Bug #19369: Small corner-case issue that breaks Ractor isolation: change cloned object from another thread

• Status: Assigned
• Assignee: ko1 (Koichi Sasada)
• Target version: -
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN

[luke-gru (Luke Gruber)]

I was looking into how objects are traversed for deep cloning and I came up with a way to break it. I don't think it'll ever happen in real life so it's not really an issue, just
an interesting case. Run with warnings disabled.

obj = Object.new
p "unshareable obj:", obj
UNSHAREABLE = obj
GO = false
SET = false
class Object
  attr_accessor :unshareable
  def initialize_clone(orig)
    puts "Clone called for #{orig.inspect}, self = #{self.inspect}"
    _self = self
    if orig == UNSHAREABLE
      t = Thread.new do
        puts "In thread"
        Thread.pass until GO
        puts "Setting unshareable!"
        # this must be done in separate thread to bypass object traversal deep-cloning
        _self.unshareable = UNSHAREABLE
        Object.const_set(:SET, true)
      end
    end
    super(orig)
  end
end

r = Ractor.new(obj) do |o|
  puts "from r#{Ractor.current.object_id} obj #{o.inspect}"
  GO = true
  loop until SET
  p "from ractor, got unshareable:", o.unshareable
end

r.take

[luke-gru (Luke Gruber)]

If you wanted to fix this one way would be to disable thread creation in the current ractor (main in this case) while deep-cloning in Ractor.new or Ractor#send. Another idea would be to not call initialize_clone during deep-cloning for Ractor.new or Ractor#send, but that would be a negative change IMO.

BTW, forgot to mention to run the above ruby code with warnings disabled.

[luke-gru (Luke Gruber)]

I made a patch for this here: https://github.com/luke-gru/ruby/commit/ad760be949c2a35aac71ac0ff8dea4ec3169f589. I'll send PR if it's an acceptable solution.

Edit: My commit doesn't actually fix this in the proper way, because you could do:

def initialize_clone(orig)
  if orig == UNSHAREABLE
    begin
      Thread.new { }
    rescue ThreadError
      t = Thread.new do
       "..."
      end
    end
  end
end

A proper is a bit tricky.

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Assigned
• Assignee set to ko1 (Koichi Sasada)