From: "byroot (Jean Boussier) via ruby-core" <ruby-core@...>
Date: 2026-03-04T21:55:55+00:00
Subject: [ruby-core:124921] [Ruby Bug#21941] Local variable becomes nil when YJIT enabled mid-method with fork/signal/ensure

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

Assignee set to jit
Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN to 3.2: DONTNEED, 3.3: DONTNEED, 3.4: REQUIRED, 4.0: REQUIRED

Reduced even further:

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

  read, wakeup = IO.pipe
  wakeup.write("!")

  begin
    while true
      begin
        next if read.wait_readable
      rescue Interrupt
      end
    end
  ensure
  end
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-116580

* 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



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