[ruby-core:124922] [Ruby Bug#21941] Local variable becomes nil when YJIT enabled mid-method with fork/signal/ensure

4 Mar 2026

      Issue #21941 has been updated by byroot (Jean Boussier).

Reduced some more, no IO or anything:

```ruby
def run
  fork_safe = ->(t) { t }
  RubyVM::YJIT.enable

  i = 0

  begin
    while i < 100
      i += 1
      p i
      begin
        next if i
      rescue Interrupt
      end
    end
  ensure
  end
  p :ok
end

run
```

----------------------------------------
Bug #21941: Local variable becomes nil when YJIT enabled mid-method with fork/signal/ensure
https://bugs.ruby-lang.org/issues/21941#change-116581

* Author: nicholasdower (Nick Dower)
* Status: Open
* Assignee: jit
* ruby -v: ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin25]
* Backport: 3.2: DONTNEED, 3.3: DONTNEED, 3.4: REQUIRED, 4.0: REQUIRED
----------------------------------------
The following code results in the `read` local variable becoming nil, even though it is never reassigned:

```
def run
  fork_safe = ->(t) { t }
  RubyVM::YJIT.enable

  read, wakeup = IO.pipe
  Signal.trap("SIGCHLD") { wakeup.write("!") }

  begin
    while true
      begin
        fork { exit }

        next if read.wait_readable
      rescue Interrupt
      end
    end
  ensure
  end
end

run
```

Error:
```
repro.rb:13:in 'Object#run': undefined method 'wait_readable' for nil (NoMethodError)

        next if read.wait_readable
                    ^^^^^^^^^^^^^^
	from repro.rb:21:in '<main>'
```

See also:
https://github.com/puma/puma/issues/3620
https://github.com/Shopify/ruby/issues/625

-- 
https://bugs.ruby-lang.org/