From: "mame (Yusuke Endoh) via ruby-core" <ruby-core@...>
Date: 2026-10-14T19:31:04+00:00
Subject: [ruby-core:127110] [Ruby Bug#22442] Lazy loading of error_highlight etc. is skipped in threads and Ractors

Issue #22442 has been reported by mame (Yusuke Endoh).

----------------------------------------
Bug #22442: Lazy loading of error_highlight etc. is skipped in threads and Ractors
https://bugs.ruby-lang.org/issues/22442

* Author: mame (Yusuke Endoh)
* Status: Assigned
* Assignee: hsbt (Hiroshi SHIBATA)
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
#21951 made error_highlight / did_you_mean / syntax_suggest lazily loaded.
When the lazy loading is triggered concurrently from threads or Ractors, the decoration may be skipped.

```ruby
# repro.rb
rs = 2.times.map do
  Ractor.new do
    nil.upcase
  rescue => e
    e.detailed_message(highlight: false)
  end
end
rs.each { puts it.value, "" }
```

Expected:

```
$ ruby repro.rb
undefined method 'upcase' for nil (NoMethodError)

    nil.upcase
       ^^^^^^^

undefined method 'upcase' for nil (NoMethodError)

    nil.upcase
       ^^^^^^^

```

Actual:

```
$ ruby repro.rb
undefined method 'upcase' for nil (NoMethodError)

    nil.upcase
       ^^^^^^^

undefined method 'upcase' for nil (NoMethodError)

```

Cause: the first caller of `detailed_message` sets a flag when it starts loading. So when another thread or Ractor calls it during the load, it returns without decoration instead of waiting for the load to complete.

Fix: fire the `autoload` instead of calling `rb_require_string`, so that the autoload's synchronization prevents the skip.

https://github.com/ruby/ruby/pull/19352

Also, because `rb_require_string` was used, a newer version installed by `gem install error_highlight` was not used.
Firing the `autoload` respects the monkeypatch of `Kernel#require`, so the newer version of error_highlight will be used.

Note: this is why the snippet is not shown in #22439.




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