Example of adding fork back to Crystal

fork.cr

lib LibC
  fun fork : PidT
  fun wait(Int *) : PidT
  fun waitpid(PidT, Int *, Int) : PidT
end

puts "Before fork"

pid = LibC.fork

if pid == 0
  puts "Inside child"
  
  puts "Sleeping in child ..."
  sleep 4

  puts "Exiting child"
  exit 1
else
  puts "Inside parent"

  status = uninitialized LibC::Int

  puts "Waiting for child to exit ..."
  LibC.waitpid(pid,pointerof(status),0)
  puts "Child exited with #{status}"
end

Expected output:

Before fork
Inside parent
Waiting for child to exit ...
Inside child
Sleeping in child ...
Exiting child
Child exited with 256

Hmm appears there is a Process.fork, but is :nodoc:ed.
https://github.com/crystal-lang/crystal/blob/1.0.0/src/process.cr#L62-L98

Is there a solution for

spawn do
  mutex.synchronize {  }
end

fork do
  # * Fibers are no longer running
  # mutex is stuck in a locked state
end

with -Dpreview_mt?

Not sure, but I suspect Fiber/mutex issue happens with Crystal's Process.fork? After the process forks, both the parent and child processes should keep running the fibers and unlock any mutexes?

Building with -Dpreview_mt seems to work with LibC.fork.