[ruby-core:108478] [Ruby master Bug#18663] Autoload doesn't work with fiber context switch.

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


https://github.com/ruby/ruby/pull/5788 fixes this issue.

I've confirmed that my PR fixes the given examples here.

There is a tiny bit of extra overhead; using a mutex has an object allocation, mutex lock and unlock, etc.

A light weight concurrency construct `rb_condition` or `rb_fiber_condition` that behaves like a "pessimistic mutex", i.e. avoids the allocation until it's actually needed might be a better solution but let's get it correct first, and we can optimise it later.

Just a few notes:

```c
struct rb_condition {
  ... waitq;
}

VALUE rb_condition_wait(struct rb_condition *condition, VALUE timeout);
rb_condition_signal(struct rb_condition *condition, VALUE result); // wake up all waiters
```

----------------------------------------
Bug #18663: Autoload doesn't work with fiber context switch.
https://bugs.ruby-lang.org/issues/18663#change-97521

* 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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