[ruby-core:114253] [Ruby master Bug#19531] ObjectSpace::WeakMap: replaced values still clear the key they were assigned to

Issue #19531 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.7: WONTFIX, 3.0: REQUIRED, 3.1: REQUIRED, 3.2: REQUIRED to 2.7: WONTFIX, 3.0: REQUIRED, 3.1: REQUIRED, 3.2: DONE

ruby_3_2 46b62f44ce30bf234a76114c8249081e47ce3da4 merged revision(s) 3592b24cdc07ed89eecb39161f21fe721a89a5de.

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Bug #19531: ObjectSpace::WeakMap: replaced values still clear the key they were assigned to
https://bugs.ruby-lang.org/issues/19531#change-103942

* Author: byroot (Jean Boussier)
* Status: Closed
* Priority: Normal
* Backport: 2.7: WONTFIX, 3.0: REQUIRED, 3.1: REQUIRED, 3.2: DONE
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### 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




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