UnixWorker is used only on platforms where fork is available. The way this code works is, it forks a preconfigured number of workers and then It starts preconfigured number of threads that write to the connected pipe.
# File lib/bundler/parallel_workers/unix_worker.rb, line 17 def initialize(size, job) # Close the persistent connections for the main thread before forking Net::HTTP::Persistent.new('bundler', :ENV).shutdown super end