Issue #16278 has been updated by cristiangreco (Cristian Greco).


Hi Jeremy, thanks for these details!

I don°«t know the details of ruby°«s GC, seems to me it might behave unpredictably sometimes. I guess what confuses me now is that although that object is retained we don°«t observe unbounded memory growth when calling the method in a loop: is it eventually collected? Is it somehow re-used, or what? 

Another thing that intrigues me is that this behaviour is not always consistent: it might vary across different ruby builds (you mentioned that memory layout and compiler optimisations might affect the output) and is also subtly influenced by code changes. For example, if I add this line `puts ObjectSpace.count_objects[:T_HASH]` just after the call to `create` then `h4` is almost never found after GC (or its reference is found associated to another object). Not sure why this happens though!


nobu (Nobuyoshi Nakada) wrote:
> Adding this method and calling it after `create` clears the output.
> 
> ```ruby
> def garbage
>   h1 = h2 = h3 = h4 = h5 = h6 = h7 = h8 = h9 = h10 = nil
> end
> ```
> 
> So a °»shadow°… seems staying on the VM stack.

Hey Nobu, what is going on here? What means that a shadow is retained on the stack? 


----------------------------------------
Bug #16278: Potential memory leak when an hash is used as a key for another hash
https://bugs.ruby-lang.org/issues/16278#change-82346

* Author: cristiangreco (Cristian Greco)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin18]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Hi,

I've been hitting what seems to be a memory leak.

When an hash is used as key for another hash, the former object will be retained even after multiple GC runs.

The following code snippet demonstrates how the hash `{:a => 1}` (which is never used outside the scope of `create`) is retained even after 10 GC runs (`find` will look for an object with a given `object_id` on heap).


```ruby
# frozen_string_literal: true

def create
  h = {{:a => 1} => 2}
  h.keys.first.object_id
end

def find(object_id)
  ObjectSpace.each_object(Hash).any?{|h| h.object_id == object_id} ? 1 : 0
end


leaked = create

10.times do
  GC.start(full_mark: true, immediate_sweep: true)
end

exit find(leaked)
```

This code snippet is expected to exit with `0` while it exits with `1` in my tests. I've tested this on multiple recent ruby versions and OSs, either locally (OSX with homebrew) or in different CIs (e.g. [here](https://github.com/cristiangreco/ruby-hash-leak/commit/285e586b7193104989f59b92579fe8f25770141e/checks?check_suite_id=278711566)).

Can you please help understand what's going on here? Thanks!



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