From: Joel VanderWerf <joelvanderwerf@...>
Date: 2010-03-23T02:36:03+09:00
Subject: [ruby-core:28868] Re: Can DRb be used across a fork()

Chris Schlaeger wrote:
> I'm trying to use DRb to communicate between a parent and child
> processes. The DRb server is started before I fork-off the children.
> When I connect back from the child to the parent DRb server, I only
> seem to talk to a zombie copy of the DRb server in the child.
> 
> The attached program demonstrates the problem. Is that a bug or is DRb
> not designed to work across a fork?
> 
> With current ruby 1.9.2 I get the following output:
> 
> #ruby19 drb_over_fork.rb
> 13151: Parent
> 13155: Child
> 13155: a: 1
> 13155: A: 1
> 13151: A: 0
> 
> Chris

There are a couple of subtleties. See if this helps:

require 'drb'

class Foo
   attr_reader :a
   def initialize
     @a = 0
   end

   def inc
     @a += 1
     debug "a: #{@a}"
   end
end

def debug(msg)
   puts "#{Process.pid}: #{msg}"
end

foo = Foo.new
uri = DRb.start_service(nil, foo).uri
unless (pid = fork)
   DRb.start_service() # Because parent's DRb thread dies across fork
   debug 'Child'
   # child
   foo_c = DRbObject.new(nil, uri)
   # This call modifies a copy of foo in the child, not the parent foo!
   foo_c.inc
   debug "A: #{foo_c.a}"

else
   debug 'Parent'
   Process.wait
   debug "A: #{foo.a}"
end

__END__

The output I get (1.8.6) is

$ ruby drb_over_fork.rb
17245: Parent
17246: Child
17245: a: 1
17246: A: 1
17245: A: 1