[ruby-core:105314] [Ruby master Bug#17048] Calling initialize_copy on live modules leads to crashes

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


Here a repro that that always crashes for me on cbbda3e:

```ruby
mod = Module.new { define_method(:foo) {:first} }
klass = Class.new { include mod }
instance = klass.new
p instance.foo
new_mod = Module.new { define_method(:foo) { :second } }
mod.send(:initialize_copy, new_mod)
4.times { GC.start }
p instance.foo # [BUG] unreachable
```

----------------------------------------
Bug #17048: Calling initialize_copy on live modules leads to crashes
https://bugs.ruby-lang.org/issues/17048#change-93727

* 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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