Bug #4409: DRb: discrepency between DRb.here? and DRb.uri leads to failure to dereference a DrbObject
http://redmine.ruby-lang.org/issues/show/4409

Author: Nicolas Bondoux
Status: Open, Priority: Normal
Category: lib
ruby -v: ruby 1.9.2p0 (2010-08-18 revision 29036) [i486-linux]

When a reference on a local object is sent back by a client to a server, it may not be recognised as a local object during marshalling:
An object instance being sent as a reference is wrapped in a DRbObject built by make_proxy with DRbObject.new(obj); the DRbObject's uri will be the uri returned by DRb::uri.
When this reference is sent back, DRbObject::_load checks if it matches uri of current server with DRb::here? method.

The problem is that DRb.here? checks that a DRbObject uri is the same as the uri which was used by current DRbServer at his creation, while DRb.uri will return an uri created from the local socket from current thread's connection.

So, when a DRbObject it sent back, it may not be recognized as referencing a local object

Fixing DRb.here? so that it compares with the uri returned by DRb.uri should be enough to quick fix this problem, and seems safe to me.
(I call it a quick fix because I am sure that even with this change, it is possible to build some less realistics cases with the same kind of problem, due to the fact that a same reference may be valid or not depending which client sent it back ...)


Here is a very simple proof of concept:
The server:

#!/usr/bin/ruby
require 'drb'
class A
  include DRb::DRbUndumped
end

class TheServer
  include DRb::DRbUndumped
  def initialize
    @a = A.new
  end

  def get_a
    @a
  end

  def is_a(iA)
    puts "inside is_a: current DRb.uri:#{DRb.uri}; iA.class = #{iA.class} #{iA.__drburi if DRb::DRbObject === iA}"
    @a.eql? iA
  end
  
  def getToken
    retVal = @@token.clone
    @@token.inc
    return retVal
  end
  
end

theServer = TheServer.new
DRb.start_service(nil,theServer)
puts "DRb.uri just after start_service: #{DRb.uri}"
DRb.thread.join


#############
The client:

#!/usr/bin/ruby
require 'drb'


theServer = DRb::DRbObject.new(nil,ARGV.shift)

a = theServer.get_a
puts "a.__drburi == #{a.__drburi}"
puts "theServer.is_a(a) == #{theServer.is_a(a)}"


#############
the outputs:
server side:
DRb.uri just after start_service: druby://Venus:60106
inside is_a: current DRb.uri:druby://127.0.0.1:60106; iA.class = DRb::DRbObject druby://127.0.0.1:60106


client side:
a.__drburi == druby://127.0.0.1:60106
theServer.is_a(a) == false


Cheers,

Nicolas


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