Question

Quick question about Ruby forking – I ran across a bit of forking code in Resque earlier that was sexy as hell but tripped me up for a few.

I’m hoping for someone to give me a little more detail about what’s going on here. Specifically – it would appear that forking spawns a child (expected) and kicks it straight into the ‘else’ side of my condition (less expected. Is that expected behavior? A Ruby idiom?

My IRB hack here:

def fork
  return true if @cant_fork

  begin
    if Kernel.respond_to?(:fork)
      Kernel.fork
    else
      raise NotImplementedError
    end
  rescue NotImplementedError
    @cant_fork = true
    nil
  end
end

def do_something
  puts "Starting do_something"

  if foo = fork
    puts "we are forking from #{Process.pid}"
    Process.wait
  else
    puts "no need to fork, let's get to work: #{Process.pid} under #{Process.ppid}"
    puts "doing it"
  end
end

do_something

Answer 1

This is how fork works. From the documentation:

Otherwise, the fork call returns
twice, once in the parent, returning
the process ID of the child, and once
in the child, returning nil.

So, in your parent ‘foo = fork’ is the PID of the child, and in the child ‘foo = fork’ is nil, thus it takes the ‘else’ branch in the child.