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


In principal I agree that allowing initialization only once is a good way to go, but the way `Module.allocate` is currently setup makes implementing this a bit complicated. At the moment there is not really a pre-init state for modules and the result from `Module.allocate` is the same as `Module.new`. If we want to do this we would have to implement a pre-init state for modules, and make sure that all the operations on modules (adding methods, constants, etc) are aware of this state so they can do the initialization in case they receive a pre-init module.

It's doable, but I don't know if the extra consistency is worth the added complexity. It would slow things down a small bit for operations like method addition and has the potential to introduce crashes if we miss places where we need the initialization check.

----------------------------------------
Bug #17048: Calling initialize_copy on live modules leads to crashes
https://bugs.ruby-lang.org/issues/17048#change-86712

* 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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