[ruby-core:98693] [Ruby master Bug#16942] instance_method causes an infinite loop with prepend, include and private

Issue #16942 has been updated by jeremyevans0 (Jeremy Evans).


This issue first occurred in Ruby 2.3.

The problematic code is the `goto again` loop in `mnew_internal`.  You can end up with a situation where `klass` is the same or an ancestor of `RCLASS_SUPER(me->owner)`, so `klass` never changes.  This is possible to work around by detecting this case and using `RCLASS_SUPER(klass)` instead of `RCLASS_SUPER(me->owner)` in this case.  However, this approach is O(m*n), where m is the number of ZSUPER methods and n is the length of the super chain of `RCLASS_SUPER(me->owner)`.  Unfortunately, I don't have enough knowledge of the internals to determine if there is a more efficient solution, or if the work around will cause other problems.

I submitted a pull request for the work around: https://github.com/ruby/ruby/pull/3201

----------------------------------------
Bug #16942: instance_method causes an infinite loop with prepend, include and private
https://bugs.ruby-lang.org/issues/16942#change-86039

* Author: pocke (Masataka Kuwabara)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-06-09T13:51:54Z master 7f3efee102) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
`instance_method` causes an infinite loop with the following code.


```ruby
module M
  def x
  end
end

module M2
  include M
  private :x
end

::Object.prepend(M2)

# infinite loop
p Object.instance_method(:x)
```


I've confirmed it on the master branch. The commit is 7f3efee102.
But `instance_method` works correctly with Ruby 2.7.1.



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