[ruby-core:111355] [Ruby master Feature#19078] Introduce `Fiber#storage` for inheritable fiber-scoped variables.

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


Eregon (Benoit Daloze) wrote in #note-30:
> Also this is a necessary condition to avoid synchronization on Fiber[]/Fiber[]= for non-GIL Rubies.

Correction: that is not the case since at least storage cannot be shared between threads.

But it's still a problem for all the other reasons.
These are fiber-scoped variables, hence it should never be possible to affect a parent fiber by a change in a child fiber (otherwise that just becomes global state).
If it's wanted to change something and have all fiber referencing it to see it, it's very easy, just an use an indirection and not the fiber-scoped variable itself.

----------------------------------------
Feature #19078: Introduce `Fiber#storage` for inheritable fiber-scoped variables.
https://bugs.ruby-lang.org/issues/19078#change-100730

* 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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