Ruby

Thanks @nobu (Nobuyoshi Nakada) for looking at the issue!

This method doesn't wait these threads which run start_adder.

It does: initialize waits for all threads to have called Adder#add at the last line at @qu.deq . It only returns after this event.

larskanis (Lars Kanis) wrote:

It does: initialize waits for all threads to have called Adder#add at the last line at @qu.deq . It only returns after this event.

It waits just once.

larskanis (Lars Kanis) wrote:

It waits just once.

Yes, but the one event is sent after all 10 threads have been called, so that @count is decremented to 0.

Why do you think so?

@nobu (Nobuyoshi Nakada) I updated the bug report, so that it should be more understandable now, what's going wrong.

I agree, this appears to a be a bug in WeakRef. The Adder instance is not garbage collected, but you still get a WeakRef::RefError when referencing it through the WeakRef instance. Here's a slightly modified example that just prints when the WeakRef::RefError is raised and prints inside Adder#initialize showing that the Adder instance has not yet been garbage collected after the WeakRef::RefError is raised (and the object ids for the instance and @qu match).

require "weakref"

class Adder
  def self.start_adder(obj, oid, q)
    obj.add
  rescue WeakRef::RefError
    p [WeakRef::RefError, oid, q.object_id]
    q.push [oid, q.object_id]
  end

  def initialize
    @qu = Queue.new
    count = 10
    count.times do
      Thread.new(WeakRef.new(self), object_id, @qu, &self.class.method(:start_adder))
    end
    count.times do
      oid, q_oid = @qu.pop
      if oid
        p [:initialize, self, oid, object_id, oid == object_id, q_oid == @qu.object_id]
      end
    end
  end

  def add
    @qu.push nil
  end
end

def test_adder
  oids = 10.times.map do
    Thread.new do
      Adder.new
    end
  end.map(&:join)
end

1000.times do
  test_adder
end

Output on ruby 2.5.3:

[WeakRef::RefError, 16073782609840, 16073782609800]
[:initialize, #<Adder:0x00001d3cf0348f60 @qu=#<Thread::Queue:0x00001d3cf0348f10>>, 16073782609840, 16073782609840, true, true]
[WeakRef::RefError, 16073295767400, 16073295767360]
[:initialize, #<Adder:0x00001d3cb62b4ed0 @qu=#<Thread::Queue:0x00001d3cb62b4e80>>, 16073295767400, 16073295767400, true, true]

The reference error doesn't appear when WeakRef is replaced by _id2ref like so:

  def self.start_adder(oid)
    ObjectSpace._id2ref(oid).add
  end

So it appears to be a bug in ObjectSpace::WeakMap.

This issue appears to be fixed in ruby-2.7.0 by the recent WeakMap modifications. It is still present on:

ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-linux]
ruby 2.5.7p206 (2019-10-01 revision 67816) [x86_64-linux]
ruby 2.6.5p114 (2019-10-01 revision 67812) [x64-mingw32]

However since the exact change isn't known, I think backporting is not necessary. So the issue can be closed (I don't have an option for closing).