From: "byroot (Jean Boussier) via ruby-core" <ruby-core@...>
Date: 2026-03-03T09:14:44+00:00
Subject: [ruby-core:124901] [Ruby Bug#21931] GC Crash in `String#%` (backport 726205b354d1068147719fb42e1de743f1838ef1)

Issue #21931 has been reported by byroot (Jean Boussier).

----------------------------------------
Bug #21931: GC Crash in `String#%` (backport 726205b354d1068147719fb42e1de743f1838ef1)
https://bugs.ruby-lang.org/issues/21931

* Author: byroot (Jean Boussier)
* Status: Open
* Backport: 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED, 4.0: REQUIRED
----------------------------------------
Reproduction

```ruby
GC.stress = true

class Args
  def to_ary
    Array.new(64) { "x" * 1024 }
  end
end

fmt = "%s" * 64
100_000.times do
  fmt % Args.new
end
```

Patch that should be backported https://github.com/ruby/ruby/commit/726205b354d1068147719fb42e1de743f1838ef1



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