[ruby-core:78352] [Ruby trunk Bug#12688] Thread unsafety in autoload

Issue #12688 has been updated by Shyouhei Urabe.


We looked at this issue in todays developer meeting and had 2 feelings in common.

1.  The autoload should not render NameError.  It definitely is a bug that must be fixed.  Ko1 is assigned.
2. Besides, we want to discourage people from doing something like `1 until go`. JRuby+Truffle might optimize this out (as far as I understand). Also generally speaking, sharing local variables across threads is something difficult to do properly.

----------------------------------------
Bug #12688: Thread unsafety in autoload
https://bugs.ruby-lang.org/issues/12688#change-61724

* Author: Charles Nutter
* Status: Assigned
* Priority: Normal
* Assignee: Koichi Sasada
* ruby -v: 2.3.0
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
I need clarification here. I expected, based on Ruby's assertion that autoloads are thread-safe, that the following code would never error. Instead, it gets a couple iterations in and raises NameError:

```ruby
loop do
  class Foo
    autoload :Bar, 'bar.rb'
  end

  go = false
  threads = (1..50).map {Thread.new { 1 until go; print '.'; Foo.const_get(:Bar) }}
  go = true
  threads.each(&:join)
  puts

  self.class.send :remove_const, :Foo
end
```

And the output with Ruby 2.3.0:

```
$ ruby23 -I. autoload_breaker.rb 
..................................................
..................................................autoload_breaker.rb:7:in `const_get': uninitialized constant Foo::Bar (NameError)
Did you mean?  Foo::Bar
	from autoload_breaker.rb:7:in `block (3 levels) in <main>'
```

Is there something wrong with my script? Is my expectation incorrect?



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