[ruby-core:108113] [Ruby master Bug#18663] Autoload doesn't work with fiber context switch.

Issue #18663 has been updated by ioquatix (Samuel Williams).


I've tracked down the root of this bug, being that it's not yielding to the fiber scheduler and implements it's own condition variable like semantics. I'll propose a fix.

----------------------------------------
Bug #18663: Autoload doesn't work with fiber context switch.
https://bugs.ruby-lang.org/issues/18663#change-97078

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
As discussed most recently here: https://github.com/ruby/debug/issues/580

The following program appears to work:

```ruby
#!/usr/bin/env ruby

require 'tempfile'

Tempfile.create(['foo', '.rb']) do |file|
  file.write(<<~RUBY)
    #
    $stderr.puts 1; q = Queue.new
    $stderr.puts 2; t = Thread.new{q.pop}
    $stderr.puts 3; q << :sig
    $stderr.puts 4; t.join

    sleep 1

    class C
    end
  RUBY
  file.close

  autoload :C, file.path

  Thread.new do
    threads = 3.times.map do |i|
      Thread.new do
        $stderr.puts "LOADING C" 
        $stderr.puts C
      end
    end

    threads.each(&:join)
  end.join
end
```

This one doesn't:

```ruby
#!/usr/bin/env ruby

require 'tempfile'
require_relative 'lib/async'

Tempfile.create(['foo', '.rb']) do |file|
  file.write(<<~RUBY)
    #
    $stderr.puts 1; q = Queue.new
    $stderr.puts 2; t = Thread.new{q.pop}
    $stderr.puts 3; q << :sig
    $stderr.puts 4; t.join

    class C
    end
  RUBY
  file.close

  autoload :C, file.path

  Async do |task|
    3.times do |i|
      task.async do
        $stderr.puts "LOADING C" 
        $stderr.puts C
      end
    end
  end.wait
end
```

Semantically, they should be very similar. It feels like someone is checking the current thread rather than the current fiber or there is a poor implementation of locking somewhere, however I don't actually know for sure yet, investigation is required.



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