[ruby-core:108079] [Ruby master Bug#18663] Autoload doesn't work with fiber context switch.

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


Autoload uses a spin lock:

```c
static VALUE
autoload_sleep(VALUE arg)
{
    struct autoload_state *state = (struct autoload_state *)arg;

    /*
     * autoload_reset in other thread will resume us and remove us
     * from the waitq list
     */
    do {
	rb_thread_sleep_deadly();
    } while (state->thread != Qfalse);

    return Qfalse;
}
```

It basically spins on `state->thread`.

I hesitate to call it poorly implemented, but it doesn’t look great. I don’t fully understand the implementation yet.

----------------------------------------
Bug #18663: Autoload doesn't work with fiber context switch.
https://bugs.ruby-lang.org/issues/18663#change-97037

* 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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