Example how how stubbing methods could be threadsafe (IDK if this is how RSpec does it or not)

threadsafe_stubbing_example.rb

# really rudimentary stubbing mechanism (eg doesn't support unstubbing)
def stub(obj, method, *expected, result)
  Thread.current[:stubs] ||= {}
  Thread.current[:stubs][method] ||= {}
  Thread.current[:stubs][method][expected] = result
  obj.define_singleton_method method do |*actual| # we'll ignore keyword args here for simplicity
    Thread.current[:stubs][method].fetch(actual) do
      raise "#{method} invoked with unexpected args: #{actual.inspect}"
    end
  end
end

t1 = t2 = nil

t1 = Thread.new do
  Thread.current.abort_on_exception = true
  stub ENV, :[], 'KEY', "value from line #{__LINE__}"
  puts "THREAD1: ENV['KEY'] = #{ENV['KEY'].inspect}"
  Thread.pass until t2.stop?
  t2.run
  sleep
  puts "THREAD1: ENV['KEY'] = #{ENV['KEY'].inspect}"
  Thread.pass until t2.stop?
  t2.run
end

t2 = Thread.new do
  Thread.current.abort_on_exception = true
  stub ENV, :[], 'KEY', "value from line #{__LINE__}"
  sleep
  puts "THREAD2: ENV['KEY'] = #{ENV['KEY'].inspect}"
  Thread.pass until t1.stop?
  t1.run
  sleep
  puts "THREAD2: ENV['KEY'] = #{ENV['KEY'].inspect}"
end

t1.run
[t1, t2].each &:join

# >> THREAD1: ENV['KEY'] = "value from line 17"
# >> THREAD2: ENV['KEY'] = "value from line 29"
# >> THREAD1: ENV['KEY'] = "value from line 17"
# >> THREAD2: ENV['KEY'] = "value from line 29"