From: "skhrshin (Shintaro Sakahara) via ruby-core" <ruby-core@...>
Date: 2024-05-13T08:25:08+00:00
Subject: [ruby-core:117857] [Ruby master Bug#20485] Simple use of Fiber makes GC leak objects with singleton method

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


I asked my co-workers to try this script and some of them gave me their results. The following table includes my results.

|Environment|# of people|Reproducibility|
|--|--|--|
|ruby 3.3.1 (2024-04-23 revision c56cd86388) [x86_64-linux] on Ubuntu/WSL2|1|Probably 100%|
|ruby 3.3.1 (2024-04-23 revision c56cd86388) [x86_64-linux] on Ubuntu/virtualbox|2|Very high but less than 100%|
|ruby 3.3.1 (2024-04-23 revision c56cd86388) [x86_64-linux] on Docker Desktop/Windows|1|High but less than 100%|
|ruby 3.3.1 (2024-04-23 revision c56cd86388) [x86_64-linux] on Ubuntu/Hyper-V|1|Low (about 10%)|
|ruby 3.3.1 (2024-04-23 revision c56cd86388) +YJIT [arm64-darwin23]|1|0%|
|ruby 3.3.1 (2024-04-23 revision c56cd86388) [x86_64-darwin22]|1|0%|

The person who tried it on `ruby 3.3.1 (2024-04-23 revision c56cd86388) +YJIT [arm64-darwin23]` also gave me the results on several Ruby versions. He said it was reproducible on 3.2.4, but not on 3.2.2.

I created a dump log by putting `ObjectSpace.dump_all(output: :stdout)` before `raise "NG"` and uploaded it to GitHub. This log doesn't contain `ObjectSpace.dump(work)` you've suggested because with putting something like `ObjectSpace.dump(work)`, `puts 0` or `sleep 1` between `work.add_method` and `work = nil` the script doesn't reproduce the problem.

https://gist.github.com/skhrshin/f639e387578db8faf431adfb7ac06631#file-bugs-ruby-lang-org_issues_20485_dump_all-log

As far as I investigated, I couldn't find any OBJECT that prevented `work` from being GCed. The address of `work` looks to be 0x7f2b552d0bb8. I don't have any knowledge about what IMEMO is. I would appreciate it if you could help me.

----------------------------------------
Bug #20485: Simple use of Fiber makes GC leak objects with singleton method
https://bugs.ruby-lang.org/issues/20485#change-108266

* 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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