From: "nagachika (Tomoyuki Chikanaga) via ruby-core" <ruby-core@...>
Date: 2024-11-10T04:06:09+00:00
Subject: [ruby-core:119870] [Ruby master Bug#20868] Method#hash changes after compaction

Issue #20868 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 3.1: WONTFIX, 3.2: REQUIRED, 3.3: REQUIRED to 3.1: WONTFIX, 3.2: DONE, 3.3: REQUIRED

ruby_3_2 commit:ee3428aa0e3a11ed1574b7d99222f6f08737f818 merged revision(s) commit:56ecc243e230e8e99761ec0ffc5116601f094bb0.

----------------------------------------
Bug #20868: Method#hash changes after compaction
https://bugs.ruby-lang.org/issues/20868#change-110561

* Author: peterzhu2118 (Peter Zhu)
* Status: Closed
* Backport: 3.1: WONTFIX, 3.2: DONE, 3.3: REQUIRED
----------------------------------------
Fix: https://github.com/ruby/ruby/pull/12004

The hash value of a Method may change after a compaction but must remain constant because otherwise it may not work as the key in a hash table.

For example:

```ruby
def a; end

# Need this method here because otherwise the iseq may be on the C stack
# which would get pinned and not move during compaction
def get_hash
  method(:a).hash
end

puts get_hash # => 2993401401091578131

GC.verify_compaction_references(expand_heap: true, toward: :empty)

puts get_hash # => -2162775864511574135
```



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