[ruby-core:111357] [Ruby master Feature#19078] Introduce `Fiber#storage` for inheritable fiber-scoped variables.

Issue #19078 has been updated by Eregon (Benoit Daloze).


I had call with @ioquatix, we agreed the changes of https://github.com/ruby/ruby/pull/6972 are OK.

It's unclear if Fiber#storage and Fiber#storage= need to be allowed to be called for a different Fiber of the same thread (e.g., might be useful for a debugger).
One concern is if Fibers become no longer fixed to a given Thread in the future then this condition wouldn't mean anything anymore, and we would have the issue of accessing state being concurrently mutated in another thread (on Rubies without GIL).
I think for debugging we should have a new API to "transfer" to a Fiber but not execute any code of that Fiber, just whatever the debugger wants to run there (e.g. get the backtrace, the stack, local variables, get the fiber locals, etc). A bit like `thread N` in `gdb`.

----------------------------------------
Feature #19078: Introduce `Fiber#storage` for inheritable fiber-scoped variables.
https://bugs.ruby-lang.org/issues/19078#change-100733

* 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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