[ruby-core:116468] [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).


While in threads Ruby controls context switching and coordination for autoloading constants or loading files with `Kernel#require` is built-in, the problem I see with fibers is that Ruby has no control, the user has the control by design.

For example, consider this script:

```ruby
File.write('/tmp/bar.rb', <<~RUBY)
  Fiber.yield
  Bar = 1
RUBY

autoload :Bar, '/tmp/bar.rb'

Fiber.new { Bar }.resume
Fiber.new { Bar }.resume
```

produces a deadlock:

```
deadlock; lock already owned by another fiber belonging to the same thread (ThreadError)
```

While yielding like that is artificial, the analogous situation in threads does not deadlock, and does not err either. Threads wait as needed until the one autoloading finishes.

In the abstract, this is also a potential issue I see with the fiber scheduler interface, because it does not establish a contract for concurrent autoloads or requires. If a fiber does a non-blocking operation while the file is being loaded, is that was part of an `autoload` or `require`, what can we assume about that situation?


----------------------------------------
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-106489

* 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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