[ruby-core:111348] [Ruby master Feature#19078] Introduce `Fiber#storage` for inheritable fiber-scoped variables.

Issue #19078 has been updated by lloeki (Loic Nageleisen).


This is really nice for some use cases, and I was planning to propose basically exactly what @ioquatix proposed here to tackle these.

One example: we create context objects that allow us to track what happens across a given HTTP request execution. Typically with Rack we can store that in the Rack environment, but it so happens that elements we instrument via module prepending may not have access to the Rack environment. To get the context we then have to store that context object in a thread local (which is actually fiber local). The consequence is that if some bit code then goes async/concurrent (another thread or fiber) then context is (of course) lost. This would allow to keep the context within which we want to track things. As it turns out one of our team members from Sqreen implemented AsyncLocalContext which is incredibly similar in its purpose: https://nodejs.org/api/async_context.html

Another example: [Rails is doing unhappy things to thread locals](https://github.com/rails/rails/blob/1512cf2ba578282c898b8eb68ac401ea5243b7b5/actionpack/lib/action_controller/metal/live.rb#L261-L263) when using `ActionController::Live`: it blindly copies thread locals to the new thread in an attempt to have the new thread behave like the old one. This is not good as it assumes none of these thread locals would be concurrently accessed or mutated, when by design thread locals are supposed to be, well, local to threads, thus can eschew thread safetiness safe nets. With this feature, Rails may be in a position to use it for what it needs and maybe stop its blind copying.

----------------------------------------
Feature #19078: Introduce `Fiber#storage` for inheritable fiber-scoped variables.
https://bugs.ruby-lang.org/issues/19078#change-100722

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
----------------------------------------
Pull Request: https://github.com/ruby/ruby/pull/6612

This is an evolution of the previous ideas:

- https://bugs.ruby-lang.org/issues/19058
- https://bugs.ruby-lang.org/issues/19062

This PR introduces fiber scoped variables, and is a solution for problems like <https://github.com/ioquatix/ioquatix/discussions/17>.

The main interface is:

```ruby
Fiber[key] = value
Fiber[key] # => value
```

The variables are scoped (local to) a fiber and inherited into child fibers and threads.

```ruby
Fiber[:request_id] = SecureRandom.hex(16)

Fiber.new do
  p Fiber[:request_id] # prints the above request id
end
```

The fiber scoped variables are stored and can be accessed:

```ruby
Fiber.current.storage # => returns a Hash (copy) of the internal storage.
Fiber.current.storage= # => assigns a Hash (copy) to the internal storage.
```

Fiber itself has one new keyword argument:

```
Fiber.new(..., storage: hash, false, undef, nil)
```

This can control how the fiber variables are setup in a child context.

To minimise the performance overhead of some of the implementation choices, we are also simultaneously implementing <https://bugs.ruby-lang.org/issues/19077>.

## Examples

### Request loop

```ruby
Thread.new do
  while request = queue.pop
    Fiber.new(storage: {id: SecureRandom.hex(16)}) do
      handle_request.call(request)
    end
  end
end
```

OR

```ruby
Thread.new do
  while request = queue.pop
    Fiber.current.storage = {id: SecureRandom.hex(16)}
    handle_request.call(request)
  end
end
```




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