From: "skhrshin (Shintaro Sakahara) via ruby-core" <ruby-core@...>
Date: 2024-05-12T06:09:40+00:00
Subject: [ruby-core:117842] [Ruby master Bug#20485] Simple use of Fiber makes GC leak objects with singleton method

Issue #20485 has been updated by skhrshin (Shintaro Sakahara).

Subject changed from Simple use of Mutex and Fiber makes GC leak objects with singleton method to Simple use of Fiber makes GC leak objects with singleton method
Description updated

Update: Using Mutex was not necessary.

----------------------------------------
Bug #20485: Simple use of Fiber makes GC leak objects with singleton method
https://bugs.ruby-lang.org/issues/20485#change-108246

* Author: skhrshin (Shintaro Sakahara)
* Status: Open
* ruby -v: ruby 3.2.3 (2024-01-18 revision 52bb2ac0a6) [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
I found a possible memory leak which occurs only when several conditions are met.

The code to reproduce the problem is below:

```
class Work
  def add_method
    singleton_class.define_method(:f) {}
  end
end

1.times { Fiber.new {}.resume }

work = Work.new
work.add_method
work = nil
GC.start

num_objs = ObjectSpace.each_object.select { |o| o.is_a?(Work) rescue false }.size
unless num_objs.zero?
  raise "NG"
end
```

Expected result: The script exits normally.
Actual result: RuntimeError "NG" is raised.

If I change `1.times { Fiber.new {}.resume }` to `Fiber.new {}.resume` or remove `work.add_method`, GC works as expected.
Is there any problem at the way to Fiber in this code, or is it a bug due to Ruby?

I tested ruby 3.3.1 (2024-04-23 revision c56cd86388) [x86_64-linux] too and the result was same.



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