[ruby-core:116478] [Ruby master Bug#20188] `Module#const_source_location` returns wrong information when real constant was defined but autoload is still ongoing

Issue #20188 has been updated by fxn (Xavier Noria).


To me, this is related with `autoload`, because it is the `autoload` the one making the fibers deadlock. If the last line was

```ruby
Fiber.new { p 1 }.resume
```

instead of a reference to the constant being autoloaded, there would be no deadlock, right? Same with `require`, if we try this without autoloads:

```ruby
File.write('/tmp/bar.rb', <<~RUBY)
  Fiber.yield
RUBY

Fiber.new { require '/tmp/bar.rb' }.resume
Fiber.new { require '/tmp/bar.rb' }.resume
```

we get a deadlock.

So that is my point, what we say about threads does not translate to fibers. In fibers, you can deadlock.

`Kernel#require` is thread-safe, things that would typically trigger a context switch (like I/O or `sleep`) won't make you deadlock if the other threads are waiting for that `require`. And same for `autoload`. And makes sense, because Ruby owns the thread schedulur mod hints API.

> fibers being cooperative

That is less the case with fiber schedulers, no?

Can Falcon trigger deadlocks?


----------------------------------------
Bug #20188: `Module#const_source_location` returns wrong information when real constant was defined but autoload is still ongoing
https://bugs.ruby-lang.org/issues/20188#change-106502

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin23]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
Ref: https://github.com/fxn/zeitwerk/issues/281

`const_source_location` keeps returning the location of the `autoload` call, even after the real constant was defined. It only starts returning the real constant location once the autoload was fully completed.

```ruby
# /tmp/autoload.rb
File.write("/tmp/const.rb", <<~RUBY)
module Const
  LOCATION = Object.const_source_location(:Const)
end
RUBY

autoload :Const, "/tmp/const"

p Const::LOCATION
```

Expected Output:

```ruby
["/tmp/const.rb", 1]
```

Actual Output:

```ruby
["/tmp/autoload.rb", 8]
```

Potential patch: https://github.com/ruby/ruby/pull/9549



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