From: "byroot (Jean Boussier) via ruby-core" <ruby-core@...>
Date: 2023-03-16T08:55:30+00:00
Subject: [ruby-core:112911] [Ruby master Bug#19531] ObjectSpace::WeakMap: replaced values still clear the key they were assigned to

Issue #19531 has been updated by byroot (Jean Boussier).


Patch: https://github.com/ruby/ruby/pull/7540

----------------------------------------
Bug #19531: ObjectSpace::WeakMap: replaced values still clear the key they were assigned to
https://bugs.ruby-lang.org/issues/19531#change-102428

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Backport: 2.7: WONTFIX, 3.0: REQUIRED, 3.1: REQUIRED, 3.2: REQUIRED
----------------------------------------
### Reproduction script

```ruby
wmap = ObjectSpace::WeakMap.new

a = "A"
b = "B"

wmap[1] = a
wmap[1] = b # the table entry with 1 is still in the list of entries to clear when `a` is GCed

a = nil

GC.start

p wmap[1] # Should be `"B"`, but is `nil`
```

### Explanation

What happens is that when we set `wmap[1] = "A"`, WeakMap internally keeps a list of keys to clear when `"A"` is GCed, e.g. pseudo code:

```ruby
class WeakMap
  def []=(key, value)
    @hash[key] = value
    @reverse[value] << key
  end
end
```

But it doesn't clear previously kept mapping when a key is overwritten.

I'll work on a fix.

### References

https://github.com/protocolbuffers/protobuf/pull/12216




-- 
https://bugs.ruby-lang.org/
 ______________________________________________
 ruby-core mailing list -- ruby-core@ml.ruby-lang.org
 To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org
 ruby-core info -- https://ml.ruby-lang.org/mailman3/postorius/lists/ruby-core.ml.ruby-lang.org/