[ruby-core:69182] [Ruby trunk - Bug #11153] [Open] Defining singleton methods using `class << self` sometimes fails when using threads

Issue #11153 has been reported by Matt Brictson.

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Bug #11153: Defining singleton methods using `class << self` sometimes fails when using threads
https://bugs.ruby-lang.org/issues/11153

* Author: Matt Brictson
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin14]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
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Defining singleton methods using this syntax occasionally produces unexpected results when run in parallel with multiple threads:

~~~
class << obj
  def method_one
  end
  def method_two
  end
end
~~~

Sometimes not all of the methods are defined, leading to this result:

~~~
obj.respond_to?(:method_one) # => true
obj.respond_to?(:method_two) # => false
~~~

Here is a short script that demonstrates the bug.

~~~
require "thread"

def define_singleton_methods_in_thread
  Thread.new do
    100_000.times do
      obj = Object.new
      class << obj
        def one
        end

        def two
        end
        
        def three
        end
      end
      puts "FAIL" unless obj.respond_to?(:three)
    end
  end
end

# Test with 2 threads
2.times.map { define_singleton_methods_in_thread }.map(&:join)
~~~

On my machine this script prints "FAIL" 3 times.

I am able to reproduce this bug in 2.0.0-p451, 2.1.6, 2.2.2, and the current ruby_2_2 branch. I *cannot* reproduce in trunk.

More information here: https://github.com/net-ssh/net-ssh/pull/240



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