From: "byroot (Jean Boussier) via ruby-core" <ruby-core@...>
Date: 2024-05-13T06:40:23+00:00
Subject: [ruby-core:117856] [Ruby master Bug#20485] Simple use of Fiber makes GC leak objects with singleton method

Issue #20485 has been updated by byroot (Jean Boussier).


@skhrshin if you can reproduce consistently, what could be helpful would be to provide a heap dump like this (use some service list GitHub gist because the output might be big:

```ruby
require 'objspace'

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

Mutex.new.synchronize { Fiber.new {}.resume }

work = Work.new
work.add_method
puts ObjectSpace.dump(work)

work = nil
GC.start

num_objs = ObjectSpace.each_object(Work).count
unless num_objs.zero?
  puts '-' * 40
  puts ObjectSpace.dump_all(output: :stdout)
  raise "NG"
end
```

That would allow us to trace back what's preventing the object from being garbage collected.

----------------------------------------
Bug #20485: Simple use of Fiber makes GC leak objects with singleton method
https://bugs.ruby-lang.org/issues/20485#change-108265

* Author: skhrshin (Shintaro Sakahara)
* Status: Open
* ruby -v: ruby 3.2.4 (2024-04-23 revision af471c0e01) [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 just `Fiber.new {}.resume` or remove `work.add_method`, GC works as expected.
Is there any problem at the way to use 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 a little different. The code above didn't reproduce the problem, but if I changed `1.times` to `Mutex.new.synchronize`, it was able to reproduce.



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