From: ufuk@...
Date: 2020-11-24T19:44:42+00:00
Subject: [ruby-core:101053] [Ruby master Bug#17321] Having a singleton class makes cloning imperfect

Issue #17321 has been updated by ufuk (Ufuk Kayserilioglu).


@jeremyevans0 Thanks for closing the issue. 

Can we get this fix backported to 2.5, 2.6 and 2.7 please? As noted in the original report, this bug is present on all the way back to 2.0.

Thank you!

----------------------------------------
Bug #17321: Having a singleton class makes cloning imperfect
https://bugs.ruby-lang.org/issues/17321#change-88725

* Author: ufuk (Ufuk Kayserilioglu)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.0.0dev (2020-11-11T09:11:09Z master fa3670e6e4) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------

## Problem
---
Running the following reproduction script:
```ruby
class Foo
  def self.foo; end
end

def report(klass, name)
  puts "  #{name}.instance_methods(false): #{klass.instance_methods(false)}"
end

def clone_and_compare(obj)
  cln = obj.clone

  report(obj.singleton_class, "obj.singleton_class")
  report(cln.singleton_class, "cln.singleton_class")
  report(obj.singleton_class.singleton_class, "obj.singleton_class.singleton_class")
  report(cln.singleton_class.singleton_class, "cln.singleton_class.singleton_class")
end

puts "## Case 1"
obj = Foo.new
clone_and_compare(obj)

puts "## Case 2"
obj = Foo.new
obj.singleton_class
clone_and_compare(obj)

puts "## Case 3"
obj = Foo.new
obj.singleton_class.singleton_class.send(:define_method, :method_on_s2) {}
clone_and_compare(obj)
```
gives the following output:
```
## Case 1
  obj.singleton_class.instance_methods(false): []
  cln.singleton_class.instance_methods(false): []
  obj.singleton_class.singleton_class.instance_methods(false): []
  cln.singleton_class.singleton_class.instance_methods(false): []
## Case 2
  obj.singleton_class.instance_methods(false): []
  cln.singleton_class.instance_methods(false): []
  obj.singleton_class.singleton_class.instance_methods(false): []
  cln.singleton_class.singleton_class.instance_methods(false): [:foo]
## Case 3
  obj.singleton_class.instance_methods(false): []
  cln.singleton_class.instance_methods(false): []
  obj.singleton_class.singleton_class.instance_methods(false): [:method_on_s2]
  cln.singleton_class.singleton_class.instance_methods(false): [:method_on_s2]
```

`Case 2` is surprising, because the cloned object has different contents to the original. 
It is surprising that `clone.singleton_class.singleton_class` has the method `:foo` whereas 
`obj.singleton_class.singleton_class` does not have any methods. 

`Case 3` suggests, however, that `clone.singleton_class.singleton_class` should have the 
same methods as the ones on `obj.singleton_class.singleton_class`.

This reproduction script gives the same output all the way from Ruby 2.0 up to Ruby-HEAD: 
https://wandbox.org/permlink/hRM9OMgtd6nuscRz

## Fix
---

@alanwu and me have put together a PR that makes `Case 2` output identical to `Case 1` output:
https://github.com/ruby/ruby/pull/3761





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