Issue #17048 has been updated by alanwu (Alan Wu).


I'm not proposing that we remove `initialize_copy`, but to make it raise when the receiver has children. So this is what it would look like:

```ruby
module A
end

A.send(:initialize_copy, Module.new) # fine, no one inherits from A

class B
  include A
  A.send(:initialize_copy, Module.new) # raise, since A now has one child
end
```
Admittedly this proposed behavior is designed to sidestep the problem that the current implementation has.
Though maybe it's fine since this is a seldom used private method and other Ruby implementations don't have to follow this behavior?

Now that you've mentioned it, removing the method and moving the logic into `Module#{dup,clone}` feels like the cleanest solution. It's more likely to be a breaking change though.

----------------------------------------
Bug #17048: Calling initialize_copy on live modules leads to crashes
https://bugs.ruby-lang.org/issues/17048#change-86707

* Author: alanwu (Alan Wu)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-07-23T14:44:25Z master 098e8c2873) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------


Here's a repro script
```ruby
loop do
  m = Module.new do
    prepend Module.new
    def hello
    end
  end

  klass = Class.new { include m }
  m.send(:initialize_copy, Module.new)
  GC.start

  klass.new.hello rescue nil
end
```

Here's a script that shows that it has broken semantics even
when it happens to not crash.

```ruby
module A
end

class B
  include A
end

module C
  Const = :C
end

module D
  Const = :D
end

A.send(:initialize_copy, C)
p B::Const # :C, makes sense
A.send(:initialize_copy, D)
p B::Const # :D, makes sense
A.send(:initialize_copy, Module.new)
p (begin B::Const rescue NameError; 'NameError' end) # NameError, makes sense
A.send(:initialize_copy, C)
p B::Const # still NameErorr. Weird
```
This example shows that the problem exists [as far back as 2.0.0](https://wandbox.org/permlink/4dVDY9sNXJ803jh8).

I think the easiest way to fix this is to forbid calling `:initialize_copy`
on modules that have children. Another option is to try to decide on
the semantics of this. Though I don't think it's worth the effort as this
has been broken for a long time and people don't seem to to be using it.
Thoughts?




-- 
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