov.minimum_coverage 97
  SimpleCov.start
end

desc "Run all tests"
task test: [
  :simplecov_setup,
  :execute_tests,
  :simplecov_report
]